Strange grid / stains pattern in background

No promises, but I've had similar issues before. Try stacking without your bias frames and see what happens.

Hi,

Thanks for your suggestions. In the meantime I tried different ways and also made another observation.

Blue Smith:
No promises, but I've had similar issues before. Try stacking without your bias frames and see what happens.

Unfortunately this did not do the trick. I also tried to only stack different smaller subsets, with the same result

Alex Nicholas:
This can also happen if you're drizzling the data with not enough sub exposures or insufficient dither settings. Drizzle works of a weighting map, and if you don't have enough subs you get better weights in some areas than you do in others, resulting in a mesh pattern appearing throughout the image.

Also this idea did not work out unfortunately. I tried to stack without drizzling, with the same result. And there were absolutely no signs of this pattern in any of the calibration frames. I tried stretching them very much and still no sign. I have about 1800 subs, so this should be sufficient amount.

What I tried next:
I took the data over a total of five nights and photographed a different object in the second half of each night. So now I tried to stack the data from the second object with the same master calibration frames. The result: no visible pattern.

I'm really at a loose end. I can't see any recurring pattern in the individual calibrated raw images either.

Konrad Krebs:
Hello everyone,
i get a strange pattern in the background of my final stack. 
I used a ZWO ASI 183 MC Pro at -20°C, gain 50, 30s per shot, 1800 shots in total. Dithered every few shots, blinked through all images, nothing obvious.
Flats, darks and dark flats, all in WBPP. The individual images all look normal, even the master flat and master dark are completely normal. does anyone have any idea what the problem could be?

I think its a calibration issue.
I didn't have this before but try different combination of calibration settings also check your calibration frames.
CS!