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Originally Posted by PRejto
I wonder if it's possible to take flats using just the night sky? i.e., long exposures using light pollution as the light source and then process out the stars? Is that too far fetched? I imagine gradients would be a quite large issue.
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Some professional observatories have used this in the past, not sure if they still do. The idea is that thousands of images from all across the sky are used, sigma clipped so that the resulting stack has nothing but the sky flat. This works well if you do a lot of LRGB imaging with star fields, not so much if you do a lot of nebulae shots as there isn't ever much in the way of true background in this case.