You can also scale your darks and you need a bias for that. Its an option in CCDstack.
Best practice is to use a temperature on your camera you can achieve all year round except perhaps for the very hottest nights.
CMOS cameras these days are pretty clean so no real gain going -20. 0 to -10 is a common temperature setting. Sounds like you need to use around 0C so its achievable.
Greg.
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