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Yes, dark frames can be successfully scaled, but you have to take bias frames. This is because the bias is constant, no matter what the length of the dark frame exposure, so to start with a level playing field you need to remove the bias. My usual exposure is 600sec, which I use for galaxies and nebulae, but if I image a bright star cluster I shorten the exposure to prevent stars saturating. I scale my 600 sec dark and things work fine. That said, scaling may not work quite as well as darks of the same length as your exposure. Scaling assumes darks scale linearly with time (eg twice the exposure gives twice the dark current). This works pretty well for reasonable changes-- scaling a 600s to 300s, but I would hesitate to scale a 1000s dark to 50s. It is important to do all darks and lights at the same temperature, but this is a non- issue with current ccd cameras as the temperature control is usually good.
Geoff
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