I've spent the last couple of weeks mostly with the new mount and didn't take any flats, so I don't know what change this caused. My first thought was that maybe the filter wheel isn't lined up correctly. But I checked it, and nothing was wrong. So, I checked all elements of my optical train starting from the back:
I recently installed the cooling enhancer from Starlight Xpress to make sure that I can cool the CCD camera down to -10 Celsius - even when it's very warm outside. I turned it off and took another flat:
Clearly that wasn't it.
2. CCD Camera
Next, I rotated the CCD camera by 90 degrees to check if it's the cause of this:
If it were the CCD camera, the artifact would have stayed in the same corner. So, next:
3. Filter wheel + OAG
I turned the CCD camera back and turnd the filter wheel + OAG:
Aha! It's the filter wheel or the OAG!
Just because it's easier, I pull out the OAG:
Gone! So, it is the OAG. Maybe I can push it back in:
Good! 3/4 in:
Yei! And once more, all the way in:
OK, I know how to fix this. But where did this suddenly come from???