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Hi Martin,
A good question. What you’re trying to achieve in a flat is a panel of luminance to provide your processing software with a “ map “ of the way that light is hitting the flat surface of your sensor. Variances in curvature and interferences with scratches or debris In your optical train will effect the luminance across the map of your sensor. With this in mind, it is easier to understand how you need to go about taking your flats. You need to illuminate the sensor as much as possible to expose the faults but not so high that you end up clipping the top end of the histogram. An easy was to do this although not perfect is to set your camera to Exposure Priority. Your camera will then assess the light coming in and set your camera up accordingly to revive the highest dynamic range without clipping.
I hope this helps
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