Glossary
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A

Aberration
Chromatic: 
An optical image distortion conditional on the varying refraction of light rays of different wavelengths on a lens. Thus light rays of shorter wavelengths have longer focal distances than light rays of longer wavelengths.
Spherical: 
An optical image distortion conditional on the varying distance of paraxial light rays of the same wavelength from the optic axis. Light rays that travel through outer lens zones have shorter focal distances than rays that travel through the lens center (optic axis). The corrector lens on the front of a Schmidt-Cassegrain, for example, is an aspheric lens which corrects the aberration from the spherical primary mirror.
ABG - Anti-Blooming Gate
An electronic drain structure on a CCD chip which assures that electrons/voltage exceeding the full-well capacity of a pixel do not spill over to adjacent pixels. 
Absolute Zero
The lowest possible temperature, attained when a system is at its minimum possible energy. The Kelvin temperature scale sets its zero point at absolute zero (-273.15 on the Celsius scale, and -459.67 on the Fahrenheit scale). 
The idea of a true minimum temperature has been confirmed by many experiments. Given the concept of temperature as molecular energy it follows that there must be a point at which no further energy can be extracted from a system. 
Although it is possible to approach ever closer to absolute zero, the "third law" of thermodynamics holds that it is impossible to attain absolute zero in a system. 

The present temperature of the cosmic background radiation is about 2.7 degrees above absolute zero. If the universe expands forever, this temperature will asymptotically approach absolute zero. 
AC - Alternating Current
Electrical current that reverses (or alternates) at regular intervals.
Achromat (achromatic objective)
Describes a correction class for objectives. The chromatic aberration for two wavelengths is corrected for objectives of this type. Usually an objective of this type is corrected to a wavelength below 500nm and above 600nm. Furthermore, the sine condition for one wavelength is met. The image curvature aberration is not corrected.
Accretion Disk
A disk of gas which accumulates around a center of gravitational attraction, such as a white dwarf, neutron star, or black hole. As the gas spirals in, it becomes hot and emits light or even X-radiation. 
Age of the Universe, The
An expanding universe must have been smaller in the past, and in fact the distance between any two points approaches zero roughly 13 billion years ago. This moment of ultra-high density is called the Big Bang, and marks the birth of the universe (at least to all intents and purposes). The age of the universe is therefore about 13 billion years. For about half a million years after the Big Bang, the universe was opaque to electromagnetic radiation. This sets a maximum distance that we can see: radiation emitted just as the universe became transparent (the cosmic microwave background), that is reaching us now, has traveled 13 billion light years. 
AGN - Active Galactic Nucleus [Plural: Active Galactic Nuclei, (also AGN)].
To try to summarise what we know about AGN is to step into a minefield. There is a "standard model" which everyone agrees is at least partly wrong, but every expert has his or her own proposal for either fixing it up, or replacing it with something completely different. To avoid getting too bogged down in these controversies, the following description is deliberately vague in many places. 

AGN are exclusively found in the centres of large galaxies; the galaxy containing an AGN is said to be its host. The central light-year or so of an AGN contains an enormous mass, equivalent to at least a million suns, and sometimes ranging up to a few billion suns. This region also contains something that shines brightly in the part of the electromagnetic spectrum from the ultraviolet through to X-rays. There is pretty good (but disputed) evidence that all this is actually concentrated on a much smaller scale, sometimes less than a few light-days across. Certainly the X-rays originate in an astronomically tiny volume, smaller than our own solar system. To put this into perspective, within a light-year of us there is just one sun (ours), and the host galaxies are typically 100 thousand light-years across or more. 

The thing (if it is one thing) at the heart of an AGN is often called the "monster". If general relativity is correct it seems almost inevitable that nearly all the mass of the monster is contained in a spinning black hole. Black holes don't radiate, by definition, so the radiation from an AGN is believed to come from gas clouds falling into the hole. In a way, the monster works as a sort of engine, fuelled by matter falling in (accreting, in the jargon). To be more precise, the gravitational potential energy of the accreted stuff is ultimately converted to radiation, and to kinetic energy in the form of jets. We don't know just how this happens, although there are dozens of competing theories. 

At larger distances from the center swirl clouds of gas and dust which are lit up and heated by the central heart, producing the characteristic emission lines and infra-red radiation. The outermost regions can be imaged from Earth in some of the nearest AGN, but mostly AGN appear just as points of light. 
ADC - Analog-Digital-Converter
An electronic device, often an integrated circuit, that converts an analog voltage to a digital value. All digital instruments use an A/D converter to convert the input signal into digital information. The output signal does only change at special times and can only take special values - quantization. Compare with DAC
ADU - Analog-to-digital Unit
ADUs are employed as a measurement of pixel value or brightness. Pixel voltages (numbers of electrons) stored during CCD integrations are converted to ADU integers representing the measured voltage compared to maximum (full pixel) voltages in terms of the full Base 2 dynamic range of the CCD system (12 bit = 2^12th, 16 bit = 2^16th, etc.). 
Afocal
A photographic method that uses an eyepiece in the telescope focused normally (you look through it) and a camera with its lens focused at infinity. You then just point the camera into the eyepiece. The camera can be on a separate tripod or attached to the telescope with a bracket or attached with a threaded adapter or even hand held. It is a great method to use for photographing the moon and planets. 
Ah 
Abbreviation for "ampere-hour". Designates an amount of electric charge. Used for accumulators to denominate their capacity. Because the voltage of an accumulator is nearly constant, you can calculate the stored energy from the given Ah-rating, e.g. 12Vx100Ah=1.2kWh.
Airy Disc
The Airy disc refers to the inner, light circle (surrounded by alternating dark and light diffraction rings) of the diffraction pattern of a point light source. The diffraction discs of two adjacent object points overlap some or completely, thus limiting the spatial resolution capacity.
Algorithm
In CCD imaging, an algorithm usually refers to a software procedure, often for image processing. An algorithm is the mathematical function which tells the computer what to do with an image.
Aliasing
An image distortion caused by a sampling frequency that is too low in relation to the signal frequency.
Where the sampling rate is less than twice the input signal's highest frequency content.
Alt-Azimuth (also called Alt-Az)
Short for Altitude-Azimuth . Telescopes which are mounted so that they move up-down and left-right (as opposed to equatorially) are called alt-az. This is a convenient mounting configuration for visual observing as the eyepiece is always in a convenient position. However, an equatorial wedge must be used for photography or CCD imaging.
Amplifier
A device that uses an active component to increase the voltage or power of a signal without distorting its waveshape.
analog 
A continuous, non-digital representation of phenomena. An analog voltage, for example, may take any value. Opposite to "digital".
AND Gate
A gate whose output is ON only if all input signals are ON.
Angle of Incidence 
Angle between the incident ray of light and a normal drawn to the point of reflection. I.e. The angle between the optical axis of the light incident on the surface of a filter and the axis normal to this surface.
Angle of Reflection
Angle between the ray of light and the normal drawn to the point of refraction.
Angle of Refraction
Angle between the refracted ray of light and the normal drawn to the point of reflection.
Angle of view
The amount of a scene that can be recorded by a particular lens; determined by the focal length of the lens; also field of view FOV.
Angular deviation
A shift in the direction of light beam from the true optical axis of the system, measured in units of angle such as arcminutes (1/60 of a degree) or arcseconds (1/60 of an arcminute).
Angstrom (Ċ) 
A unit of length. 1/10,000 of a micrometer (10-4µm).
Anti-Blooming
This feature is added to a CCD chip to prevent pixel blooming.  This feature generally reduces sensitivity, well-depth, and linear response.  For these reasons, non-anti-blooming chips are popular, and there is even software available to remove blooming streaks from CCD images
Antimatter
Particles with certain properties opposite to those of matter. Each matter particle has a corresponding antiparticle. The antiparticle has exactly the same mass and the opposite electric charge as its partner. An example is the electron (negative charge) and its antimatter version the positron (positive charge). 
When a particle and its antiparticle collide, both are annihilated and converted into photons. Similarly two photons with sufficient energy can combine to form a particle-antiparticle pair. 
The universe is made almost entirely out of matter. This means that in the big bang there was an excess of matter over antimatter so that when matter and antimatter combined and annihilated, some matter was left over. 
Apparent Field of View
A characteristic of eyepieces. The apparent field of view is the angle through which your eyeball rotates when you look through an eyepiece and transfer your gaze from one edge of the field to the other. 
Aperture
The lens opening formed by the iris diaphragm inside the lens. The size of the hole can be made larger or smaller by the auto focus system or a manual control. The size is indicated as a 'f-number' or 'f-stop' i.e. f/4, f/5.6, or f/8.
Aperture diaphragm
An adjustable diaphragm located in the illumination optics, which controls the numerical aperture of the illuminating beam and affects the brightness of the beam.
Aperture, maximum
The largest size of the hole though which light enters the camera.
Aperture, numerical
The aperture is the sine of the angle under which light enters into the front lens of a microscope objective; its symbol is NA. The aperture influences both the light gathering capacity and the resolution capacity of an objective. Since various media can be present between specimen and objective lens (such as the embedding medium for the specimen), the numerical aperture (NA = n * sin a) is usually applied as the unit of measurement for the light gathering capacity and the resolution capacity.
Aperture Synthesis
The technique of combining the signals from a collection of individual antennas or telescopes to provide an image with a resolution equivalent to a single telescope with a size roughly equal to the maximum distance between the individual antennas. This may be quite large, e.g. 217 km for MERLIN, and up to the size of the Earth for VLBI.
Apochromat (apochromatic objective)
Describes a correction class for objectives. The chromatic aberration for three wavelengths is corrected for objectives of this type (usually 450nm, 550nm and 650nm) and the sine condition for at least two colors is met. The image curvature aberration is not corrected.
Artifacts
Erroneous pixels created during the capture phase of imaging, caused by electrical interference or physical barriers such as dust.
Aspheric Surface
A lens or mirror surface that is altered slightly from spherical to reduce aberrations.
Aspect ratio
The relationship of the X and Y scales of a 2-dimensional grid. Non-square CCD pixels are represented as square by video monitors and other output devices, yielding an aspect ratio not in accord with true sky coordinates unless the images are resampled to an aspect ratio that, in effect, squares the pixels. 
I.e. The ratio between the width and height of an image or image sensor.
Astronomy
The science that studies the natural world beyond the earth.
Asynchronous
In hardware, it is an event that occurs independent of other events; it is not synchronized with a clock signal. 
In software, it refers to a function that begins an operation and returns to the calling program prior to the completion or termination of the operation.
Atom
The smallest component of matter which retains its chemical properties. An atom consists of a nucleus composed of at least one proton, some number of neutrons, and at least one electron
The atomic number of an atom corresponds to the number of protons present in the nucleus of an atom. This determines its elemental identity. The number of neutrons determines the isotope of the atom. 
Attenuation Level (also Blocking level)
A measure of the out-of-band attenuation of an optical filter, over an extended range of the spectrum. The attenuation level is often defined in units of optical density
Aurora
Spectacular array of light in the night sky, caused by charged particles from the Sun hitting the Earth's upper atmosphere. The aurora borealis is seen in the north of the Northern hemisphere; the aurora australis in the south of the Southern.
Autoguiding / Guiding
Telescope tracking controlled by feedback from real-time sensing of star movements within the field of view (FOV). Movement may be sensed by an electro-optical device, such as a CCD chip, or by the human eye comparing star movement to a eyepiece reticle intersection or a reticle grid. Autoguiding refers to automatic feedback to telescope drives provided by electronic devices, while manual guiding is accomplished by human feedback intervention using slow-motion controls on telescope drives. 
Automatic exposure
A mode of camera operation in which the camera automatically adjusts the aperture, shutter speed, or both for proper exposure.
AutoStar
A brand name for Meade's hand-held computerized controller.
Average transmission
The average calculated over the useful transmission region of a filter, rather than over the entire spectrum. For a bandpass filter, this region spans the FVMM of the transmission band.
Averted Vision
When you look squarely at something, you are using a part of the retina of your eye that is not as sensitive to low light levels as the parts that are off to the side. Thus to see faint objects, don't look straight at them. Center them in the field of view of your telescope, but fix your stare part way out to the edge of the field. 
Azimuth
a) Directional bearing around the horizon, measured in degrees from north (0°).
b) Angular distance from the north point eastward to the intersection of the celestial horizon with the vertical circle passing through the object and the zenith.
 

B

Bandpass
An optical filter that has a well-defined short wavelength cut-on and long wavelength cut-off. Bandpass filters are denoted by their center wavelength and bandwidth.
Bandwidth
Also FWHM. For optical bandpass filters, typically the separation between the cut-on and cut-off wavelengths at 50% of peak transmission. Sometimes a bandwidth at, for example, 10% of peak transmission is specified. I.e. The highest frequency signal component that can pass through input amplifiers and/or filters without being attenuated.
Barlow
An extra lens you can add to an eyepiece to amplify the magnification. Usually a 2 times multiplier. 
Base
The control portion of a bipolar transistor. In an NPN transistor, the P-type material forms the base.
Bayer pattern
A pattern of red, green, and blue filters on the image sensor's photosites. There are twice as many green filters as the other colors because the human eye is more sensitive to green and therefore green color accuracy is more important.

Bias Signal
The electrons and subsequent ADU generated by the voltage maintained over the CCD array during integration. 
Big Bang
The state of extremely high (classically, infinite) density and temperature from which the universe began expanding. The beginning point of time and space for the universe. 
Big Crunch
One hypothesized future for the universe in which the current expansion stops, reverses, and results in all space and all matter collapsing together; a reversal of the big bang.
Binary 
A system of numbers using 2 as a base, in contrast to the decimal system which uses 10 as a base. The binary system requires only two symbols: 0 and 1.
Binary Star
A system of two stars orbiting around a common center of gravity. Visual binaries are those whose components can be resolved telescopically (i.e., angular separation > 0'.5) and which have detectable orbital motion. Astrometric binaries are those whose dual nature can be deduced from their variable proper motion; spectroscopic binaries, those whose dual nature can be deduced from their variable radial velocity. At least half of the stars in the solar neighborhood are members of binary (or multiple) systems.
Binning
Binning involves combining pixels on a CCD chip to create larger pixels.  For example, taking a 2x2 square of pixels and creating one pixel that is twice the width and four times the area of the original pixel.  This is done to increase sensitivity or to match a long focal length telescope to a CCD camera with small pixels. 
Binocular Viewer
A set of prisms that allows you to use two eyepieces on a telescope. Using both eyes is particularly good for the moon and planets. 
Bipolar
1. An analog signal range that includes both positive and negative values.
2. An electronic device whose operation depends on the transport of both holes and electrons.
Bipolar Transistor 
BJT (bipolar junction transistor). Most important semiconductor device, foundation for integrated circuits. You may think of it being an electrically controlled valve or current amplifier. Modern bipolar transistors work up to the three-digit GHz-range.
Bipolar Switch
An electronic switch which is able to control bipolar signals. The switch consist of two identical in/outputs and a control line which does open and close the switch.
There are usually in 2 or 4 switches packed in one IC  e.g. the CMOS IC 4016 contains four.
Bit 
A binary digit. A bit is the smallest unit of storage in a digital computer, and is used to represent one of the two states in the binary number system.
Bit Depth
A measurement of the number of bits used to create a single pixel in a digital image. A 24-bit RG8 image is created from a palette of 16.7 million colors.
Bit-mapped
Images formed from pixels with each pixel a shade of gray or color. Using 24-bit color, each pixel can be set to any one of 16 million colors.
Black Hole
An object so dense that its escape velocity exceeds the speed of light. According to general relativity, such an object must collapse to an infinitely dense point, a singularity. The singularity is surrounded by a surface called the event horizon, within which objects and information can only move inwards, quickly reaching the singularity (and being crushed to a point in the process, of course). Therefore nothing can escape from a black hole. A technical exception is Hawking radiation, a quantum mechanical process first described by Steven Hawking, but this is unimaginably weak for the massive black holes of interest to astronomers. 
Blocking range
The range of wavelengths over which an optical filter maintains a specified attenuation level.
Blooming
Each photosite of a CCD chip can contain a certain amount of electric charge. This amount is determined by the well depth of the CCD. When the well-depth is exceeded, electric charge "bleeds" out of the photosite appearing in an image as a bright streak extending vertically from a bright source in the image (usually a star).  This effect can be minimized or eliminated by using a CCD with an anti-blooming gate.
Blueshift
A shift in the frequency of a photon toward higher energy and shorter wavelength. Blueshifts can be produced by relative motion of the emitter toward the observer (doppler blueshift), light falling in a gravitational field from the emitter to the observer (gravitational blueshift), or in a contracting universe (cosmological blueshift). For further details, see Redshift
Boolean Algebra
A logical calculus named for mathematician George Boole, using alphabetic symbols to stand for logical variables, and 0 and 1 to represent states. AND, OR, and NOT are the three basic logic operations in this algebra. NAND and NOR are each combinations of two of the three operations.
Brightness
The business of observational astronomy boils down to measuring the brightness of celestial objects. Unfortunately, the English word "brightness" covers three quite different concepts, each of which covers several subtle variations. 
that is: Luminosity, Flux Density and Intensity (or Surface Brightness)
Brown Dwarf
A low-mass substellar object that is near the minimum mass for nuclear fusion reactions to occur in its core. Brown dwarf objects are a possible source of baryonic dark matter. Brown dwarfs are possible dark matter halo objects.
Byte
A group of eight bits (in the telecommunication field also octet).
Multipliers are Kilobyte (1024 bytes), megabyte (1024x1024 bytes) etc.

