halo problems

[JC Astronomia]

Hello everyone.
I have been having problems with my photos for a long time.
I attach a 600s Ha shot of the Dolphin Head Nebula (SH2-308) taken from my house, in Malaga Capital, with a bortle of 8-9.
You can see a kind of donut in the photo that I later cannot remove with processing.
Could anyone give me a clue as to what this is due to? Could it be a problem with internal reflexes?
I have a heating tape.
Equipment:
Tube: OS RH200 AT
ESATTO 3.5” LP ROBOTIC FOCUSER
QHY600 PH-M SBFL CAMERA
Ultra high speed filters 3.5/4 nm f/3: optimized for CMOS (H-alpha/O-III/S-II)
FILTER HOLDER WHEEL: QHYCFW3XL
Capture program: NINA
Say that I have the problem with all the filters.
Thank you

Link to full photo in fits

[Robert Mercer]

This is a problem with your calibration frames - your flats are not being processed properly.  I would suggest you look at how you process those.

Robert

[Robert Mercer]

I will add that the heavier the light pollution the more severe the effects of vignetting will show on your light frames.  For a CMOS camera the minimum requirement is to use both bias and flat frames (flats of adequate intensity).  For a CCD darks are a must dependent on camera.

[JC Astronomia]

Thank you Roberto for your response.
I have used Bias, Flats and Darks, but without a solution. The gradient elimination processing improves but does not eliminate it completely and it is still seen.
I use a Flats RB Focus eXcalibur generator. Both at 30% and 50% of ADU.
I'll try to try sky flats when the weather improves.
Anything else to eliminate the circles?