I have an ASI183MM-Pro which I've used on two telescopes. In both cases, the flats come out fine, except for Ha and SII. With those two filters, the flats show an odd pattern. The patter seems to be coming from the camera, as I've changed the light source, the telescope, and rotated the camera, and the pattern stays the same.
The image below shows SII, Ha, and OIII respectively.
All broadband flats come out fine, including red, which I would think should have the same issue if it's wavelength related.
All of my flat are taken a 2 seconds. The narrowband filters obviously require a brighter setting on my light panel. SII and Ha require a higher brightness setting that the other filters. I'm not sure if that's relevant or not.
When I use these bad Ha and SII flats to calibrate my light frames, the light frames come out worse. Below is one such example.
Is this a defective sensor?