C

Calibration 
1. Generally, normalizing a system to a set of standards or constants. 
2. Specific to CCD imaging, to eliminate unwanted signal and reduce noise components by subtracting a dark frame and dividing by a flat-field frame.
Capacitance
1. The capability of storing electrical charge. Unit of measure is the Farad (F).
2. In a capacitor or system of conductors and dielectrics, the property that permits the storage of electrically separated charges when potential differences exist between the conductors. Capacitance is related to charge and voltage as follows: C = Q/V, where C is the capacitance in farads, Q is the charge in coulombs, and V is the voltage in volts.
Cassegrain
Telescope devised by Cassegrain in which an auxiliary convex mirror reflects the magnified image, upside down, through a hole in the center of the main objective mirror - i.e., through the end of the telescope itself.
Catadioptric
Any of a number of compromise telescope designs, using both a lens and mirrors. Examples are the Schmidt-Cassegrain and Maksutov-Cassegrain. Because the light path is folded twice, the telescope is very compact. 
CCD - Charge Coupled Device
A semiconductor device used for signal filters or as sensor elements. Electronic cameras get their picture from a CCD-sensor. The inner consists of a matrix of light sensitive elements, converting light into current. Because of the huge amount of elements, a direct wiring is not suitable. Therefore the charge packets generated during light exposure are passed by electric fields until they reach a connection point. I.e. "charge coupled". It operates by storing charge on capacitors and selectively moving that charge through the device by manipulating voltages on its electrodes.
Usually, CCDs are micromanufactured into two-dimensional grids of rows and columns, each intersection comprising a pixel several microns in both dimensions.
CCD raw format
The uninterpolated data collected directly from the image sensor before processing.
Cepheid Variable
A type of luminous giant star whose luminosity varies in a periodic fashion. Cepheids are characterized by a rapid rise in luminosity followed by a slow decline. The period of the cycle is related to the luminosity of the Cepheid by the Period-Luminosity relationship. The more luminous the Cepheid, the longer the period. This property makes Cepheids useful for obtaining distances. One determines the pulsation period and uses the relationship to get the luminosity. The apparent brightness of the star then gives you the distance. Cepheids come in two types, Type I which are metal rich and Type II which are metal poor. Type I Cepheids are more luminous than Type II. 
Circuit Layout
The physical arrangement of all the circuit elements on the surface of the device.
Chrominance
The color parameters of an image. Usually represented by hue and saturation. 
Clear Aperture
The surface area of an optical filter which is free of any defects or obstructions. On interference filters the clear aperture is often delimited by an annulus of metal or opaque material.
CG-5
A style of GEM made by Celestron. 
CMM - Color Management Modules
This is software, such as Apple's Color Sync and Kodak's Color Management System, that attempts to regulate the display of color.
CMOS - Complementary Metallic Oxide Semiconductor
Circuit technique initiating the high integration level of today's integrated circuits. Nearly all modern microprocessors are produced in CMOS-technology. Conceptual not the fastest design technique, but allowing for complex circuits and automatic layout.
CMOS image sensor
An image sensor created using CMOS technology. It requires more light but it is cheaper than an CCD sensor.
CMY - Cyan, Magenta, Yellow. 
Named the subtractive colors of the human visual spectrum, since cyan = white - red, magenta = white - green, and yellow = white - blue.
CMYK- Cyan, Magenta, Yellow, Key
Cyan, Magenta, Yellow, and Black (historically called the key color, hence K) are the standard inks used in the lithographic printing industry to reproduce color images.
Collector
The region of a bipolar transistor that "collects" the emitted electrons and then passes them on through a conductor, completing the electrical circuit.
Collimated Light
Light in which the rays are parallel.
Collimation
This refers to how correctly the optics are pointing towards each other. If a telescope is out of collimation, you will not get as clear an image as you should. Refractors generally have fixed optics, so you don't have to collimate them. Reflectors and catadioptrics usually have screws that you turn to collimate.
Color balance
The overall accuracy with which the colors in a photograph match or are capable of matching those in the original scene.
Color depth
The number of bits assigned to each pixel in the image and the number of colors that can be created from those bits. True Color uses 24 bits per pixel to render 16 million colors.
Color Space
Describes the mode used to represent color such as RGB, CMYK, or Lab. Each space has its own unique limitations.
Color Temperature
A stellar temperature determined by comparison of the spectral distribution of the star's radiation with that of a blackbody.
Coma
This refers to the blurring of objects at the edge of the field of view, most common in short focal ratio Newtonian telescopes (at f/10 and longer, Newtonians are very well corrected for coma).
CompactFlash
One of three competing memory card formats used in digital cameras. Holds between 2Mb and 128Mb of picture data.
Compression
The process of reducing the size of a file. Compression techniques are distinguished by whether they remove detail and color from the image. 'Lossless' techniques compress image data without removing detail; 'lossy' techniques compress images by removing detail.
Compression, lossless
A file compression scheme that makes a file smaller without degrading the image. This method is generally less effective than lossy methods in terms of resulting file size, but retains the entire original image.
Compression, lossy
A file compression scheme that reduces the size of a file but degrades it in the process so it can't be restored to its original quality. Once deleted the data cannot be recovered. The higher the compression the more noticeable the artifacts of the compression.
Conductor, Electrical 
A material capable of carrying (conducting) electricity. Silver is the best electrical conductor. Copper, gold, and aluminum are also popular conductors. Aluminum is the conductor most commonly used in IC fabrication.
Confocal (Confocality)
While the optical design of conventional microscopes results in the detection of both focused and unfocussed image components, the confocal principle suppresses the structures outside of the focal plane of the microscope objective. To achieve this pinholes are implemented in optically conjugated locations in the optical path. They function as point light source (excitation pinhole) and point detector (detection pinhole). The diameter of the detection pinhole, along with the wavelength and numerical aperture of the objective being used, determines the axial extension of an optical section.
Constellation
Precisely defined area of the celestial sphere, associated with a grouping of stars, that the International Astronomical Union has designated as a constellation.
Continuum Emission
Any type of electromagnetic emission which produces radiation over a relatively wide range of frequencies. c.f. line emission
Coordinates
Quantities which provide references for locations in space and time. A typical coordinate system consists of a point of reference (the origin), a set of directions (axes) that span space, and a set of labels that indicate how points are related to the origin. Coordinates in and of themselves are user defined and arbitrary, although certain simple, regular coordinate systems (e.g. Cartesian coordinates) are widely used. 

A Coordinate Singularity is a location at which a particular coordinate system fails, such as the Schwarzschild metric coordinates at the Schwarzschild radius of a black hole, or lines of longitude at the North pole. This failure doesn't indicate a breakdown in the underlying geometry. It is merely a failure of the coordinate system to give a unique well-defined label to a point in that geometry. 
Corrector Plate
Thin lens-like optical piece which removes certain optical aberrations.
Corrosion
An unwanted electrochemical process that affects device (e.g., semiconductor, telescope-mounts etc.) reliability.
Cosmic Background Radiation (CBR)
The Cosmic Background Radiation (CBR) consists of relic photons left over from the very hot, early phase of the Big Bang. It now peaks in the microwave band, corresponding to blackbody radiation with a temperature of about 2.7 degrees Kelvin. The CBR is also sometimes called the Microwave Background, or the Cosmic Microwave Background (CMB). 
Cosmic Rays
Cosmic rays are really charged particles such as protons, alpha-particles (i.e. helium nuclei) and electrons, traveling at almost the speed of light, c. From the theory of special relativity, cosmic rays carry a very high energy (tending to infinity as the speed tends towards c). This energy is much larger than their rest-mass energy (mc ²), and so they are also known as high-energy or relativistic particles. 
Cosmological Constant, the
A physical constant that appears in the theory of general relativity. It corresponds to a force between particles which increases with separation, and only has an effect over cosmological distances, hence the name. Although the constant fits well into the equations, they are more elegant when the constant is dropped (set to zero). Einstein originally put the constant in because without it he found that general relativity predicted an expanding or contracting universe, and he assumed that the universe must be static. By doing this he missed the chance to claim the expansion of the universe as a prediction of GR. To rub salt into this wound, it later turned out that Einstein's model of a static universe held balanced by the cosmological constant could never have worked: the balance is precarious and the slightest disturbance would send the universe into accelerating contraction or expansion. 

Not surprisingly, Einstein called the cosmological constant `my greatest blunder'. 
Cosmological Distance Ladder, The 
Distances to galaxies are found by a long chain of arguments called the cosmological distance ladder. 
1. A scale model of the solar system can be constructed from observations of the motions of the planets in the sky. All distances are known in terms of the radius of the Earth's orbit, the Astronomical Unit, (AU). Copernicus made the first roughly accurate solar system model, using data taken in ancient times, in his famous De Revolutionibus (1543). Modern models are exquisitely accurate. 
2. The actual distances to most of the planets can be measured by radar, and since we also know the distance to them in AU, the length of the AU can be found (to nine significant figures!). 
3. Distances to nearby stars can be found by various geometrical methods. The simples is via their annual parallax, i.e. their apparent change of position in the sky caused by the motion of the observer on Earth around the sun. The best precision is now about three significant figures in a handful of cases. More usefully, the final results of ESA's Hipparcos satellite (released in April 1997) give the parallaxes of around 10,000 stars to within few percent. Before Hipparcos, annual parallaxes were not as important as some more subtle geometric methods, but the new data will change the situation completely. 
4. Stars of similar type have similar luminosities. Thus if we know a star's type (from its colour and/or spectrum) we can find its distance by comparing its apparent with its absolute magnitude; the latter derived from geometric parallaxes to nearby stars. Unfortunately nearby stars are not very bright in absolute terms, so we cannot see distant versions very far away (certainly not in other galaxies). 
5. Distances to the super-bright stars that can be seen in other galaxies (especially Cepheid variables and RR Lyrae stars) are found by searching for distant star clusters in our Galaxy that contain both a Cepheid (say) and some fainter stars whose absolute magnitudes are known directly. 
6. Distances to the nearest galaxies are found using Cepheids, RR Lyraes, etc. The Hubble Space Telescope is now finding Cepheids in galaxies about ten times more distant than was possible from the ground. 
7. For more distant galaxies, we need objects even brighter than Cepheids. Examples are supernova explosions, planetary nebulae, and globular star clusters. The absolute magnitudes for such things can't be easily found in our own Galaxy, so they are measured in nearby galaxies (or clusters of galaxies) with Cepheid or similar distances. 
8. At the furthest limits, only whole galaxies are detectable. Galaxies come in a very wide range of luminosities, so we need a way to find their luminosity before we can get their distance. Various methods exist. For instance galaxy luminosity is related to the speed of internal motions; most radio galaxies seem to have similar luminosities; the range of brightnesses in clusters of galaxies do not vary much from one cluster to another. 
At every step of the distance ladder, errors and uncertainties creep in. Each step inherits all the problems of the ones below, and also the errors intrinsic to each step tend to get larger for the more distant objects; thus the spectacular precision at the base of the ladder degenerates into an uncertainty of a factor of several at the very top. 

To find Hubble's Constant, the ratio of the cosmological recession speed to the distance, we need to go up to Step 7 of the ladder. This is because we can only measure the sum of the recession speed and the random motion of a galaxy, and so we need to go far enough away that the the random motions are small compared to the recession speed. 

More details of individual methods are given in Ned Wright's The ABC's of Distances. A good book describing the Distance Ladder in detail is Rowan-Robinson (1985) (although a lot has happened since it was written). 
Object  Distance (pc Method
Sun, Solar System 10-6 Radar, Orbits
Alpha Centari parallax
Hyades Cluster 40  Hipparcos parallax
Galaxy 104 Cepheids, Main Sequence Fitting
Andromeda 105 Cepheids, Supernovae, OB star
Virgo Cluster 107 HST Cepheids, OB stars, SN
Beyond  108 and up Brightest Galaxies, Tully-Fisher
 
Counter Weights
Many telescope motors are not very powerful, if you don't balance the system (telescope & camera) the motors cannot drive accurately. 
CPU - Central Processing Unit
see Processor
Crayford
A style of eyepiece focuser that does not use gears; very smooth motion. 
Critical Angle
Angle of incidence for which the angle of refraction is 90° when light goes from one medium of high index of refraction into one of lower index.
CRT - Cathode Ray Tube
The CRT is a vacuum tube used as a display screen in a monitor or television set. The inner surface of the CRT is coated with phosphors, which glow and produce light when hit by an electron beam.
Curvature Constant (k)
A constant (k) appearing in the Robertson-Walker metric which determines the curvature of the spatial geometry of the universe. The three standard Friedmann models have 
k > 1 for positive curvature (spherical geometry) 
k < 1 for negative curvature (hyperbolic geometry) 
k = 1 for zero curvature (flat geometry) 
Curvature of image field
The curved surface to which a microscopic image is to be clearly and distinctly mapped is described as image curvature aberration. It is conditional on the convex shape of the lens and makes itself apparent as an error due to the short focal distances of microscope objectives. Here the object image is not in focus both in the center and at the periphery at the same time. Objectives that are corrected for image curvature aberration are called plane objectives (plane = flat image field).

