Quote:
Originally Posted by Merlin66
Marc,
Why?
The flats and the lights both contain bias....
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It’s the difference between a division and subtraction. If you don’t use a bias or dark frame to calibrate your flats to make a Master Flat you end up dividing noise instead of subtracting it from your lights. As Marc says, it ends up under/over correcting your lights and doesn’t remove the bias.