"Donuts" after synthetic gradient correction

[AstroBernd]

I am taking images with a C11 Schmidt-Cassegrain in Hyperstar configuration. So, i do have quite some vignetting plus a relatively large central obstruction.
When i try to correct the images for vignetting, i am ending up most of the time with a donut shaped background in the middle of my images. Here is a stretched example (smoothed, but still noisy; stars removed):

I am working with synthetic flats (out of normal sky images with the stars removed and the "Points" method) or with the adaptive gradient removal (subtraction) on the finished color image most of the times.
I also do take FF images, usually with a luminescent foil, but usually the donut still appears.
Is there a way to take the central obstruction better into account? Am i missing something fundamental.
The effect appears similarly with different OSC cameras (CCD and CMOS).
Any idea or help?
Thanks in advance
Bernd

[fabdev]

Hello, I would start to solve the problem with real flat fields first.
Usually this happens when the dark frames of the flat fields are not subtracted.

"Image / Flat" is different from "(Image + Dark) / (Flat + Dark)".

if your cameras do not need dark frames then use bias as darks. If the bias are also almost noiseless then use a constant image where every pixel is the average value of the dark frame (bias frame). This constant is necessary for the formula above.

About the synthetic flat fields methods, I don't understand well the question, it seems impossible to me that all 5 methods all provide a donut shape. Maybe that screenshot is the result of the "Vignetting" method, but that method require an image corrected for dark frame and usually cannot be applied on processed images. The "Points" method does not require to delete stars first. Again, I'd suggest to fix first real flat fields before using the synthetic ones. By the way the "Adaptive" ones can be applied several times if needed, even after stretching.
Fabio.

[AstroBernd]

Thanks Fabio for the quick and comprehensive reply. I will focus on the respective dark frames first and also double check where I have “real” flats.