Image processing
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Basic stuff together with more advanced techniques

Image processing - my way...
This writing is based on experience but does not necessarily have to be the way it should be done ;-) 

Use _your_ favorite capture program!

I usually capture RGB BMP-files as raw images, each session in  separate directories which I rename later with some helpful information.


which means the session was on 17.Sep.2002 using the ToUCamPro (740k) with the LX90 no cooling. The item captured was IC1396 with 20 sec exposure and a focal reducer.

Now that we have some images - I separate the RGB images into the 3 color channels and a grayscale (L).

I use a small program which does process a whole directory at once leaving 4 TIFF images for each raw file.

If you capture FITS files you already have the 3 RGB images, a grayscale (L) can be done in most image processing programs.

It is time to register and stack the channel images - I usually do it channel by channel and not with the RGB color image. This gives more options later in the process.

Before registering you may subtract a dark frame, flat, bias etc. There are other sites describing why and how this can be done.


Now comes the fun stuff..

This writing is Photoshop (PS) based but PaintShop Pro may have similar features.

In PS put the three RGB images into one new image as separate layers. Then let each of the 3 RGB images only drive the corresponding color channel (it is available in the Blending Properties window - double click on a layer and remove the channel checkmarks for the two colors which are not driven by this image)

Insert a 'Levels' layer mask above the three (the black and white circle at the bottom of the layer window) and adjust the black and white level using the colorpicker - select the black and click on a background portion of the image - click around until you are satisfied with the background level. Do the same for white and click into a white star - again try until you find a image which pleases you.
Now we have a stacked, level adjusted color image - sometimes this is just enough.
For nebulosity and galaxy images one want to do some more processing.

It is time to insert the grayscale (L) image. 

Open it and try to 'push' it a bit i.e. apply an unsharp mask and/or drive the contrast a bit - it may look quite 'dry' for the purpose we use it later.

One can even blur the three RGB channels to get a smother image when applying a luminosity channel.

Insert it as topmost layer and use it as luminance channel. Either click the pull down menu and switch from 'Normal' to 'Luminosity' or open the Blending Options dialog.

You have to reduce the blending level for this layer to approx. 25%-50% but again try until it pleases you.

Click to enlarge

For the final insert a second 'Levels' layer above the L channel and use it to finalize the background and white levels - sometimes one want to stretch the histogram and move the middle marker a bit to the left to get most out of the image stack.

Adjust the Luminosity level and the second Levels layer until you are either proud of the image or tired because you cannot improve it (BTW: if it is not captured it cannot be seen neither can it be squeezed out...) 

Time to add text and capture information
(Create a layer set and put all text info into it - you may close the set having all texts put aside with one click)

That's it ...............
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