I’m sure it would help if I understood the math behind preprocessing with the master flat - but I don’t and I have run into a perplexing case.
Ever since obtaining my QHY183C I have taken flats with a target ADU of 19,000, because I read somewhere that it was a good value for this camera. And I never had any reason to doubt it, I never saw any issues in my images that seemed like a problem with the flats.
Last weekend I was at the club dark(ish) site. While waiting for the moon to set, I took an hour’s worth of 30-second exposures of Caroline’s Rose cluster at gain 5, offset 8. I had not disassembled the imaging train since my previous session, so I have that master flat and I took a new set that evening anyway before it was dark enough to image.
When I stacked the images, I got this (with the histogram screen stretch):
I processed again with the earlier master flat (same result) then took a new set of flats (still had not disassembled) - same result.
Then I processed the 5-minute subs I took later that evening (same gain / offset) - and they looked normal with the normal master flat workflow.
So I did what I thought one was to never, ever do - I processed the 30s subs without flat-field correction:
Next I ran a series of flats with lower and lower ADU values, all the way down to just 1k, which seemed to be getting pretty close to the limit. Less reverse vignetting, but still obvious:
So - is this normal behavior? Did I manage to repeatedly do something wrong this time?