While taking pictures, I noticed a strange artifact on the subs. Blue circle with a green border. It was also visible after the meridian flip, but it moved from the position in the image compared to the before the flip.
I noticed that I had a somewhat transparent stain on the filter, but I did not clean it. I suspect that this stain is the cause of this artifact. I was hoping that calibrating with flat frames would remove this artifact, but that didn't happen. No stain is visible on the flat frames. I have not yet had a chance to clean the filter and try again to confirm this assumption.
Has anyone had a similar case or has more knowledge about this? If it's not this stain, then I don't know what the problem would be because I've never had this kind of appearance in pictures before.
Attached is an integrated image and master flat frame.
