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With CCDs you set a temperature which is regulated by software and usually maintains that temp +/- 1 degree or usually a fraction of a degree.
So you take a library of darks usually about 6 -16 at a particular temperature (one that you can attain in winter and perhaps a 2nd set for the temp you can attain in summer). You don't need to take a fresh
set of darks each night. Even if your camera does not have accurate temperature regulation you can use adaptive darks where software measures the noise and adjusts your dark to match. It works very well and I can see no difference between exact temp/time darks and close but not the same darks set using adaptive darks.
DSLRs are great instruments but they have trouble with capturing star colours very often and also as they are one shot colour they tend to have trouble with colour noise. Their main advantage is they are portable, don't require a computer to operate and are easy to use and give great results without a lot of fuss. No filter wheels etc.
Also easy to use with a variety of lenses and you can use them for daytime work as well.
In addition they seem more affected by light pollution than cooled CCDs again because of the noise factor.
A one shot cooled CCD is more similar to a cooled DSLR except the accurate temperature regulation is an advantage. I am not sure how accurate the temp regulation is on a cooled DSLR.
The main advantage of a cooled DSLR would be it does not require a computer to operate (I think).
One shot colour main advantage is cooling and thus lower noise. Cooled DSLRs do not get as cold as a cooled CCD.
I like to get my cameras down to -35C when I can as I see the CCDs even a relatively noisy KAI11002 is pretty clean around that temp.
So yeah it is a matter of noise mainly. A Canon 5D has quite deep wells and relatively high QE and is no slouch in the specification department. But then that is a $5000 camera.
As far as how long an exposure it would be a matter of how dark is your site you are taking it from. In a light polluted area that may be a max of 3-5 minutes. At a dark site maybe 10. A cooled CCD you can go up to 30 minutes or more. Depending on your tracking. I don't see why you couldn't go 15 minutes with a DSLR set at ISO800 on a cold night.
Greg.
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