D

DAC - Digital-Analog-Converter
Opposite of the ADC. An electronic device, often an integrated circuit, that converts a digital value to an analog voltage. D/A converters are used in many instruments to convert digital reading information into an analog signal for analog output. A time- and value-discrete signal is converted into a time-continuous, value-discrete signal.
Dark Current
The electronic signal generated by the thermal characteristics of the CCD even in the absence of impinging light. 
Dark Frame
An image of the dark current and camera readout and bias signal made by integrating an image while keeping the CCD array in total darkness. 
Dark Matter
Term used to describe any astronomical mass that does not produce significant light and hence is hard to observe. Examples of dark matter include planets, black holes, white dwarfs (because they are low luminosity) and more exotic things like weakly interacting particles.
Dark Subtract
This is the process of removing noise (generated by dark current) from a CCD image.  A dark frame is digitally subtracted from an image to eliminate dark noise.
Darlington Amplifier
An amplifier in which the collectors are tied together, and the emitter of the first directs current to the base of the second.
DC - Direct Current 
The flow of electrons only goes in one direction.
DDP - Digital Development Processing.  
This an image processing routine. DDP processing allows both bright and dim parts of an astronomical object to be displayed at the same time. DDP essentially compresses washed-out regions of an object into a range that the computer can display. This process is especially useful on galaxies which have bright cores and faint spiral arms.
Deconvolution
An iterative image processing filter that uses Fourier transform mathematics to restore a blurred image as nearly as possible to an unblurred state. 
Density
The density of an object is equal to the mass of that object divided by its volume. Substances (like lead, water, iron, granite) have a certain density under normal pressures. In such cases the density of a substance can also be used to determine how much mass will be present given a certain volume of the substance. For example, water has a density of 1 gram per cubic centimeter (gm/cm3) so a cube of water 10 centimeters on a side weighs 1000 gm (1 kilogram). Some substances (like gases) are compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible (and hot!) gases and is much denser at its center than near its surface. 
Depth of field
The distance between the nearest and farthest points that appear in acceptably sharp focus in a photograph. Depth of field varies with lens aperture, focal length, and camera-to-subject distance.
Depth of focus
The focal length of a lens system to maintain a precise image size.
Dew Heater
An electric strap that wraps around the front of a telescope and heats up when plugged in to a power supply. Keeping the front lens warm prevents dew condensing. 
Dew Shield
A cylinder extending out from the front of refractors, SCTs, and Maks to prevent stay light from entering the telescope and to capture air in front of the lens to delay the formation of dew. 
Dichroic
Dichroic filters are interference filters at an angle of incidence of light of 45°. The transmissivity and reflectivity of dichroites depend on a specific wavelength of light. For an RSP 510 filter (reflection short pass), for example, the excitation light below 510 nm is reflected and the excitation light above this value is transmitted. The transmission values are generally between 80% and 90% and the reflection values between 90% and 95%.
Dielectric
An insulating layer. A material that has high resistance. This term is usually used when the insulating layer separates the plates of a capacitor.
Diffraction
Spreading or bending of a wave upon passing around an obstacle or through a narrow opening.
digital
A method of representing information in an electrical circuit by switching the current ON or OFF. Only two output voltages are possible, usually represented by "0" and "1." In clocked circuits a digital signal may change its logic state once during a clock cycle. Opposite to analog.
Digital Circuit
A circuit that operates like a switch and can perform logical functions. Used in computers or similar logic-based equipment.
DIL - Dual In-Line
See DIP below
Diode
Simple semiconductor element, comparable to a valve. The ideal diode is blocking electric current in one direction and conducting it in the other. The real diode causes a voltage drop and energy loss in the forward direction, therefore power electronics sometimes use active diodes.
DIP - Dual In-line Package also DIL
The most common type of IC package; circuit leads or pins extend symmetrically outward and downward from the long sides of the rectangular package body. Usual package form for ICs in the past, where the pins lie in two rows with 2.54mm (1/10") distance. The pins are put through holes in the circuit board and are soldered from the back side. Nowadays in series fabrication the SMD-package is used.
Discrete Device
A semiconductor containing only one active element, such as a transistor or a diode.
Dispersion
a) The separation of a beam of light into the individual wavelengths of which it is composed by means of refraction or diffraction.
b) Resolution of white light into its component wavelengths, either by refraction or by diffraction.
DMM - Digital Multi Meter
An electronic instrument that measures voltage, current, resistance, or other electrical parameters by converting the analog signal to digital information and display. The typical five-function DMM measures DC volts, DC amps, AC volts, AC amps, and resistance.
Dobsonian (Dob)
A very simple and stable mount usually used with reflectors; especially very large (greater than 10-inches) telescopes. There are usually no motors or tripods. A simple rotating base and a tilting tube make for an easy to push or pull viewing session. 
Doppler Effect
The change in frequency of a wave (light, sound, etc.) due to the relative motion of source and receiver. Things moving toward you have their wavelengths shortened (blueshift). Things moving away have their emitted wavelengths lengthened (redshift). 
Doppler Shift
Change in the apparent wavelength of radiation (e.g., light or sound) emitted by a moving body. A star moving away from the observer will appear to be radiating light at a lower frequency than if at rest; consequently, lines in the star's spectrum will be shifted toward the red (lower frequency) end of the spectrum. The existence of a direct relationship between the redshift of light from galaxies and their distances is the fundamental evidence for the expansion of the universe.
Dot pitch
Describes the distance between the perforations on the monitor's shadow mask- The better displays usually have a dot pitch under 0.28mm. As with fine halftone screens, the smaller the dots, the sharper the image.
Double Star
A "system" of two stars that appear - because of coincidental alignment when viewed from Earth - to be close together; it is, however, an optical effect only, and therefore not the same as a binary star system (although until the twentieth century there were few means of distinguishing double and binary stars).
Doublet
Two simple lenses used in combination, placed close together or in contact. If they are cemented together, they constitute a "cemented doublet". If they are merely closely adjacent, they are a "separated doublet". 
DRAGN (Double Radiosource Associated with Galactic Nucleus)
i.e. extragalactic radio sources - are clouds of radio-emitting plasma which have been shot out of active galactic nuclei (AGN) via narrow jets
The very best example is Cygnus A, the brightest DRAGN in the sky.
DRAM - Dynamic Random Access Memory
A semiconductor read/write memory chip, in which the presence or absence of a capacitive charge represents the state of a binary storage element (zero or one). The charge must be periodically refreshed.
Driver
Program operating as an interface between an application and a special hardware. A graphics card driver has to insure, that every program can display itself independently from the actual graphics board by means of a special command set.
DSP - Digital Signal Processor
A versatile all purpose chip, used in cameras to handle basic contrast and brightness adjustment and image compression.
Dust
Tiny grains of stuff, e.g., carbon grains (soot) and silicate grains (sand) that are about 0.1-1.0 micron in size. Dust grains are a major component of the interstellar medium. Dust blocks visible light causing interstellar extinction. Dust scatters incident starlight, particularly the blue wavelengths of light (blue light has a wavelength comparable to the dust grain's size) causing interstellar reddening. The dust itself is cold, and cools even further by giving off infrared emission. 
Dynamic Range
The ratio of a CCD pixel's full-well capacity to the readout noise. Useful in determining the appropriate number of digitization levels that the analog-to-digital conversion system should use. 

E

Earth
Third planet from the Sun. First forms of life appeared about 3.2 to 3.5 × 109 years ago (Homo sapiens appeared as a species about 105 years ago).
Eclipse
The obscuration of a celestial body caused by its passage through the shadow cast by another body.
EEPROM OR EPROM. Electrically-Erasable Programmable Read-Only Memory. 
Similar to PROM, but with the capability of selective erasure of information through special electrical stimulus. Information stored in EEPROM chips is retained when the power is turned off.
Electromagnetic Spectrum
The distribution of light separated in order of some varying characteristic such as wavelength or frequency. The "electromagnetic spectrum" refers to the full range of possible frequencies and wavelengths of light. If we "take a spectrum" of a star we analyze its light according to wavelength or frequency by, say, passing the light through a prism. A "spectral line" refers to emission or absorption at a particular wavelength of light. 
Electron
A negatively charged particle revolving round the nucleus of an atom. 
An elementary particle (of the type known as a lepton) with a negative charge. One of the components of atoms, the electrons orbit around the nucleus, and the distribution and number of electrons determine the chemical properties of an element. 
Electronic Shutter
Electronic shuttering is the process of controlling the exposure period of the CCD by electronic methods (compared to conventional shuttering, which involves an electro- mechanical shutter system that opens, allowing light to fall momentarily on the CCD, and then closes). In most interline transfer CCDs the process of electronic shuttering involves "dumping" accumulated charge from the imaging photosites to the substrate for a predetermined amount of time and then stopping the "dumping" process for the actual exposure period. This is followed by the normal charge transfer during vertical blanking and then readout during the next field.
Element
A particular type of atom, with specific atomic number and chemical properties. The smallest unit into which matter may be broken by chemical means. 
EMF - Electromagnetic Force
The force between charged particles, which accounts for electricity and magnetism. One of the four fundamental forces of nature, it is carried by photons, and is responsible for all observed macroscopic forces, except for gravity. 
Emission Filter
Also Barrier filter, Emitter. A color filter that attenuates all of the light transmitted by the excitation filter and very efficiently transmits any fluorescence emitted by the specimen.
Emission Spectrum
The bright lines seen against a darker background, created when a hot gas emits photons characteristic of the elements of which the gas is composed. 
Emitter
The region of a bipolar transistor that serves as a source or input end for carriers. N-type for NPN, P-type for PNP.
Encke's Division
A region of decreased brightness in the outermost ring of Saturn.
Energy
Energy is usually defined as "the capacity to do work" but just what does that mean? Work is defined in physics as the exertion of a force over some distance, e.g., lifting a rock up against the gravity of the Earth. You probably have a pretty good colloquial grasp of the idea of "work" as something that takes effort. Energy is also something that is conserved within a closed system. This means that it is neither created nor destroyed but simply moved about (possibly changing from one form of energy to another). Light is basically a form of energy, one that radiates through space. So the Sun can release nuclear energy, creating light which travels through space to the Earth, where it can be absorbed by, say, a photocell, which in turn permits a motor to run propelling a solar-powered car forward. 
Entrance Pupil
The apparent size of the limiting aperture of a lens or lens system (properly that of the diaphram), as seen from the object plane. This can shift and become a complex matter in some circumstances. 
Entropy
A quantitative measure of the disorder of a system. The greater the disorder, the higher the entropy. 
Equatorial (Eq)
a) A special kind of telescope mount that has its axes tilted up to match the latitude of your observing site and is pointed at the North (or South below the equator). 
b) The classic type of telescope mount with one axis parallel to the Earth's polar axis (i.e. pointing at the celestial pole) and the other at right angles. Once the object is located, only the polar axis need be driven by a motor to counteract the Earth's rotation.
See also GEM.
Equilibrium
A balance in the rates of opposing processes, such as emission and absorption of photons, creation and destruction of matter, etc. so that there is no net change. 
Escape Velocity
The outward velocity required to leave the surface of a body mass M and radius R and escape to infinity (not fall back). The formula for the escape velocity is (2GM/R) 1/2
Euclidean Geometry
Flat geometry based upon the geometric axioms of Euclid. 
Event
A point in four-dimensional spacetime; a location in both space and time. 
Event Horizon
An event horizon is a lightlike surface in spacetime which divides spacetime into two regions: that which can be observed, and that which cannot. 

In the case of a black hole, the event horizon is that surface surround the region out of which light itself cannot escape. No signal or information from within the event horizon can reach the outside universe. For a nonrotating black hole, the horizon is located at the Schwarzschild radius, corresponding to 

Rs = 2GM/c2
Excitation Filter
Also Exciter. A color filter that transmits only those wavelengths of the illumination light that efficiently excites a specific dye. See Emission filter.
Exit Pupil
The exit pupil of a lens system is an image of the entrance pupil (hence conjugate to it) and normally should be the image of the limiting diaphram. In both the microscope and the telescope it is the eyepoint where the beam has its smallest cross-section. It is also called the Ramsden circle (q.v.) or eyepoint. 
Experiment
A controlled trial for the purpose of collecting data about a specific phenomenon. 
Explosive
A chemical that causes a sudden, almost instantaneous release of pressure, gas, and heat when subjected to sudden shock, pressure or high temperature.
Exposure
1. The act of allowing light to strike a light-sensitive surface. 
2. The amount of light reaching the image sensor, controlled by the combination of aperture and shutter speed. In photographic terms this is the product of the intensity of light and the time the light is allowed to act on the sensor, or the film. In practical terms, the aperture controls the effective diameter of the hole that allows light through, and shutter speed controls the length of time the shutter is open.
Extinction
As light from a star travels through interstellar space it encounters some amount of dust. This dust scatters some of the light, causing the total intensity of the light to diminish. The more dust, the dimmer the star will appear. It is important to take this effect into account when measuring the apparent brightness of stars. 

The dark bands running across portions of the milky way in the sky are due to extinction by copious amounts of dust in the plane of our Galaxy
Eyepiece
The lens system used in an optical instrument for magnification of the image formed by the objective.
 
Eyepieces come in various types. Every eyepiece has a focal length. The magnification that results when a given eyepiece is used with a given telescope, is equal to the focal length of the telescope divided by the focal length of the eyepiece. Thus if the telescope has a focal length of 1000 mm and the eyepiece has a focal length of 25 mm, the magnification will be 1000 / 25, or 40. 
 
There are several types of eyepiece designs. The most popular are: 
Ramsden
The Ramsden is a very old design, with a rather narrow apparent field of view -- perhaps as little as 30 degrees. Ramsdens do not work well at focal ratios shorter than about f/9, but good ones make surprisingly nice eyepieces for Lunar, planetary, and double-star observation, at longer focal ratios. Ramsdens often have prominent ghosts. The simplest form of Ramsden consists of two identical simple lenses, each flat on one side and convex on the other, facing each other with convex sides inward, spaced apart by a distance equal to or slightly less than their focal length. 
Kellner 
In essence, the Kellner is an achromatized Ramsden. It has a slightly larger apparent field of view than the Ramsden, and works at slightly faster focal ratios. Kellners tend to have rather prominent ghosts. Kellner eyepieces consist of a small achromat -- a cemented doublet -- near your eye, and a simple lens at the far end of the eyepiece. 
Orthoscopic 
Orthoscopics have moderate apparent fields of view -- 40 or 45 degrees -- and work well at fast focal ratios. Many consider them the best eyepieces for Lunar, planetary, and double-star work. 
There are actually several designs called "orthoscopic". The most common kind has a simple lens nearest your eye, and a cemented triplet further away. Another kind resembles a Plossl. 
Erfle 
The Erfle has a rather wide apparent field of view -- perhaps 68 degrees or more. The image quality at the edges of the field, at small focal ratios, is not as good as for more modern wide-field eyepieces. Erfles are generally composed of five or six simple lenses, grouped into two doublets and a singlet, or three doublets. 
Plossl 
Plossls have moderate apparent fields of view -- 50 degrees is typical -- and work well at fast focal ratios. Plossls consist of four simple lenses, grouped as two cemented doublets. 
Ultra Wide 
Ultra Wide Angle is a "house brand" of Meade. These eyepieces are well corrected, with very large apparent fields of view, of 84 degrees. Ultra Wide Angle eyepieces are reported to be generally similar in design to Naglers. 
Koenig 
Koenigs have a rather wide apparent field of view -- perhaps as much as 70 degrees. 
The various eyepieces commonly labeled "Koenig" contain anywhere from four to seven simple lenses, grouped into various combinations of cemented doublets and singlets. 
Speers-Waller
Noted for a very wide apparent field of view -- almost 80 degrees -- and for excellent correction at fast focal ratios. Speers-Waller eyepieces are reported to be similar to Naglers.
Naglers 
Nagler is a "house brand" of Tele Vue. These eyepieces are noted for a very wide apparent field of view -- 82 degrees -- and for excellent correction at fast focal ratios. Naglers are big, heavy, and expensive, and consist of seven or eight simple lenses grouped together into four singlets or doublets. 
Eye Relief
The distance from the surface of the rearmost lens of the eyepiece, to the exit pupil. When the eyepiece is in use, that distance should be the distance from the rearmost lens of the eyepiece to the iris of the observer's eye. The remaining distance is the space between the observer's eye and the eyepiece.
It is the clearance available for moving the observer's head without bumping the telescope, and is also the place where the observer's spectacles must fit, if they are worn while observing. 

F

Fastar
Fastar is Celestron's high-speed CCD imaging system. Fastar involves removing a Schmidt-Cassegrain telescope's secondary mirror and placing the CCD camera at the front of the telescope. This provides a wide field of view and a vary fast imaging system. A telescope which is said to be "Fastar compatible" has a removable secondary mirror. The actual accessories needed for Fastar imaging are sold separately.
FET - Field Effect Transistor
A solid-state device in which current is controlled between source and drain terminals by voltage applied to a gate terminal, which is insulated from the semiconductor substrate.
Fermi Mechanism
A process hypothesised by Enrico Fermi whereby a small fraction of the charged particles in space can be boosted up to the high energies of cosmic rays. The mechanism works on particles which are already distinctly more energetic than average. Fermi imagined the high-energy particles scattered from dense clouds in the interstellar gas, or more correctly from the magnetic field embedded in the clouds. In these collisions the particle gains energy if the cloud is moving towards it, and loses if the cloud is moving away, but Fermi realised that the first case happens slightly more often than the second, so the particles slowly gain energy. 

