Darks can just be taken with the camers itself. Lens cap on, preferably in a very dark room, or covered by something to block out any potential stray light. Personally, at least during winter, I actually stick my 5D III or 7D in the fridge or freezer, after normalizing the temperature closer to the real ambient I had when imaging, and let it rip. ;P It's the only way I can get similar temperatures, and when the refrig/freezer door is closed, the lights go off and it's nice and dark in there. I would let the camera normalize in temperature for about 30 minutes, then do a warmup cycle to get the sensor temperature normalized (take a few dark frames that you'll just throw away)...then take your actual darks.
Haha, you should see the look on my Wife's face, when she comes home and finds a 30' USB cable running from the fridge into my office. "No honey, dinner will have to wait for awhile. The light in the fridge will ruin everything". On a serious note, thanks for all the info, Jon and others. I did not know that Bias and Flats were related. I ASSumed bias frames were only associated with darks, as they are frequently mentioned in the same breath. Good info! Looks like another thread to be pinned.
LOL. Yeah, just wait till your running darks for multiple cameras at once! (I tried that once...BYE seemed fine with it at first...then it decided to start barfing all over the place.)
Anyway, I am actually working on some of my own master darks again. I'm doing -6°C, 0°C, 6°C and maybe 12°C. I am going to be taking 10x20m darks for each, which will be integrated into the four masters. I'm choosing 20 minutes because with my current equipment, I will never be taking subs longer than that. I'm choosing to do only 10x, because with the darks being longer than any of the subs they will be calibrating, they will always have to be scaled down, which should suppress the random read noise much like stacking.
My primary goal with doing this, for now at least, is to use one of these master darks for cosmetic correction in PixInsight. I may or may not calibrate the lights with them...I guess it would depend on whether I'm seeing amplifier glow. I do have some glow, but I am generally not seeing it until I get to really high ISOs...maybe that's just because of the cold, and the overall thermal contribution is so low that it takes ISO 6400 and up to really make it visible. Anyway, cosmetic correction can help with removal of hot pixels. I'm spending a good deal of time at my dark site on dithering. The other night I spent about 50 minutes on the dithering process alone, which could have been better spent getting more lights (as much as 12 additional 240s lights, to be exact, a non-trivial number, and a non-trivial increase in total photons captured.) I am thinking I may start dithering every 3 frames (something BYE can do), so that I can still get some of the benefit...but use cosmetic correction on top of that to help deal with the hot pixels.
Anyway, I thought I'd detail another option for you, the cosmetic correction path. All that really does is use a scaled master dark to identify the hot and cold pixels in the frame, and algorithmically correct them rather than correct them via dark subtraction.