| Following is a description of
the 740K cam housing.
Remembering the design considerations: Can attach to 1.25"
eyepiece and to CANON SLR lenses.
I usually connect the cam via a flip mirror diagonal
with a T2 thread to the telescope and therefore the CANON adapter is not
only used for the SLR lenses.
The housing
Due to an upgrade for digital satellite reception I had an old alu LNB
(the satellite antenna receiver part which is mounted in front of the
antenna) at hand which looked like it would make a new housing for my 740K
cam.
After some modification it was able to receive the components from the
cam as well as a Peltier and a cold-finger with the CCD.
The tube of the feed horn was extended to fit into a standard
1.25" eyepiece tube.
The SLR attachment is based on my
'old' design which has a removable front part with the CANON bayonet
ring. It is only a alu part which fits onto the eyepiece tube and which
accepts the bayonet ring front part.
The Peltier cooling
The Peltier cooling part consist of
 | a 30x30mm Peltier element (second hand so I don't know which
manufacturer but it seems pretty common in specs). |
| an good quality alu CPU cooler with fan (ball bearing) |
| a cold-finger made from copper |
| the power supply to drive the Peltier |
The cooler is mounted on the outside of the alu die cast LNB housing of
course with thermal transfer compound (actually a double sided thermal
transfer tape which provides a very strong hold for the cooler - trade
name is BOND PLY).
On the inner side the Peltier is mounted with nylon screws going through
the LNB and the cooler.
The CCD is mounted with the double sided thermal transfer tape on top
of the cold-finger which in turn sits on the cold side of the Peltier.
By design the CCD is pressed to the cold-finger and the Peltier when
the cam housing is closed.
The power supply has 12V input and a LM317 linear regulator to provide
3 different power settings for the Peltier. The fan is driven by the same
circuit with the intention if the Peltier does run on low power the fan
can do the same...
The settings are:
| Low: Ipelt=0.63A; Upelt=3.56V; Uvent=4.12V |
| Med: Ipelt=0.94A; Upelt=4.54V; Uvent=5.32V |
| High: Ipelt=1.42A; Upelt=6.12V; Uvent=7.22V |
I could not place a thermometer to the cold-finger but I recognize the
following effects with 20°C ambient:
| Low: icing on the raw pelt without any parts mounted |
| Med: just no icing on the CCD window - optimal ?? |
| High: icing on the CCD of the completed and running cam (this is
where I have to seal the CCD window)21 |
Cam assembly
Putting everything together with insulation of the LNB
housing and some pads and double sided tape to hold the PCB is shown to
the right. The CCD is wired using a flat computer cable with 8 wires each
side.
The front part of the housing builds a shield and window
which accepts the CCD and put a slight pressure to the whole assembly to
avoid screws and other mounting parts.
And finally............
This is a picture stack of M3 taken on the day of the first light of
the cam with all the modifications shown
above but yet un-cooled mounted on my Lomo Astele 133.5mm telescope in
prime focus.
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