In a strange coincidence, in 1977 four independent groups of researchers realised that in shock waves the high-energy particles find that every scattering is nearly head-on, giving much more efficient acceleration (called "first-order Fermi acceleration"). According to simplified calculations, the particles end up with a "power-law" distribution of energies, very like that of the electrons which actually produce cosmic synchrotron radiation. It therefore seems quite likely that the first-order Fermi mechanism is responsible, although things don't agree so nicely when the calculations are done more carefully.
FFT - Fast Fourier Transform.  
FFT filters are image processing algorithms applied to CCD images, usually to sharpen or smooth and image. FFT filters have the same function as kernel filters, but are applied differently so the processing is faster. Applying an FFT to an image changes the image so that applying a kernel filter is easier. The FFT function is then reversed and a filtered image results. 
Field
A mathematical representation of a quantity describing its variations in space and/or time. 
Field Rotation
Rotation of the FOV over time. With an Alt-Az system that does not control for field rotation, if tracking is otherwise perfect, the stars and other objects in the FOV will pivot around the center of the FOV during a CCD or film exposure or during a visual observing session. Equatorially mounted and driven systems will suffer from field rotation if polar alignment is imperfect. 
FIFO - First In, First Out
Electronic pendant of the "stand in a queue". Used to synchronize data sources with different speed or unsynchronized data streams.
Filter Wheel
Black and white CCD cameras use color filters to separate the three primary colors (red, green, and blue) in order to produce full color images. Usually an automated mechanical wheel holds the filters in front of the camera and rotates the appropriate filter into place before an exposure.
Filtering
1. Using color-dyed or interference-layered glass inserted into the optical path to restrict the passage of full-spectrum wavelengths. 
2. Applying a mathematical function to the pixels in an image array that modifies each pixel's value according to the values of an assigned set of neighboring pixels. Primarily used to blur or sharpen localized aspects of an image. 
Firewire
Apple's name for the communication standard IEEE 1394.
FITS - Flexible Image Transport System. 
The standard data file format for astronomical CCD images. FITS images use file extension names of FTS, FIT, or FITS.
FLASH MEMORY
It is a non volatile memory technique with fast access times; rewriteable many times and uses a block erase technique as opposed to EEPROM, which erases one bit at a time.
Flat-Field Frame
A CCD image of the irregularities in the optical system and the CCD chip. The image is integrated while the optical/CCD system is pointed at a wide-field, evenly illuminated source, such as that provided by a specially manufactured light box, the inside wall of an observatory dome, a large poster board positioned in front of the telescope, or a twilight sky. 
Flip-Flop
An electrical circuit having two stable states: on and off. A basic logic circuit component.
Fluorite Objectives
Describes a correction class for objectives. Fluorite lenses are semi-apochromatic, meaning their degree of correction lies between the achromatic and apochromatic.
Flux
A flux is the rate at which something is transferred through a surface, like 10 flies per minute through the 1 square inch hole in the busted screen door. In astronomy flux is used to express the amount of energy radiated per second across an area like a square centimeter. 
Flux Density (see also Brightness)
When we describe Sirius as the brightest star in the sky, we are talking about flux density. This is a measure of how bright things seem to us from our platform on Earth. It depends both on the luminosity and the distance, since distant objects appear fainter. Radio astronomers measure this in units of Jansky, optical astronomers use apparent magnitude. 
Frequency
This is a property of a wave, and it is the number of wave crests that pass a given point per second. Frequency is is measured in units of inverse time (e.g., "cycles per second''). A cycle per second is the unit of frequency and it is known as a "Hertz.'' Since light moves at the constant speed of light, the frequency of a light wave is related to the wavelength: the frequency is given by the number of wavelengths that go by per second at the speed of light, hence frequency is wavelength (distance) divided by speed (c). The higher the frequency of light the greater its energy. 
It designates the incidence of a change in signal or state. Given in Hz, that is number per second. FM-radio works in the MHz-region (M=Mega=106), fast integrated circuits in the GHz-region (G=Giga=109), as does the mircowave oven
Focal Length
The distance from the optical center of the lens to the image sensor when the lens is focused on infinity. The focal length is usually expressed in millimeters (mm) and determines the angle of view (how much of the scene can be included in the picture) and the size of objects in the image. The longer the focal length, the narrower the angle of view and the more that objects are magnified.
Focal Ratio
The effective focal length of an optical system divided by the diameter of the primary optical component. I.e. The ratio between the focal length of a telescope and the aperture.
A telescope with an 8" aperture and 80" (2000mm) focal length has a focal ratio of f/10. Smaller focal ratios equate to shorter exposure times. An f/4 system is faster than an f/6 system, for example.
Focal Reducer
An optical component or system for changing the image scale of a telescope to achieve a better match between the seeing disk and the pixel size.
Focus
The process of bringing one plane of the scene into sharp focus on the image sensor.
Fork Mount
A fork mount is a type of mount where the telescope is held by two arms, and swings between them. A fork mount can be either alt-azimuth or equatorial (through the use of a wedge). Fork mounts are most commonly used with Schmidt-Cassegrain telescopes, and are almost always equatorial.
FOV - Field of View
The size of an imaged or visual scene in terms of sky dimensions, usually stated in degrees, arcminutes, or arcseconds. The approximate width of the FOV in arcseconds can be calculated by dividing the width of the CCD chip in microns by the focal length of the optical system in millimeters and multiplying the result by 206.
Frame Grabber
A device that lets you capture individual frames out of a video camera or off a video tape.
Frame Rate
The number of pictures that can be taken in a given period of time.
f-stop
A numerical designation (f/2, f 2.8, etc.) indicating the size of the aperture (lens opening). A small f-number ( large aperture diameter) gives a small depth of field, and a large f-number gives a large depth of field.
Full Well Capacity
Each photosite on a CCD chip can contain a certain number of electrons. This number is called the Full Well Capacity. Within a certain chip full well capacity is a function of binning, so that binning 2 times, for example, quadruples the full well capacity. Of course, 2x binning also makes the CCD effectively 4 times more sensitive and so the wells will fill in the same amount of time. Chips with anti-blooming generally have lower full well capacity, but will not bloom when the well is filled
Future
Those events which could be influenced by a given event. All events located within the future light cone. 
FWHM - Full Width Half Maximum
A measurement of the size of a point source image, such as a star, in terms of the width of the 50% peak value circumference. Usually stated in units of arcseconds or pixels. 

G

GaAs - Gallium-Arsenide
A compound semiconductor material in which active devices are fabricated. GaAs has a higher carrier mobility than silicon, thus it has the capability of producing higher speed devices.
Gain
The factor by which an incoming signal is multiplied.
Galactic Equator
The primary circle defined by the central plane of the Galaxy.
Galaxies
Galaxies are often described as huge collections of stars. This is about as accurate as describing a city as a huge collection of street-lights: stars are certainly the most obvious feature of galaxies when see far away in the dark, but they represent a fairly small fraction of the total mass, 90% or so of which consists of the famous dark matter, whose nature is one of astronomy's biggest mysteries. As well as stars and dark matter, galaxies contain gas and dust in the inter-stellar medium between the stars. The may also contain an active galactic nucleus at their center. 

Galaxies come in a wide range of sizes (meaning diameter, brightness, and mass, all of which roughly go together), ranging from tiny dwarfs with only a few million stars, through normal galaxies like our own Milky Way, with a hundred billion stars, to the trillion-star cD galaxies that sit at the centers of the great clusters of galaxies. As is often the case in nature, the smallest are the most common; on the other hand the size of the larger galaxies more than makes up for their rarity, so that a typical star is likely to be in a galaxy with a size approaching that of the Milky Way or larger, and large galaxies are also the easiest to find in the sky, due to Malmquist bias. 

Normal galaxies come in two basic types: spiral and elliptical. 
Gamma
The logarithmic brightness value assigned to video monitors to allow replication of the logarithmic visual range of the eye. 
Gate
1. A signal which in the active state enables an operation to occur; and when in the inactive state inhibits an operation from occurring.
2. The basic digital logic element - where the binary value of the output depends on the values of the inputs.
3. The primary control terminal of a field effect transistor.
GEM - German Equatorial Mount
A telescope mount that can easily counteract the Earth's rotation and track the stars. 
General Relativity (GR)
Einstein's theory of gravity. Einstein describes a gravitational field in terms of the "curvature" of space. J. H. Wheeler boiled the theory down to the slogan 
Space tells matter how to move; matter tells space how to curve. 
Because of the curvature of space, the various ways we use to define length end up giving different answers when general relativistic effects are important, for instance in cosmology, or near a black hole. 

More... 

When Einstein proposed the theory in 1916, there was very little evidence in its favor. The situation is very different today: the predictions of General Relativity have been tested with high precision in many ways, in laboratories, in the Solar System, and by observations of distant pulsars. The results agree very well with General Relativity, showing that it must be, at the least, very close to the right answer. 

... A full appreciation of general relativity involves mastering some difficult mathematics (unlike special relativity). To be honest, even professional astronomers often never get around to learning GR properly! 
Geocentric
Taking the Earth to be the center, e.g., of the solar system, or of the universe. 
Geodesic
In geometry, that path between two points/events which is an extremum in length. In some geometries, such as Euclidean, the geodesics are the shortest paths, whereas in others, such as in the spacetime geometries appropriate to general relativity, the geodesics are the longest paths. 
Giant Molecular Cloud
A region of dense interstellar medium that is sufficiently cold that molecules can form. They are very cold (10-20K) with relatively high densities (trillion particles per cubic meter), and huge. Even though the temperatures are very cold the molecules in these molecular clouds emit radio radiation which can be detected on Earth. These regions are believed to be where new stars can form. 
GIF - Graphics Interchange Format
An image file format used heavily used on the Web (256 colors, patented by UNISYS)
Globular Cluster
A tightly packed, symmetrical group of thousands of very old (pure Population II) stars. The stellar density is so great in the center that the nucleus is usually unresolved. The stars within a globular cluster orbit each other because of their mutual gravity.
GoTo
A motorized & computerized telescope.
Gravitational Lens
A massive object which causes light to bend and focus. This occurs because light falls in a gravitational field.
Graviton
A hypothetical massless carrier boson which is the carrier of the gravitational force.
Gravity
The weakest of the four fundamental forces; that force which creates the mutual attraction of masses. 
Gray scale
A series of shades of gray ranging from pure white to pure black.
The linear array of brightness values assigned to a monochrome image represented in black and white, where 0 = black and the maximum array value = white. For example, in an 8-bit dynamic range, 0 = black, 256 = white, and medium gray = 128. 
Ground
A common reference point for an electrical system.
Guiding
No telescope mount can track perfectly, yet for CCD imaging it is necessary to very accurately track the object being imaged. This is done by guiding on a star to make small corrections to the mount to accurately follow the star. This makes up for any errors in the telescope's drive system. Guiding can be done manually by watching a star through a crosshair eyepiece, or, more commonly, by using an autoguider to automatically guide. See also, Unguided Exposure, Autoguider, Self-Guiding, and Track & Accumulate.

H

H-alpha Solar Filter
A very special filter that allows a very small portion of the sun's total spectrum through the telescope. It is on the order of 1.7 to 0.7 Angstroms. It allows you to view solar prominences. 
Hadron
A class of particles which participate in the strong interaction. Hadrons consist of those particles (baryons, mesons) which are composed of quarks. 
Halftone
A method of printing a continuous tone image, such as a photograph, using a screen of tiny dots - Used in inkjet and lithographic printers only four CMYK process colors are used. Once printed and when viewed from a distance, the human eye perceives these tiny dots as a continuous tone.
Hardware
Generic term for electronic devices, that is, things you can touch. Contrary to software. (My definition: throw it out of the window, if it is broken it was hardware....)
Hawking Radiation
Emission of particles, mostly photons, near the event horizon of black holes due to the quantum creation of particles from the gravitational energy of the black hole. 
Heat Sink
An assembly that serves to dissipate, carry away, or radiate into the surrounding atmosphere heat that is generated by an active electronic device.
Heliocentric
Taking the Sun to be the center, e.g., of the solar system. 
Histogram
A histogram is a graph of number of pixels versus pixel value.  
Pixel values run from lowest (displayed as black) to highest (displayed as white). A bar is plotted for each pixel value showing the number of pixels in the image with that value. An astronomical image typically has more bars toward the lower (darker) end of the histogram since most astroimages contain are large amount of dark sky around a brighter (but small) object.
HSL - Hue, Saturation, Luminance
A particular conformation of color theory.
HST - Hubble Space Telescope
Hubble Constant
The constant of proportionality (designated H) between recession velocity and distance in the Hubble law. It is a constant of proportionality but not a constant in time, because it can change over the history of the universe. 
The Hubble constant is defined as the rate of change of the scale factor with respect to time divided by the scale factor. 
Measuring the Hubble constant is difficult and remains and important task for astronomers. Present best values lie between approximately 50 km/sec/Mpc and 100 km/sec/Mpc, with a value around 70 km/sec/Mpc favored. 
Hubble's law
In 1929 Edwin Hubble showed that the further a galaxy is away from us, the faster it is moving away; this is now called Hubble's law, and it means that the universe is expanding. 

What Hubble actually measured was the redshift, usually written z , of the spectrum of the galaxies: he found that the spectrum was `stretched out' so that a feature that should have been at a certain wavelength was actually detected at a wavelength longer by a factor (1 + z). The usual cause of such a redshift is the Doppler effect, and by assuming this it is possible to calculate the recession velocity of the galaxy. In the cosmological case, it turns out that the Doppler formula is only an approximation; the deeper meaning of the redshift is that the factor (1 + z) gives the ratio of the size of the universe at the time the light was emitted, to the size it is today. 

This expansion is quite a subtle concept. It does not mean that everything (galaxies, planets, people, ants, atoms) is getting bigger; if that were the case, we would never know, because our tape measures and rulers would be growing at the same rate. The idea is clearest for a toy universe consisting of `dust' spread evenly through space, with individual particles not interacting at all with each other. Expansion means that the distance between any two particles is getting larger. Now think of a row of particles: since each is moving away from its immediate neighbours (at the same rate, from homogeneity), the relative motion must be proportionately larger for particles separated by larger distances: Hubble's law. Of course the real universe is lumpy on `small' scales; instead of being a smooth soup, matter is condensed into galaxies and stars and all the rest (just as well for us!). Within these objects the original expansion has been overcome by gravity. This is a runaway process. Start with a small region in the early universe with a slightly higher-than-average density. Its gravity will slow down both its own expansion and that of surrounding matter, so both the size of the region and its relative over-density increase. Eventually the expansion near the centre is stopped altogether. The size of the region affected continues to grows as outlying particles have time to move in. At present the largest regions in which the expansion has been stopped are the clusters of galaxies a few million light years across. For points with larger separations, the motions induced by local density peaks are small compared to the relative motion from the overall expansion. 
Hue
Distinct color or shade.
Hybrid Circuit
Mounting technique, where several elements are placed on a substrate (Al2O3-ceramics) by special pastes by silk-screen print, followed by a burning process. Semiconductors are bonded "naked" (without package) or soldered in SMD packages. Complete modules are often founded in synthetic resin.
Hyperbolic Geometry
A geometry which has negative constant curvature. Hyperbolic geometries cannot be fully visualized, because a two-dimensional hyperbolic geometry cannot be embedded in the three-dimensional Euclidean space. However, the lowest point of a saddle, that point at which curvature goes both "uphill'' and "downhill,'' provides a local representation.
Hypothesis
A proposed explanation for an observed phenomenon. In science, a valid hypothesis must be based upon data and must be subject to testing.  

