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Mike, I use a median combine to create a master dark due to the possibility of random cosmic ray strikes, Otherwise the combine method is largely unimportant. I use one master for each ISO, but I also use a set of bias frames so that the darks can be scaled for different exposure times.
10 darks is sufficient to make a good master dark, 30 would be better, 60 will gain you nothing except wasted time. I use 120 seconds for darks, to reduce the degree of scalling, and renew them monthly due to temperature changes as the seasons vary.
cheers,
Doug
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