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Hi Sandy
Vignetting is just one of the imaging system defects that flat fielding attempts to remove.
However, depending on what else you have in the optical train, such as filters for example, when these other optical surfaces are close to the sensor, any dust on them will show up as a doughnut or “O” ring and flat fields will also remove these.
Here are some examples, taken through separate RGB filters, along with a guess at the possible sources of the dust.
Cheers
Dennis
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