I

IC - Integrated Circuit
consists of several (up to some million) transistors, resistors and capacitors (sometimes planar inductors, too). These are built in a numerous process steps on a ultra-clean silicon disk (the wafer). These processes include the fabrication of "masks", used for structuring a light sensitive varnish. After etching the enlightened parts of the varnish, so called dotants are brought in by diffusion of ion implantation. This modifies the electrical behavior of the silicon.
ICC profiles - International Color Consortium
A profile is a list of characteristics that describes a particular color space Pixel such as the space of an Apple 13-inch monitor. ICC profiles are interpreted by Color Management Modules, when your image is viewed on a monitor in a different workstation.
IEEE - Institute of Electrical and Electronics Engineers
Abbreviation for Institute of Electrical and Electronics Engineers. Also a standardization organization.
IEEE 1394
A port on the computer capable of transferring large amounts of data very fast. (Firewire)
i.Link
Sony's name for IEEE 1394.
Image sensor
A solid-state device containing a photosite for each pixel in the image. Each photosite records the brightness of the light that strikes it during an exposure.
Inch
One inch (1") is 25.4mm. The inch is the commonly used measure in electronics, mostly 1/10" or 1/20" of distance is found for pin distances of components.
Index of Refraction
Ratio of the speed of light in a vacuum to the speed of light in a particular substance; ratio of the sine of the angle of incidence to the sine of the angle of refraction.
Insulator
A material that is a poor conductor of electricity - used to separate conductors from one another or to protect personnel from electricity.
Intensity (or Surface Brightness)  (see also Brightness)
This is what we mean when we say that the center of a galaxy is brighter than the outer regions; an image is essentially a map of intensity. It measures the flux density we receive, not from the object as a whole, but from each unit area of the sky (technically, solid angle). The flux density of an object is thus the product of its intensity times its solid angle.
Integration
Acquisition of electronic data from a CCD array. Synonymous with image acquisition or exposure. Analogous to exposure in film terminology. 
Interference
A wave is something that moves along and has high points (crests) and low points (troughs). If two (or more) different wave trains pass over one another the crests and troughs can add together to make bigger crests and troughs, and a crest and a trough can add together to produce zero. So if light is a wave phenomenon, then two light sources produce waves that in some places produce large amplitudes and other places produce zero. When to point sources of light are projected onto a screen this wave interference effect produces alternating light and dark spots. This demonstrates the wavelike nature of light. 
Interpolation
In an image interpolation adds extra pixels. It's done with some zoom lenses. 
A process that uses software to add new pixels to the mosaic-like bitmap of an image or part of an image. The color of the new pixels is derived from the original adjacent pixels. Interpolation appears to increase the original resolution and quality of an image.
Interstellar Medium
The name given to the stuff that floats in space between the stars. It consists of gas (mostly hydrogen) and dust. Even at its densest the interstellar medium is emptier than the best vacuum humanity can create in the laboratory, but because space is so vast, the interstellar medium still adds up to a huge amount of mass.
Ion
An atom which has gained or lost an electron and thereby acquired an electric charge. (Charged molecules are called radicals, not ions.) 
I/O - Input/Output
The process of transferring data to and from a computer-controlled system using its communication channels, operator interface devices, data acquisition devices, or control interfaces. The computer may exchange information in several ways, that is, it has to accept data or to provide data. At lowest level, the circuit level, the computer has a set of so called port addresses and memory addresses. A 8-bit port holds 8 binary signals, for example.
Ion
This is what an atom becomes when an electron is separated from the atom, leaving it with a net positive charge or, if an electron is added, leaving it with a net negative charge.
IRQ - Interrupt ReQuest
With the help of interrupts a device can get the attention of the processor, e.g. because an error appeared or new data is ready. This way the processor doesn't have to look at all available ports all the time, which is called polling. IRQs provide flexibility and can enhance performance.
ISO - International Standards Organization
ISO is taken from the Greek word isos, meaning equal. The speed or light sensitivity of conventional photographic film and of the CCDs found in digital cameras, is measured in ISO values. Lower ISO numbers indicate slower films, or lower CCD ratings which require less light to be correctly exposed.
Isotope
One of the forms in which an element occurs. One isotope differs from another by having different numbers of neutrons in its nucleus. The number of protons determines the elemental identity of an atom, but the total number of nucleons affects properties such as radioactivity or stability, the types of nuclear reactions, if any, in which the isotope will participate, and so forth. 

J

Jansky
Unit of flux density, named after Karl Jansky, who first discovered extra-terrestrial radio waves (from the Milky Way) at Bell Telephone Laboratories in Holmdel, New Jersey, in 1932. 

1 Jy = 10-26 Watt Hertz-1metre-2
Jets
Perhaps the weirdest feature of AGN is that they can produce narrow jets of material streaming outwards from the centre. The jets are usually produced in pairs, pointing in opposite directions; occasionally there only seems to be one jet. These jets can sometimes be "seen" directly by radio telescopes, but more often we deduce their existence because far outside the galactic nucleus they produce a DRAGN. In one sense, we should not be too surprised to find jets, since in every other case where accretion is thought to occur in astronomy, jets are also common. These include young stars forming out of gas clouds, and binary stars where matter is falling from one of the pair onto the other. Unfortunately, we don't know how jets are produced in any of these circumstances! 
JPEG - Joint Photographic Experts Group
A very popular digital camera file format that uses lossy compression to reduce file sizes. Developed by the Joint Photographic Experts Group. JPEG compression provides the best results with continuous-tone images, such as photographs, when the size of the file is an important factor.
JPEG images are stored with a file extension name of JPG or JPEG. 
Jupiter
Fifth planet from the Sun. It is more massive than all other planets and satellites combined; if it were about 80 times more massive, it would become self-luminous due to fusion reactions. The heat flux to from the center to the surface is mainly convective. For both Jupiter and Saturn it is necessary to invoke a substantial source of internal heating (presumably gravitational contraction) to account for the surface temperature (Jupiter radiates about 2 1/2 times as much heat as it receives from the Sun). Jupiter's surface shows pronounced horizontal striations: the light layers (zones) are at a slightly higher altitude and about 15° cooler than the dark layers (belts). It is surrounded by a partial torus of atomic H in the orbit of Io. Thirteen satellites, the four outermost of which have retrograde motion, high eccentricity, and high inclination. (Jupiter XIII, discovered in 1974, has a period of 239 days; i = 26°.7, e = 0.147.)

K

Kelvin Scale 
This is the temperature scale which uses the same size of degree as the Celsius or Centigrade system, but which begins at absolute zero, the coldest temperature possible corresponding to the lowest possible energy state of a system. Temperature in degrees Kelvin gives a measure of the average energy of a system.
Kepler's Laws
The three laws of planetary motion discovered by Johann Kepler. 
1. Planets orbit on ellipses with the Sun at one of the focii of the ellipse 
2. Equal area is swept out by the planets motion as it moves around the ellipse (a planet moves fastest when it is nearest the Sun), 
3. The square of the period of the orbit is equal to the cube of the semimajor axis (half the long axis of the ellipse). This is written as P2 = a3, where P stands for the period of the orbit (the sidereal period) and a is the size of the semimajor axis of the ellipse. 
Kernel Filters
Kernel filters are image processing algorithms applied to a CCD image, generally to smooth or sharpen an image. A kernel is a small grid which tells the computer how to change the value of a certain pixel based on the values of neighboring pixels. The most common types of kernel filters are low-pass ("smoothing" and "blurring" filters) and high-pass ("sharpening" filters).
Kinetic Energy
The energy associated with macroscopic motion. In nonrelativistic physics the kinetic energy is equal to one half the mass times the velocity squared, i.e., 1/2 mv2

L

Lab color mode
Lab mode describes a theoretical color space where color and brightness are split into three different channels. two for color and one for brightness.
LED - Light Emitting Diode
Semiconductor, emitting light of a defined wavelength when current flows. Because of its high efficiency, its robustness and its fast reaction LED have replaced bulbs for displays.
Lepton
A member of a class of particles which do not participate in the strong interaction (the force that binds atomic nuclei together). The best-known lepton is the electron. Another example is the neutrino. 
Libration
Any of several oscillations in the apparent aspect of the Moon as seen from Earth, which, when combined, enable Earth-based observers over a period of time to see altogether about 59 percent of the Moon's surface. Physical librations are angular motions about the center of mass due to gravitational torques on the Moon. Optical librations are the apparent rotations of the Moon, caused by our observing it from somewhat different directions at different times.
Light
Electromagnetic radiation that may or may not be visible to the human eye.
Light Box
An internally illuminated container that uses indirectly placed light sources to evenly illuminate a translucent diffuser screen on one side of the container. The screen is sized to cover at least the entire aperture of a telescope so that accurate flat-field frames can be made. 
Line Emission
An electromagnetic emission process which produces radiation at a number of specific frequencies, c.f. continuum emission. It is so-called because when the light is analyzed with a spectrograph, line emission shows up as bright lines crossing the spectrum. --picture?-- Line emission is generally produced by fluorescence, in which atoms or molecules are "excited" to a high energy state by absorbing ultraviolet radiation, and then "decay" to their "rest state" via a series of quantum jumps. In each quantum jump, a photon is emitted with a specific frequency which is determined by the quantum properties of the atom. The frequency of the photon when received on Earth is affected by the Doppler effect because the emitting atom is moving relative to us. Of course we actually receive the accumulated radiation from large numbers of atoms moving with different speeds, so emission lines have a finite width, which are usually specified in terms of the equivalent range of speeds.
Linear Circuit
A circuit whose output is an amplified version of its input or whose output is a predetermined variation of its input. See also OpAmp.
Linear Response
A CCD camera with a "linear response" has sensitivity such that doubling the exposure time of an object of a certain brightness will result in an image twice as bright. There is a linear relationship between exposure time and brightness. This is especially useful for making magnitude measurements of variable stars, comets, asteroids, or supernovae. A camera with a nonlinear response is not suitable for making magnitude estimates since a star which appears twice as bright in an image is not necessarily twice as bright in actuality. CCD cameras equipped with an anti-blooming feature are generally nonlinear. For taking pretty pictures linear response makes no difference.
LiOn
Lithium ion battery
Long-focal-length lens (telephoto lens)
A lens that provides a narrow angle of view of a scene, including less of a scene than a lens of normal focal length and therefore magnifying objects in the image.
LRGB
Images taken with a black and white CCD camera through red, green, and blue filters are combined to make RGB images. An interesting effect of human vision is that we get most of the spatial information about an image from the brightness, or luminance, portion of the image and not from the color, or hue, portion. This means it is possible to take a low resolution color image and combine it with a high resolution black and white image to make an LRGB image (the L standing for luminance). This is a definite advantage for CCD imaging; since placing a filter in front of a camera decreases its sensitivity, color images can be binned to gain higher sensitivity at the expense of resolution. As long as the low-resolution color image is combined with a high-res luminance image (taken unfiltered and thus at the camera's maximum sensitivity) the full resolution is maintained and the total exposure time is decreased.
LSTTL
Schottky-clamp TTL logic typically using one-third the power of TTL, but maintaining TTL speeds.
Lunar Eclipse
When the Earth' s shadow covers the moon. 
Lumen (lm)
Represents luminous flux  (cd·sr)
Luminance
The intensity of a source of light. Synonymous with brightness
Luminosity (see also Brightness)
When we describe Eta Carina as the brightest star in the Galaxy, we are talking about luminosity. This is a measure of how bright things are intrinsically. It can be measured in Watts, or Watts per Hertz to describe the luminosity at a specific frequency. For historical reasons in optical astronomy absolute magnitude is used, and radio astronomers quote the power, which is the luminosity per steradian (multiply by 4 pi to get back to luminosity).
Physics: Total amount of energy radiated per second. It has units of energy per second (e.g. ergs per second). Since many astronomical objects radiate away energy this is an important characteristic. We compare luminosity of an object to the solar luminosity, the total energy given off per second by the Sun. One solar luminosity is 4 × 1033 ergs per second. Luminosity has the same units as Power, e.g. energy per second. The Watt is the familiar unit of power. For comparison, a 400 Watt light bulb is 10-24 solar luminosities. 
LZW - Lempel-Ziv-Welch
A compression scheme used to reduce the size of image files (used e.g. in TIFF, PDF, GIF, and PostScript language file formats).

M

Mach Number
The ratio of the speed of a fluid flow (e.g. a jet) to the speed of sound, so that supersonic flows have Mach numbers greater than one. In jets there are two Mach numbers, depending on whether we use the speed of sound in the outside medium (the external Mach number) or the speed of sound in the jet fluid (the internal Mach number). 
Magnification
a) Ratio of size of optical image to size of the object. 
b) Determined by dividing the focal length of the telescope by the focal length of the eyepiece and multiplying by any amplifiers (Barlows). 
Magnitude
In the second century B.C., the greek astronomer Hipparchus made a catalogue of stars, and divided them into five "magnitudes", with stars of the first magnitude the brightest, and the fifth magnitude the weakest. In the 19th century it became possible to measure accurately the relative brightness of stars. Tragically, rather than starting anew with a sensible system, astronomers followed a proposal by Pogson which re-defined magnitudes in such a way that most of the traditional magnitudes of stars stayed roughly the same. The awful formula that resulted has become one of the initiation rituals of modern astronomy, and is therefore not described here. Suffice it to say that we are stuck with a logarithmic scale on which the numbers get smaller as stars get brighter, so that the brightest stars even have negative magnitudes. These are actually measures of flux density, known as apparent magnitude in this context. The equivalent measure of luminosity, absolute magnitude, is the apparent magnitude an object would have if seen from a distance of 10 parsecs. 
Mak - Maksutov-Cassegrain
A telescope that uses mirrors and lens to "fold" the light into a smaller tube. A Mak usually never needs collimation.
Mars
Fourth major planet out from the Sun. Its tiny satellites are locked in synchronous rotation with mars.
Mass
The measure of how much "stuff" something has, mass determines the inertia of an object (its resistance to being accelerated by a force) and how much gravitational force it exerts on another object. In pre-Einsteinian physics mass was conserved, neither created nor destroyed. Einstein discovered that mass can be converted into energy and vice versa. The conservation of mass is still a good approximation since mass-energy conversions generally involve relatively small amounts of mass. The mass of astronomical objects is often measure in terms of the Sun's mass. The solar mass is 2 × 1033 grams. 
Maximum Entropy Deconvolution
This is an image processing routine. It is used to enhance detail in CCD images that are slightly blurred due to atmospheric effects, tracking errors, or imperfect optics. The routine works by attempting to match an ideal image (essentially a perfect star image) to the actual blurred image. The image is adjusted through a series of iterations to a closer approximation of an ideal image.
Median
Literally the middle value in a sequence of values arranged in increasing size order. A useful mathematical estimator of the true value from a set of values when one of these values is contaminated, i.e. known to be much larger than the average.
Median Combine
When combining images there are two methods employed. Most often images are summed to get as much information as possible. However, noisy images can be smoothed to some extent by taking the median value of all the pixels in the images. I.e. the middle value if all values are ordered.
Medium [Plural: media.]
Generic name for the stuff that fills space, a very dilute gas composed mainly of hydrogen and helium. When the temperature exceeds a few thousand Kelvin, the gas is ionised and is therefore technically a plasma. 

