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I have noticed that bias frames are incredibly important for good flat calibration, especially with narrowband imaging.
After bias subtraction :
If your image has an adu reading of say 4000 at the center and at the corner it is 1000 because of vignetting (4:1). Then you take the flats and your adu reading is 40000 at the center and 10000 at the corner (4:1).
This will correct well.
Before bias subtraction Bias of 500adu
If your image has an adu reading of say 4500 at the center and at the corner it is 1500 because of vignetting (3:1). Then you take the flats and your adu reading is 40500 at the center and 10500 at the corner (3.85:1).
This will over correct the corners and they will end up brighter than the center.
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