On Saturday night I was trying to fine tune my guiding and was using the Rosette Nebula as my test subject. I wasn't really trying to get anything useful because the moon was so bright but I figured if I could get a night of decent guiding performance I'd be happy.
The next morning I processed the subs just to see what I got and it was quite strange. I was getting this very dark triangle in the lower right corner and not much real nebulosity at all. I went back and checked my flats and darks and they looked ok so I went back to the raw subs. I pulled one up and it looked ok but when I debayered it, it had these odd dark marks all over it. When I blinked the debayered subs the stars moved agains the marks. Whatever this was was in the optical path since it was fixed relative to the frame and not moving with the stars when dithering occurred.
Here's a screenshot showing the artifact:
When you apply an unlinked STF to that file, the dark spots and the very steep gradient on the lower right corner pop out but they weren't at all visible before debayering which is why I didn't notice when they were being acquired.
I've no idea what I'm seeing here. Is it condensation on the sensor perhaps?
The camera was an ASI294MC Pro being cooled to -15C and the ambient temperature was around -2C when this was shot. The optical path was a Stellarvue SV80-3SV (80mm F/6) with field flattener to the camera. I did some subs on the rosette the week before and they were fine. Whatever this is was new behavior that I've never seen before.
Anyone got any ideas?
Edited by ac4lt, 18 February 2019 - 08:25 PM.