Important examples of cosmic media are: 
Inter-stellar medium (ISM) 
The medium between the stars within a galaxy. In spirals the ISM is a complex soufflé of hot (million Kelvin) and cool (3 to 100 Kelvin), gas, where the cooler regions have higher density so that the pressure is roughly constant. In ellipticals the ISM consists almost entirely of "hot" gas (roughly ten million Kelvin) which extends seamlessly into a gaseous halo surrounding the galaxy. 
Intra-cluster medium (ICM) 
The hot gas (several tens of millions of Kelvin) filling groups and clusters of galaxies. In big clusters, the ICM can contain more material than all the galaxies put together. 
Inter-galactic medium (IGM). 
The gas filling the regions between clusters of galaxies. We don't know whether there really is a proper IGM, or whether the gas just gets thinner and thinner as you go away from one cluster, until you reach the far outskirts of the next. 
Mercury
Innermost planet of the Solar System. Transits of the Sun occur either 7 or 13 years apart - last transit 1973 November 10.
Mercury Vapor Lamp
Discharge-lamp, producing light of defined wavelengths by ionization of mercury atoms with accelerated electrons.
Messier Catalog
a) One of the earliest catalogues of nebulous-appearing astronomical objects, compiled in 1781 by the French astronomer Charles Messier. Messier's catalogue included many objects that were later realized to be galaxies.
b) List of the locations in the sky of more than 100 galaxies and nebulae, compiled by Charles Messier between 1760 and 1784. Some designations he originated are still used in identification; M1 is the Crab Nebula (in Taurus).
Micrometer
One-millionth (10^-6) of a meter. About 40 millionths of an inch. Synonymous with micron. Symbol is µm.
Microwave
Electromagnetic waves in the GHz-range. Related to radio waves or light. Used for mobile phones, RADAR, microwave ovens etc.
Mil
One-thousandth of an inch (10^-3 inch). Equal to 25.4 microns.
Milky Way, The
The common name for our own Galaxy (which is just greek for Milky Way!). Astronomers usually just call it the Galaxy, with a capital G to distinguish it from other galaxies in the universe. The Milky Way is a large galaxy of spiral type, and our Sun is in the disk, roughly 25,000 light-years (8 kpc) from the center. The disk is visible as the faint band of milky light that circles the night sky, which is made up of myriads of distant stars. 
Morphology
The systematic study of shape or structure. Often used by astronomers as a pompous synonym for "structure". 
Moon
Natural satellite of Earth. Studies of lunar rocks have shown that melting and separation must have begun at least 4.5 × 109 years ago, so the crust of the Moon was beginning to form a very short time after the solar system itself. It would have taken only 107 years to slow the Moon's rotation into its present lock with its orbital period. The Moon's orbit is always concave toward the Sun.
MOS - Metal silicon diOxide Silicon also Metallic Oxide Semiconductor
Semiconductor with isolated control terminal. By collection of electric charge on the gate a conducting channel between the source and the drain contact is build. The MOS Semiconductor is slower than a bipolar transistor, but is easy to produce and therefore cheap. The field of high-integration is based on MOS.
Mount
The assembly that supports a telescope and allows for smooth, steady movement to track the stars. The most important part in astrophotography. 
MPEG - Motion Pictures Expert Group
A digital video format developed by the Motion Pictures Expert Group.
Multitasking
The ability of an operating system to run several programs simultaneously and independent from another. If a computer only has one CPU, this is achieved by distributing the computing power by a so-called scheduler, running each program for a small amount of time, calculated by special criterions. UNIX to some extent Win.. offer a good multitasking. See multithreading also. 

N

NAND Gate (Not-AND gate). 
An AND gate followed by an inverter. The output of the AND gate is inverted to the opposite value.
Nanometer (nm)
A unit of length commonly used for measuring the wavelength of light: I nm = 10 angstroms (Ċ) = 10-9 meters; and 1000 nm = 1 micron (µ) = 10-6 meters.
ND - Neutral-density Filter
An optical filter that attenuates light by an amount independent of the wavelength within the useful range of the filter. Metal-coated filters typically have a wider neutral range than glass filters and are more heat-tolerant.
Nebula
The interstellar medium manifests itself to the astronomer in various phenomena. Many of these are amorphous, diffuse structures known as Nebula. This comes from the Latin word for "cloud." Its plural is nebulae. 

A beautiful example of a nebula is an emission nebula. Emission occurs when a cloud of gas is warmed up by some source of continuum radiation, say from a nearby star. The various atomic energy levels are excited by this radiation, and as the electrons jump back down to their lower energy states they emit photons at distinct spectral wavelengths. Since hydrogen gas is the most common form of gas, hydrogen emission lines are most often observed. In particular, a specific transition in the hydrogen atom corresponds to red light and color pictures of these emission regions appear red. This is the so called H-alpha emission line of hydrogen. 
Neptune
Eighth major planet out from the Sun. Discovered in by following predictions calculated by Urbain Le Verrier. Similar predictions had been made earlier by John Couch Adams but were not followed up.
Neutral Filter
Neutral filters are semi-reflective glass plates. They are used to distribute the light path independent of wavelength. The incident light is partially reflected and partially transmitted. Neutral filters are usually placed at a 45° angle in the path of the beam. The ratings of a neutral filter are based on its reflectivity-to-transmissivity ratio. A neutral filter RT 30/70, for example, reflects 30% of the excitation light and transmits 70%.
Neutrino
Any of three species of very weakly-interacting lepton with an extremely small, possibly zero, mass. Electron neutrinos are generated in the interior of the Sun (and other stars) in nuclear reactions. Generally such neutrinos do not interact with matter and stream out through the Sun. A very few of these many neutrinos can be detected in sophisticated detectors here on Earth, giving us a "window" into the interior of the Sun. In 1987 neutrinos from a Supernova in the Large Magellanic Cloud were detected in terrestrial neutrino experiments. 

If neutrinos have a small but nonzero mass, they would constitute an important type of dark matter in the universe. 
Neutron
A charge-neutral particle (of the hadron type) which is one of the two particles that make up the nuclei of atoms. Neutrons are unstable outside the nucleus, but stable within it. The number of protons in the nucleus determines what element that nucleus is. Different isotopes of a given element have different numbers of neutrons in the nucleus. The total number of neutrons and protons affects properties such as radioactivity or stability, the types of nuclear reactions, if any, in which the isotope will participate, and so forth.
Neutron Star
A dead "star'' supported by neutron degeneracy pressure. A neutron star is the core remnant left over after a supernova explosion. 
NGC - New General Catalog
An object assigned a number in the New General Catalog of non-stellar objects.
NIR - Near infrared
The region of the electromagnetic spectrum ranging in wavelength from approximately 750 to 2500 nanometers.
NiCad
Nickel cadmium battery.
NiMH
Nickel metal hydride battery. Ecologically safe and very efficient.
Noise
1. An undesirable electrical signal from an external source such as an AC power line, motors, generators, transformers, fluorescent lights, CRT displays, computers, radio transmitters, and others.
2. Pixels on an image which are caused by the electron movement in the sensor. The amount of movement depends on the temperature of the semiconductor and will increase with higher temperatures.
3. The degree of randomness or unpredictability in a signal. Noise components are a primary determinant of the quality of an image. 
NOR Gate (Not-OR)
An OR gate followed by an inverter. The output of the OR gate is inverted to the opposite value.
Normal incidence
An angle of incidence of zero degrees.
NOT Gate 
The output is just the opposite from the single input.
Nutation
Slight but recurrent oscillation of the axis of the Earth, caused by the Moon's minutely greater gravitational effect on the Earth's equatorial "bulge".
Nova
A star that experiences an abrupt increase in brightness by a factor of a million (in contrast to the much brighter supernova). A nova is produced in a semidetached binary system where hydrogen-rich matter is being transferred onto a white dwarf. As more and more hydrogen builds up on the surface the temperature rises. The material is degenerate so when the temperature becomes high enough for nuclear burning to take place it does so explosively producing a nova. 
Numerical Aperture (NA)
A design function of a lens system that relates the refractive index of the lens material and the cone angle of the lens such that the NA is equal to the refractive index of the lens times the sine of the cone angle of the lens. (N.A. = n siny)

O

Occams Razor Principle
Any hypothesis should be shorn of all unnecessary assumptions; if two hypotheses fit the observations equally well, the one that makes the fewest assumptions should be chosen.
Occultation
The cutoff of the light from a celestial body caused by its passage behind another object.
OP Amp - OPerational amplifier
A general-purpose IC used as a basic building block for implementation of linear functions. An operational amplifier has an inverting and a non-inverting input. The output voltage is the extremely gained difference between the inputs. By external circuitry nearly any function may be realized.
Open Cluster
A small, loose cluster of stars that typically contains several hundred members. The best examples are the Hyades and the Pleiades, both in the constellation Taurus. Open clusters line the Galactic plane, in contrast with globular clusters, which are members of the Galaxy's halo or thick disk.
Optical Center
Point in a lens through which light rays pass without refraction.
Optical Density (OD)
Property of an optical material that determines the speed with which light rays travel through it.
A logarithmic unit of transmission: OD = -log (T), where T is the transmission (0 f T f 1).
Opto-Electronics
Solid state devices capable of emitting a specific wavelength of energy, sensing a specific wavelength of energy or interacting together in a common package to perform an electronic function.
OR Gate
The output is yes if at least one input is yes.

P

Parfocal
Eyepieces with common focal planes so that they are interchangeable without refocusing. 
Parallax
Generally speaking, parallax is the apparent shift in the direction to an object as seen from two different locations. This shift can be used to determine distances (through "triangulation"). Stellar parallax occurs as the Earth orbits the Sun and our line of sight to a nearby star varies. The effect is to make the star appear to shift position over the course of the year. In reality, stellar distances are so great that parallax shifts are less than an arc second, completely unobservable to the unaided eye. 
Parallel port
A port on the computer where 8 data lines work in parallel (therefore the name) and that is faster than a serial port but slower than SCSI, USB, or IEEE 1394 ports. Often used by printers and flash card readers.
PCB - Printed Circuit Board 
A substrate on which a predetermined pattern or printed wiring and printed elements has been formed. Also called a printed wiring board (PWB).
PCI - Peripheral Component Interconnect
It is a standard for a local bus in a PC
PCMCIA - Personal Computer Memory Card International Association
A standard's association which gives its name to the credit sized devices used in notebooks and handheld PC's for memory storage, modems and other peripheral devices.
Photon
Experiments have shown that light of a given energy (frequency) is not something that can be broken up indefinitely. Rather for a given frequency it comes in discrete bundles with energy hf where h is Planck's constant and f is the frequency. These discrete bundles of light are known as photons. It is often useful to think of light as a bunch of particle photons. Other times it is useful to think of light as a wave. Astronomers do both as needed. 

The idea of the photon was first put forward by Albert Einstein in his theory of the photoelectric effect. 
Photosite
A small area on the surface of an image sensor that captures the brightness for a single pixel in the image. There is one photosite for every pixel in the image
Pixelization
An effect seen when you enlarge a digital image too much and the pixels become obvious.
Pixel
Short for picture element. Each cell that constitutes an intersection of the grid of rows and columns on a CCD array is a pixel. A pixel is a single measurable entity for charge storage, release, and ADU conversion on a CCD.
Pixel Value
A measurement (usually in terms of ADUs or DNs) of the electronic charge obtained in a pixel during a CCD integration. After image processing, a measurement of the processed brightness of a pixel. 
Plan Objectives (Flat Field Objectives)
Describes a correction class for objectives. The image curvature aberration is corrected for objectives of this type. Correcting this error requires lenses with stronger concave surfaces and thicker middles. Three types of plan objectives, planachromate, planapochromate and plan- fluorite, are based on the type of additional correction for chromatic aberration.
Planck Constant
A fundamental constant of physics, h, which sets the scale of quantum-mechanical effects. 

h = 6.625 × 10-27 erg-sec 
Plasma
Sometimes called the fourth state of matter (after solid, liquid and gas), a plasma is a gas in which some of the atoms or molecules are ionised, i.e. their electrons have been stripped off, and are floating around freely. Strictly speaking, almost all gas in space is a plasma, although only a tiny fraction of the atoms are ionised when the temperature is below about 1000 Kelvin. The gas in and around DRAGNs is much hotter than this, and so is a fully ionised plasma. The very low densities in space allow the electrons to travel without much obstruction, so paradoxically space is an almost perfect electrical conductor. Although charges can move around freely, averaged over even small volumes (say a million km across) cosmic plasmas are always neutral (i.e. they contain equal numbers of positive and negative charges), because the electrical forces which arise when charges are noticeably separated are enormously strong. 

Plasmas in space are permeated by magnetic fields, A good way to think about cosmic magnetic fields is in terms of field lines. These behave like rubber bands embedded in the plasma, so that as the plasma flows the field lines are pulled and stretched along with it. When they are stretched enough they can pull back on the plasma. Individual electrons and ions in the plasma feel a magnetic force which makes them travel in a helical path around the field lines, so that they can only travel long distances in the direction along the field. This binds the plasma together so that it behaves like a continuous medium even when (as in the plasma in DRAGNs) the individual electrons and ions almost never collide. 
Plossl
A type of eyepiece. Very common nowadays. Good field of view and easy to use. 
Pluto
The most distant known planet from the Sun. Its orbit has the highest eccentricity and highest inclination to the ecliptic of any planet and some astronomers suggest that it may be an escaped satellite of Neptune. In the mid-1970s Pluto crosses Neptune's orbit on its way in, and for the rest of this century Pluto will be closer to the Sun than Neptune (Pluto and Neptune, however, are never less than 2.6 AU apart). Its mass and radius have not been determined with any great certainty.
PNG  - Portable Network Graphics
A compressed but lossless image format which is free to use and supported by most web browsers - can fully substitute GIF and at the cost of size also JPEG.
Port
An electrical connection on the computer into which a cable can be plugged so the computer can communicate with another device such as a printer or modem.
The "outdoor" of a computer. E.g. an 8-bit-port, owning a special address, provides 8 digital lines in parallel for data interchange.
Position Angle (PA)
The direction of an "arrow" on the sky, for instance the long axis of a DRAGN, is given by its Position Angle (PA), which is zero if the direction is due north and increases as the arrow rotates to the east, i.e. to the left, if north is at the top of the image, since you are looking up. 
Primary Mirror
The first mirror encountered by incident light in a telescope system.
Prime Focus (PF)
a) The focal point of the large primary reflecting mirror in astronomical telescopes when the light source is extremely distant. This focus actually falls at a point just within the upper structure of the telescope itself and is therefore accessible to CCD cameras and other instruments; it provides a large field of view.
b) Attaching a camera to a telescope to make it a telephoto lens is sometimes called PF.
c) Placing the photosensor in the first focus point of the telescope. 
Not all telescopes can be set up this way. 
Processor or CPU (central processing unit)
heart of the computer. The CPU is the device executing the commands of a program and calculating results. In detail, we have to distinguish between the CPU for handling integer arithmetic and the FPU (floating point unit) for non-integer mathematics.
Proton
A particle of the hadron family which is one of the two particles that make up atomic nuclei. The proton has a postive electrical charge. 
Pseudocolor
In contrast to truecolor, where every point of a picture has an individual color, a pseudocolor picture has an index to a color table for every point. The length of this table is limited (e.g. to 256 entries), so that only a finite amount of colors is available to a picture. During translation of a truecolor picture into pseudocolor, the "most important" colors have to be calculated und put in the table.
PSF -- Point Spread Function
The visible and measurable smear of photons from a point source. Optical diffraction and atmospheric distortion are primary factors causing light from point sources such as distant stars to be smeared into a PSF much larger than the geometrically calculable size of the point source. 
Pulsar
A rotating magnetized neutron star that produces regular pulses of radiation when observed from a distance. A pulse is produced every time the rotation brings the magnetic pole region of the neutron star into view. In this way the pulsar acts much as a light house does, sweeping a beam of radiation through space. 

Q

QCUIAG - QuickCam and Unconventional Imaging Astronomy Group 
An International group of astronomers producing Astronomical images with unconventional electronic imaging devices such as modified webcams, video surveillance cameras and digital cameras.
http://www.qcuiag.co.uk/
Quantization
A process where the continuous range of values of an input signal is divided into non-overlapping sub-ranges and, to each sub-range, a discrete value of the output is uniquely assigned.
Quantum Efficiency
The ratio of effectiveness with which a CCD converts received photons (quanta of light) to measurable electrons. Amateur CCDs operate from about 15% up to about 75%, depending on the wavelength of light. High-end CCDs built for professional use frequently run 80% or higher. 
Quasar
From quasi-stellar object, a star-like (i.e. unresolved) object that has a very large luminosity and is located at very large distances from us (as indicated by their high cosmological redshifts). Although technically the term quasar refers to objects that are highly luminous in the radio band, the term tends to be used for both radio-loud and radio-quiet objects high-redshift unresolved objects. Quasars are believed to be powered by supermassive black holes in the centers of galaxies in the process of formation early in the history of the universe. 

