For a very long time I could not get my flats to correct the corner vignetting on my full frame ASI6200 sensor using a flat panel. There was always very dark corner vignetting. It was a bit better with sky flats on a good day if I took the flats at approximately the same angle as the target that was being imaged. Always difficult with Sky flats.
I've just gone through the process of aligning my sensor with a laser as shown in the forum and there is a detailed post around this.
After I did the sensor alignment I made a new set of flats with the flat panel and these new flats corrected the M27 image data much better than I've ever seen. That data was taken with the ASI6200 sensor that I aligned last night.
It is very possible that anybody looking at flats not calibrating correctly, especially corner vignetting, may need to consider if their sensor needs levelling and taking the somewhat painful steps to do so. On a full frame sensor this might be more critical.
However you might already know this but if not it could be a useful nugget of info learnt from my experience.
Edited by pyrasanth, 23 June 2021 - 05:32 PM.