Gardient on OIII flat frames

Have you tried putting it in a different position in your filter wheel?  I don't have near as much experience as others on here, so I can only think that maybe that position on your filter wheel is off a hair.  I would expect it to be visible in the lights, too, if it was the position of the filter wheel though.  Have you tried going to the OIII filter from both directions?  I'm reaching here, so maybe someone else has better ideas, I just know how frustrating things can get with the hobby.  I had serious issues with my calibration frames early on with my DSLR and almost quit because of it.

Hi Marc,

I am no expert, but there are some thoughts that I will write down to help you. They may not be correct. It's just a starting point for solutions. Maybe you already have thought about them.

The advice from @Alex is the one I would check first. The mean value of the flats is usually much higher than the one of the light frames. Because the images are stretched with different levels, the gradient could be much less visible in the light frames. It may not be the case, but be sure to check this.

I own a filter which is quite thick/high. I once put it into a filter wheel and I also used a distance ring with a slightly longer thread to mount the camera or the corrector to the wheel. I then realized, that the filter got slightly stuck on this thread. Maybe, this is what you are seeing. In my case, there was a slight sound of scratching metal when changing the filter.

The round edge of the dark part in your flat is quite sure the edge of the filter. You can try to look at the filters if the cam-wheel-corrector-spacer-etc-combination is not mounted to the telescope. You can see the filters change and depending on the size of your sensor, you should see the problem quite obviously.

My filter wheels have some small holes in it, where internal sensors decode the current position of the wheel itself. If one of those holes is slightly off (production issue?), then this could be problematic. But this will affect only one filter position. So again, the advice from @Alex  is helpful.

I don't know your scope, but on my Newt, I see those effects also if the mirrors are not aligned well. But then, you should experience this kind of problem on every filter.

Unless you are not using a very strange position for taking flats vs taking lights, there should be no difference in the filter position for either of those frames.

I hope, you soon will find the right solution.

Clear Skies

Christian