R

Radian
A unit of angle equal to about 57 degrees. The length along the arc of a circle covering by one radian is equal to the radius of the circle. The complete angle around the circle (360 degrees) is equal to 2 pi radians. The radian is particularly useful because if you know the distance to some object and you measure its apparent size as the angle it subtends in your field of view in radians, then the actual size is just that number of radians times the distance to the object. For example, a meter stick held up at a distance of 100 meters makes an apparent angle in your field of view of 0.01 radians. 
Radiation
The emission of particles or energy. Also the particle or energy so emitted. 
Radio Galaxy
A galaxy that is emitting most of its energy in the form of radio waves rather than light in or near the visible bands where stars emit most of their radiation. This means that radio galaxies are dominated by some non-stellar process. 
RAM - Random Access Memory
Stores digital information temporarily and can be changed as required. It constitutes the basic (read/write) storage element in a computer.
Readout Noise
ADU randomness introduced by the collection, amplification, and analog-to-digital conversion of signal data. 
Readout Register
The part of a CCD image sensor that reads the charges built up during an exposure.
Recombination
When free electrons are captured by ions to form atoms. 

The occurrence in the early universe when the temperature became sufficiently low that free electrons could no longer overcome the electrostatic attraction of the nuclei, and were captured to form atoms
Recycle Time
The time it takes to process and store a captured image.
Red Dwarf
A small, dim, low-mass Main Sequence star. 

Red dwarf stars are hard to detect because they are so dim. In principle, they could constitute a major mass constituent of the universe, if their production is heavily favored in the star formation process. In that case they could constitute a significant source of dark matter
Red Giant
A star with low surface temperature (thus red) and large size (giant). These stars are found on the upper-right hand side of the HR diagram (high luminosity, low temperature). The red giant phase in a star's life occurs after it has left the main sequence. The Sun will become a red giant in about 5 billion years. 
Redshift
A redshift is a shift in the frequency of a photon toward lower energy, or longer wavelength. The redshift is defined as the change in the wavelength of the light divided by the rest wavelength of the light, as 
    z = (Observed wavelength - Rest wavelength)/(Rest wavelength) 
Note: that postive values of z correspond to increased wavelengths (redshifts). 

Different types of redshifts have different causes. 

The Doppler Redshift results from the relative motion of the light emitting object and the observer. If the source of light is moving away from you then the wavelength of the light is stretched out, i.e., the light is shifted towards the red. These effects, individually called the blueshift, and the redshift are together known as doppler shifts. The shift in the wavelength is given by a simple formula 
    (Observed wavelength - Rest wavelength)/(Rest wavelength) = (v/c) 
so long as the velocity v is much less than the speed of light. A relativistic doppler formula is required when velocity is comparable to the speed of light. 

The Cosmological Redshift is a redshift caused by the expansion of space. The wavelength of light increases as it traverses the expanding universe between its point of emission and its point of detection by the same amount that space has expanded during the crossing time. 

The Gravitational Redshift is a shift in the frequency of a photon to lower energy as it climbs out of a gravitational field. 
Reflecting Telescope (Reflector)
Telescope that uses mirrors to magnify and focus an image onto an eyepiece. 
Reflection short pass Filter
Are interference filters that transmit short-wave light while reflecting long-wave light. An optical reflection short pass filter is characterized by the reading of the wavelength edge at which the filter changes from transmission to reflection (50% threshold).
Reflection long pass Filter
Are interference filters that reflect short-wave light but are transparent for long-wave light. An optical reflection long pass filter is characterized by the reading of the wavelength edge at which the filter changes from reflection to transmission (50% threshold).
Refracting Telescope (Refractor)
Telescope that uses lenses to magnify and focus an image onto an eyepiece. 
Refraction
The deflection from a straight path undergone by a light ray or energy wave in passing obliquely from one medium to another. The refraction of a light ray or energy wave is governed by Snell's Law.
Refractive Index
The factor by which the light velocity in an optical medium is less than in a vacuum.
The index of refraction is the ratio of the constant velocity of light in vacuum (c) to the variable velocity of light in a transparent medium (v).

n = c / v
This property is also known as optical density and expressed as nD20. The number 20 represent the Celsius temperature of the sample while D represents the monochromatic D line of the sodium spectrum (wavelength: 589.3 nm). 
The consequence of this differing velocity in different materials results in varying amount of bending of light in different substances. Snells's law illustrates a well-known relationship between the angle of bending and the refractive index:

n1sin(b)1 = n2sin(b)2

The value for the index of refraction of several materials is given below:
Material     Refractive Index
Diamond     2.4173
Polycarbonate polymer 1.6
Flint glass 1.575
Quartz 1.553
Water  1.3328
Air  1.003
 
The refractive index of a substance changes if the temperature changes or if the color of the light used changes. Refractive index measurements can be used to determine the concentration of a solution or ascertain purity and identify a substance.

The nDvalue decreases as the temperature rises. To compare experimental results with those listed in standard tables, set at 20 degrees Celsius, the correction is as follows:

nD20 = nDT + (T - 20)(0.00045) 

The temperature T is in degrees Celsius
Relativity
The rules relating observations in one inertial frame of reference to the observations of the same phenomenon in another inertial frame of reference. Casually applied only to the Einsteinian Special Theory of Relativity, but a more general term. 
Reliability
The ability of a device to perform within the desired range over a measured period of time.
Resample
Increase or decrease of the number of pixels in one or both axes of an image using a method that correlates adjacent pixel values to create a smooth pixel-to-pixel brightness transition. 
Resistance (R) 
The difficulty in moving electrical current through a conductor to which voltage is applied. Expressed in ohms.
Resistor
A discrete device which implements a resistance for electronic circuits. Available in different types, shapes and made. Higher values are expressed in kilo- or mega-ohms. A resistor indicated as 1k5 is meant 1'500 Ohm or 1.5 kOhm. Resistors may wear also colored rings which indicate value and tolerance.
Determine the Resistance of a Resistor by its Color Coded Rings
Use the table below.
  1. Turn the resistor so the gold or silver stripe is at the right end of the resistor.
  2. The first two stripes on the left correspond to the first two digits of the resistance value.
  3. The third stripe from the left corresponds to a multiplication value.
  4. Multiply the two-digit number from step 2 by the multiplier of step 3. This number is the value of the resistance in W.
The 4th ring indicates the accuracy of the resistor. (e.g. A gold stripe means that the value of the resistance may vary by 5% from the value given by the first three stripes.)
For resistors with smaller tolerance a third number stripe is added counting 5 in total.
High precision resistors (0.1% to 1%) may have a 6th stripe (temperature coefficient)
Table: Color Code for resistors.
  1. Stripe 2. (+3.) Stripe 3. (4.) Stripe 4. (5.) Stripe
BLACK 0 0 x 1 -
BROWN 1 1 x 10^1 1%
RED 2 2 x 10^2 2%
ORANGE 3 3 x 10^3 -
YELLOW 4 4 x 10^4 -
GREEN 5 5 x 10^5 0.5%
BLUE 6 6 x 10^6 0.25%
PURPLE 7 7 x 10^7 0.1%
GRAY 8 8 - -
WHITE 9 9 - -
Gold - - x 10^-1 5%
Silver - - x 10^-2 10%
Example

First stripe is YELLOW. ----> 4
Second Stripe is BLUE. ----> 6
Third Stripe is RED. ----> x 100
Therefore the resistance is 46 x 100 = 4600W

(or 4.6 KW or 4k6).
Furthermore, if the last band is Silver, you can expect that the actual value might deviate by as much as 10% of 4.6KW
(or by as much as 460 W!)
Resolution
All images are blurred to some extent; otherwise they would contain infinitely fine detail and hence an infinite amount of information. The amount of blurring is technically called the resolution of the image, with high resolution meaning little blurring. 
1. The smallest value of input (or output) signal, other than zero, that can be measured (or sourced) and displayed. Also called sensitivity or minimum resolvable quantity.
2. An indication of the sharpness of images on a printout or the display screen. It is based on the number and density of the pixels used. The more pixels used in an image, the more detail can be seen and the higher the image's resolution. 
3. Quantified by defining the Point spread function, which is the pattern produced by the spread-out light from a single point.
 
The maximum resolution available from a telescope is set by the phenomenon of diffraction, and gives a FWHM approximately equal to the ratio of wavelength to the diameter of the telescope.
Resolution, interpolated
A process that enlarges an image by adding extra pixels without actually capturing light from those pixels in the initial exposure.
Resolution, optical
The true resolution of an image based on the number of photosites on the surface of the image sensor.
Reticle
A system of cross-hairs in the eyepiece of a telescope.
RGB - Red, Green, Blue
The color system used in most digital cameras where red, green, and blue light is captured separately and then combined to create a full color image. 
Named the additive colors of the human visual spectrum, since red + green + blue = white.
Richardson-Lucy Method (RL)
Image reconstruction algorithm.
ROM - Read Only Memory
Permanently stores information repeatedly used-such as tables of data, characters of electronic displays, etc. Unlike RAM, ROM cannot be altered.

S

Sample Rate
The rate at which a continuous-time (analog) signal is sampled. It is frequently expressed as samples/sec (S/s), kilosamples/sec (kS/s), or Megasamples/sec (MS/s).
Sampling
The taking of readings from a single data source. In basic CCD imaging theory, for delivered optical system information not to be lost, the resolution of imaging CCD pixels must be at least twice as precise as the delivered resolution of the optical system (e.g., for appropriate sampling, a telescope delivering a PSF with an FWHM of 2 arcseconds calls for a CCD with pixels having a one-arcsecond FOV.) Less than this level of precision is termed undersampling. Significantly more is termed oversampling.
Saturn
Sixth major planet out from the Sun. The most spectacular of the Solar System, it is circled by a series of concentric rings. All the satellites of Saturn are locked in synchronous rotation.
Saturation
Reaching or exceeding the full-well capacity of a pixel. Also, in color theory, the degree of purity of a hue in terms of mixture with white light. 
Schottky Diode
A junction or barrier formed by the direct contact of semiconductor materials with a metal. This type of contact rectifies signals and may also exhibit some resistance.
Schottky TTL
A form of TTL logic in which Schottky diodes are used to clamp the transistors out of saturation, effectively eliminating the storage of charge within the transistor - allowing increased switching speeds.
Schwarzschild Radius
The radius of the event horizon surrounding a nonrotating black hole. Its size is given by Rs = 2GM / c2. For a one solar mass star this is about 3 kilometers. 
SCSI port - Small Computer System Interface
A port that's faster than the serial and parallel ports, harder to configure than the newer USB port. Mostly used for fast disk connections.
SCT -- Schmidt-Cassegrain Telescope
A telescope that uses mirrors and lens to "fold" the light into a smaller tube. 
A telescope with a spherical primary mirror and a thin refractive corrector plate with a complex, non-spherical shape.
An SCT occasionally needs collimation.
Secondary Mirror
The second reflecting surface encountered by the light in a telescope. The secondary is usually suspended in the beam and therefore obstructs part of the primary.
Self-Guiding
Self-Guiding is a feature of certain CCD cameras. In these cameras there are two CCD chips:  one for imaging, and a smaller one for guiding the telescope. During long exposures, errors in a telescope's drive prevent perfect star images. Guiding eliminates this problem, but for most CCD cameras a second separate CCD must be used to guide. Self-Guiding CCDs simply use their built-in guiding chips to eliminate the need for a second camera. Also, a guidescope or off-axis guider does not need to be used, avoiding some of the other problems inherent in guiding a telescope using these methods.
Semiconductor
An element such as silicon or germanium or a compound like GaAs that has an intermediate band gap. Unlike metals that freely conduct and insulators that do not conduct charge, semiconductors selectively conduct charge through the movement of holes and electrons.
Serial Cable
If it applies to your cam - this is the cable that connects your camera to your PC and allows you to export the images you have taken. Nowadays mostly replaced with USB connection.
Serial port
A port on the computer (RS-232) where the wires carry the information in a serial manner - (speed is usually low but it is some kind of the lowest denominator in computer communication). Many digital cameras came equipped with cable to download images through this port but it's slow! Both parallel and USB ports are faster connections.
Sharpening
Sharpening is a software routine used to enhance detail in a CCD image. There are numerous methods employed for sharpening images--there are even whole programs dedicated only to sharpening. Some are simple sharpening routines, while others involve more complex algorithms (such as the unsharp mask technique), and some employ iterative routines to produce successively sharper images while attempting to minimize the noise inherently generated in sharpening (the Lucy-Richardson and similar techniques).
Shielding
A metal enclosure for the circuit being measured or a metal sleeve surrounding wire conductors (coax or triax cable) to lessen interference, interaction, or current leakage. The shield is usually grounded.
Shutter
The device in the camera that opens and closes to let light from the scene strike the image sensor and expose the image.
Shutter Speed
The length of time the (mechanical) shutter is open and light strikes the image sensor. It also applies to systems with electronic shutter but here the control is just a digital signal.
Sidereal Year
A period of time based on the revolution of the Earth around the Sun, where a year is defined as the mean period of revolution with respect to the background stars.
Signal
The measured value recorded by a pixel during a CCD integration. The signal in a CCD image usually comprises input from sky, thermal, and electronic sources. The natural degree of randomness in the pixel value is the noise component of the signal. 
Simplicity
Of a scientific hypothesis, the principle that a proposed explanation must not be unnecessarily complicated !!!?! 
Singularity
In classical general relativity, a location at which physical quantities such as density become infinite. Another definition is a point in spacetime where timelike worldlines end (or begin). Singularities can be initial singularities (such as the big bang itself) or ending singularities, such as at the center of a black hole, or the big crunch. 
SN-Ratio - Signal to Noise Ratio (SNR or S/N)
The ratio of the maximum signal that can be measured to the level detected with no signal present (noise level).
SLR - Single-Lens Reflex
Cameras that have a mirror that allows the optical viewfinder to look through the main lens. With some digital cameras this means that the LCD panel cannot be used for previewing. This kind of camera offers professional quality, but at a price far above that of simpler models. There are many attachments that increase the versatility of the digital camera. Taking off the camera lens and gettting an adapter to attach some telescopes right to the camera to make it a giant lens. This technique is called Prime Focus
SmartMedia
The second memory card format used in digital cameras. Holds between 2Mb and 128Mb of picture data. (Third is Sony's MemoryStick)
SMD - Surface Mounted Device
the modern packaging system for integrated circuits. In contrast to DIL-packages no drillings are necessary for mounting, the pin distance is much smaller (typ. 1.27mm (1/20") ... 0.8mm).
Software
General term for things you cannot hold in hands, normally programs or data. A subdivision is firmware, which is built into a device and normally cannot be changed in contrast to other software.
Solar Eclipse
An eclipse in which the Earth passes through the shadow cast by the Moon. It may be total (observer in the Moon's umbra), partial (observer in the Moon's penumbra), or annular.
Solar Filter
A special filter added the FRONT of the telescope so you can safely view the Sun. It cuts the amount of light by a factor of 100,000 (ND5). Small solar filters added to the eyepiece are dangerous and should be thrown away. 
Solar Flare
Sudden and short-lived brightening of a region of the solar chromosphere.
Solar Mass, Luminosity
The Sun is a fairly average star, and its mass and luminosity serve as good standards for comparison with other objects. The luminosity and mass of stars and galaxies is generally given in terms of the solar mass and solar luminosity. The solar mass is 2 × 1033 grams. One solar luminosity is 4 × 1033 ergs per second. Luminosity has the same units as Power, e.g. energy per second. The Watt is the familiar unit of power. For comparison, a 400 Watt light bulb is 10-24 solar luminosities. 
Special Relativity
Einstein's theory of motion. The theory is "special" because it describes the special case where gravity can be neglected. For motions much smaller than the speed of light, represented by the symbol c, special relativity gives answers that are almost identical to those provided by "classical mechanics", i.e. the theory of Newton (which is much simpler to use). When objects move at speeds near c, classical mechanics gives the wrong answers and we have to use special relativity. The most important result is that it takes an infinite amount of energy to accelerate anything with finite mass to c, and therefore c acts as a universal speed limit. When the special features of Einstein's theory come into play, we describe motion (and the moving objects) as relativistic. For our purposes, motion faster than about 0.1c counts as mildly relativistic. 

