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I get this sometimes. My Planewave CDK17 setup is very sensitive to correct flat fielding.
Firstly, I take flats with a white cloth cover over the end of my scope and take about 6 flats in my observatory during the morning usually. The slight filtered light that makes its way into my observatory is perfect for flats as it turns out.
I aim for about 20,000 ADU. If the flat is too bright then I find it overcorrects.
Similarly I take a dark for the same exposure time for each filter's flat.These are subtracted when the flat is done. This is all at an exact temperature - the same as the light exposures you took. Also at the same focus and with the camera in the same orientation as the light exposures.
If you do adaptive darks - that is using a longer exposure dark for a shorter light but at the same temperature you also need a bias. That stops the weird corrections as the dark is scaled properly.
I take 5 or 6 flats darks and flats and then use median combine for the flats and subtract the sigma rejected combined darks from them. I shoot for 20,000 adu and make sure the exposure is long enough so there are no shutter artifacts from too slow an exposure that captures the shadows of the shutter opening.
I sometimes can get away with older flats if I haven't changed anything but I would update my flats if anything changed - camera orientation, filters, camera, changed telescope etc.
At a guess I would say your darks are mismatched to your lights and then your flat is not flat dark subtracted. Or your flats are overexposed.
Greg.
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