When the motion is highly relativistic, it is not convenient to talk about speed because everything is moving at almost exactly c. Instead it is useful to use the Lorentz factor defined as
Spectrum
The spectrum of an object is a graph of its brightness at each wavelength of light, versus wavelength. Sometimes, instead of wavelength, frequency or photon energy is used; all three graphs are equivalent. Here 'light' is used as a shorthand for electromagnetic radiation of all kinds, from radio through to gamma rays. 
Speed
The magnitude of the velocity. 
Speed of Light (c)
The finite speed at which light travels. Unless otherwise stated, usually refers to the fundamental constant c, the speed of light in a perfect vacuum. 

Approximately equal to 3 × 1010 cm/sec. 
Spherical Geometry
A geometry which has positive constant curvature
Spiral Galaxy
A galaxy consisting of a flattened rotating disk of stars, a central bulge and a surrounding halo. The disk is prominent due to the presence of young, hot stars which are often arrayed in spiral patterns. The characteristic appearance of these bright spirals gives the galaxy type its name. 
Spotting Scope
A small telescope, always a refractor or catadioptric, generally used for terrestrial viewing.
Stacking
Adding or averaging multiple CCD integrations of the same FOV to emulate the improved SNR of a single longer exposure.
Standard Candle
Any astronomical object of known luminosity that can thus be used to obtain a distance. Cepheid variables, Main sequence stars, and type I supernovae have all been used as standard candles.
Star
A self-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring at their cores, but may also be applied to stellar remnants such as neutron stars
Star Alignment
Telescope mounts that have computers attached to them can be aligned with a couple known stars. The computer finds a pointing/position factor that it can apply to any of the objects in its database to calculate where the telescope needs to point to find an object. Also called GoTo
Step-Up-switching regulator    
Switching regulator, whose output voltage exceeds its input voltage. Necessary in many battery powered systems, often found in solar power stations, too.
Stop
1. An aperture setting that indicates the size of the lens opening. 
2. A change in exposure by a factor of two. Changing the aperture from one setting to the next doubles or halves the amount of light reaching the image sensor. Changing the shutter speed from one setting to the next does the same thing. Either changes the exposure one stop.
Stop down
To decrease the size of the lens aperture. The opposite of open up.
Stretching
Applying a mathematical transfer function to the pixel values in an array so that they are changed to better portray the image in terms of relative brightness levels. Often used synonymously with scaling. 
Subexposure / Subintegration
One of a set of stacked CCD integrations
Sun
Star that Earth orbits. Central body of solar system. It takes about 1-10 million years for photons to diffuse from the Sun's interior to its surface. About 3% of the energy radiated is in the form of neutrinos. Every second about 655 million tons of H are being converted into 650 million tons of He. A grazing light ray is deflected 1".7 by the Sun. If the total angular momentum of the solar system were concentrated in the Sun, its equatorial rotation speed would be about 100 km s-1.
Supernova
The explosion of a star. Supernovae come in two types: 
Type I is caused by sudden nuclear burning in a white dwarf star. 
Type II is caused by the collapse of the core of a supermassive star at the end of its nuclear-burning life. In either case, the star is destroyed and the light given off in its explosion briefly rivals the total light given off by a whole galaxy. 

A Supernova Remnant is the material blown off during a supernova, now seen as a great glowing cloud expanding into space. 
Switching Regulators
regulate a value not by variation of a controllable resistor or similar methods with energy loss, but switch a source periodically on and off. The generated ripple is filtered by high quality inductors and capacitors, therefore the efficiency is very high. The regulation is similar to the pulse width modulation (PWM).
Symmetry
The property under which some quantity does not change when certain attributes, such as spatial location, time, rotation, and so forth, vary. 
Synchronous
In hardware, it is an event that occurs in a fixed time relationship to another event. In software, it refers to a function that begins an operation and returns to the calling program only when the operation is complete.
System Gain
The number of electrons represented by each ADU. Synonymous with conversion factor.

T

Tachyon
1. The name given to a speculative particle that follows a space-like trajectory through spacetime. That is, the particle must travel faster than the speed of light

2. A gluon that has not yet dried. 
Telecompression
Reduction of the effective focal ratio of an optical system via the introduction of a positive lens system into the optical path between the basic optical elements and the focal plane. 
Temperature
A measure of the average energy of random motion of the constituents (e.g., molecules, atoms, photons) of a system. 
Theory
In scientific usage, a hypothesis or related group of hypotheses which have become well established by the fundamental criteria of the scientific method. 
Thermal Radiation
Radiation emitted by any object with a temperature greater than absolute zero. A thermal spectrum occurs because some of the heat energy of the object is converted into photons. In general, a thermal spectrum depends not only upon the temperature, but also upon the composition of the object, its shape, its heat capacity, and so forth. 
Compare blackbody radiation which is thermal radiation from an ideal emitter. 
Thermodynamics
The theory of heat and temperature, encompassing both the macroscopic behavior of bodies, and the statistical description of the submicroscopic world of particle motions. 

Laws of Thermodynamics: 
First Law: A statement of the conservation of energy: The change of the energy of a system is equal to the heat added plus the work done on the system. 

Second Law: The Entropy of an isolated system can only increase with time. Also can be stated: heat never flows spontaneously from a cooler to a hotter body. (Perpetual motion is impossible.)

Third Law: Absolute Zero can never be attained, only approached arbitrarily closely. 
Thought Experiments
Experiments which could be performed in principle, but might be very difficult in practice, and whose outcome can be predicted by pure logic. Often used to develop the consequences of a theory, so that more practical phenomena can be predicted and put to actual experimental tests. 
TIFF - Tagged Image File Format
A popular lossless image format used in digital photography. TIFF images are stored with a file extension name of TIF.
Time-lapse photography
Taking a series of pictures at preset intervals to show such things as flower blossoms opening.
Tracking
This ability allows a telescope mount to follow the apparent motion of the stars. Actually it is counter acting the rotational motion of the Earth. This feature is very useful for extended and high power viewing. It is required for astrophotos longer than a few seconds. 
Transistor
A semiconductor device in which a small control signal is used to control a larger current flow. Major families are bipolar transistors and MOSFETs.
Truecolor
Contrary to pseudocolor. With truecolor, every point of a picture has an individual color.
TTL - Transistor-Transistor-Logic
A popular logic circuit family that uses multiple-emitter transistors. A low signal state is defined as a signal 0.8V and below. A high signal state is defined as a signal +2.0V and above.
TTL - Thru-The-Lens
A camera design that let's you compose an image while looking at the scene through the lens that will take the picture.

U

Uncertainty Principle
The principle of quantum mechanics which states that the values of both members of a certain pairs of variables, such as position and momentum or energy and time interval, cannot be determined simultaneously to arbitrary precision. For example, the more precisely the momentum of a particle is measured, the less determined is its position. The uncertainty in the values of energy and time interval permits the quantum creation of virtual particles from the vacuum. 
Uniform Motion
Motion in which no forces are acting: unaccelerated motion. Motion at a constant velocity. The state of rest is a special case of uniform motion. 
Underexposure
Exposing the film / image sensor to less light than is needed to render the scene as the eye sees it. Results in a too dark image.
Unipolar
An analog signal range that is always positive (through zero).
Units of Angle
You can think of an astronomical image as a map of the radiation received from some small region of the sky. The coordinates of the map therefore correspond to angles on the celestial sphere. This means that the directly-measurable "sizes" and "areas" of astronomical objects are actually angles and solid angles; to convert to real lengths we have to find the distance, which is usually not known very accurately. Astronomers still use the units of angles invented by the ancient Babylonians, in which the 360 degrees of the circle are each divided into 60 minutes of arc, or arcmin, and each minute of arc is divided into 60 seconds of arc, or arcsec. Since the Babylonians stopped here, fractions of an arcsec are quoted using decimal notation, and even S.I. prefixes, hence milliarcsec, microarcsec. Radians rarely makes an appearance in astronomy. However, solid angles are often quoted in steradians; square arcsec is a popular alternative.
Units of Length
Since meters are such a pathetically small unit of length compared to the distances between stars and galaxies, astronomers have invented other units of length: 
Light-year (ly)
9.46×1015 meters.  -  The distance light travels in a year. 
Parsec (pc) 
The preferred unit of astronomers, apparently adopted in the late 19th century because light-years were becoming too widely used by the general public! Roughly the distance to the nearest star. 
1 pc = 3.26 light years = 3.086×1016 meters. 
kiloparsec (kpc) 
A thousand parsecs. A useful unit when describing galaxies, which can be anything from a few kpc to 100 kpc in size. 
Megaparsec (Mpc) 
A million parsecs. Typically used to give distances to galaxies. 
Universe
That which contains and subsumes all the laws of nature, and everything subject to those laws; the sum of all that exists physically, including matter, energy, physical laws, space, and time. 

Also, a cosmological model of the universe. 
Unguided Exposure
There are small tracking errors inherent in every telescope system. To avoid these errors ruining the exposure there are two methods that are used.  One is to guide the telescope using either a second CCD camera or by using a Self-Guiding CCD. The other way to do it is to take an unguided exposure, short enough to keep the tracking errors from being seen. It is possible to combine multiple unguided exposures to create the effect of a single guided exposure. Often high-quality telescope mounts give tracking accuracy specifications in terms of how long an unguided exposure they are capable of taking.
Unsharp Mask (USM)
Unsharp mask is an image-processing technique used to sharpen an image and enhance detail.  True unsharp masking is done by subtracting a blurred version of an image from the original.  Unsharp mask routines in software simulate this effect and make it easier to have control over the final image.
Uranus
Seventh planet from the Sun. Has retrograde rotation.
USB port - Universal Serial Bus
A high-speed serial port which connects peripherals, different standards exists for slower and faster communication.

V

Vector
A mathematical entity which has direction as well as magnitude. Important physical quantities represented by vectors include velocity, acceleration, and force. A vector changes whenever either its direction or its magnitude change. 
Velocity
The rate of change of position with time, or, more precisely, the first derivative of position with respect to time, dx/dt. Velocity is a vector quantity, incorporating both the speed of motion (the amplitude) along with the direction of motion.
Venus
Second planet from the Sun. Has retrograde rotation. Mariner 10 has established that the cloud tops rotate every 4 hours retrograde. Radar experiments have established that the surface is somewhat smoother than the Moon, but there are mountains and there is extensive cratering. Last transit of Sun was in 1882; next one will be 2004. Venus's rotation period is in synchronism with Earth - that is, at inferior conjunction the same side is always toward the Earth.
VGA - Video Graphics Array
1. IBMs implementation of 'high resolution color graphics' in ancient times ;-)
2. Nowadays used to indicate a resolution of 640 x 480.
V-Drive chip
The V-drive (or vertical driver) chip is used to convert the TTL level vertical timing signals (V1, V2, V3, V4) to the bi-level or tri-level signals that are required by the CCD. Most V- drive chips also level shift the Vsub (or substrate drive) signal to the appropriate level that is required for norma1 substrate biasing as well as to a higher level for electronic shuttering.
Vigentting
Restriction of an FOV such that all parts of a desired imaging surface at the focal plane (such as a CCD chip, a piece of film, or the eye's pupil) are not illuminated by the entire primary optical element. Usually caused by an undersized or improperly placed secondary, or by narrow apertures in the focuser/camera assembly. 
Voxel
An acronym based on the words, volume and pixel. A voxel represents the smallest, indivisible volume element in a three-dimensional system. In this documentation, both the volume elements of the specimen as well as the 3D picture points are referred to as voxels.

W

Wave
A propagating disturbance which transmits energy from one point to another without physically transporting the oscillating quantity. A wave is characterized by a wavelength and a frequency
Wavelength
A wave is an oscillation in space, with peaks and troughs. The distance from one peak to the next is the wavelength. It is measured in units of distance. The wavelength of ocean water waves will be meters. The wavelengths of visible light corresponds to hundreds of nanometers (billionths of a meter). Wavelength is an important way to characterize a wave. For light, the shorter the wavelength, the higher the energy of the light wave. 
Wedge
An equatorial wedge is used for imaging with a fork-mounted telescope. Most computerized telescopes have alt-azimuth fork mounts where the fork point straight up and down. This configuration is easiest to use because the eyepiece is always in a convenient position and the telescope is less bulky. However, field rotation results if the telescope is not polar aligned. An equatorial wedge mounts between the telescope and tripod to allow the forks to be aimed toward Polaris so the telescope can track in just one axis, eliminating field rotation.
Well Depth
Well depth is a measure of how much charge an individual photosite on a CCD chip can contain. Well depth is generally measured in electrons. For example, a Kodak KAF-1300 CCD chip has a well-depth of 150,000 electrons. When more charge than this fills the photosite, blooming occurs unless the CCD chip has a built-in anti-blooming protection. In general, anti-blooming chips have much lower well depths, which is one of the reason non-anti-blooming CCDs are popular (they are much more sensitive).
White balance
An automatic or manual control that adjusts the brightest part of the scene so it looks white.
White Dwarf
The remnant of a star, at the end of its life, consisting of a carbon and oxygen core supported by electron degeneracy pressure. The surface has a very high temperature and radiates mainly in the ultraviolet (hence white as in white hot), but it is only about the size of the Earth (hence dwarf). The maximum mass that can be supported by electron degeneracy pressure, and hence the maximum possible mass of a white dwarf, is known as the Chandrasekhar mass and is equal to 1.4 solar masses. 
WORD
The standard number of bits that a processor manipulates at one time. Microprocessors typically use 16-, or 32-bit words. (Or 2 bytes and 4 bytes respectively.)
Work
In physics, a compound of the force exerted with the displacement produced. (More specifically, the vector product of force with displacement.) Work is not instantaneous, but is defined over the interval over which the force is applied. It has the same units as energy
Working Distance
The distance from the front lens of an objective to the focal point. The free working distance is defined as the distance between the front lens of the objective and the cover slip or uncovered sample. Usually objectives with large working distances have low numerical apertures, while high-aperture objectives have small working distances. If a high-aperture objective with a large working distance is desired, the diameter of the objective lens has to be made correspondingly large. These, however, are usually low-correction optic systems, because maintaining extreme process accuracy through a large lens diameter can only be achieved with great effort.

X

X-Rays
High-energy electromagnetic radiation (light), with short wavelength (less than, roughly, 10 nanometers) and high frequency (greater than about 1016 Hertz). X-rays would be produced by blackbody radiation at temperatures in excess of a million degrees. Sources of astrophysical X-rays include accretion disks, gas impacting on neutron stars, X-ray Bursters, and hot gas located in the centers of galaxy clusters. 

Y

YUV color space
Y stands for the luminance (lightness) information, and is compatible to black&white (and gray) signal. U and V are the so-called color difference signals B-Y an R-Y, and carry the additional color information (additive color space). The YUV representation of video information is also oriented on the human perception of visual information, whereby RGB representation is more based on the technical reproduction of color information. The human eye senses luminance and color with different receptors. There are less color receptors, and they have significant less spatial resolution.

Z

Zenith
The sky directly overhead. An object "transits" when its line of right ascension crosses the zenith.

Most entries are collected from other sources but no full copy of any existing page was made. 
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As of: Februar 2004