Began the process of setting up my QHY9M mono camera as a screw in Lum/Ha replacement for my QHY10C colour camera whenever the moon was up and ran into a serious reflection problem. The Ha flat looked like it had a major vignetting problem and when pre-processed through Nebulosity resulted in an image with bright corners and a dark inner area. Nebulosity couldn't handle the very sharp drop in intensity. Immediately googled high and low and could only find this link
http://saratogaskies.com/articles/ha-ff/index.html
but it seemed to be the exactly same problem. Sent an email off to Jim Solomon at Saratogaskies to see if he solved it but no he hadn't so plodded on. Should point out my system config was
10” F5 Serrurier truss
Onag
Mpcc II
spacer
Astronomik 6nm Ha filter
QHY9M with IR window
My first thought was the IR window on the camera so swapped it for a clear window with no effect. Then swapped the Ha filter and the spacer, putting the filter on the back of the coma corrector and noticed the circle got slightly larger. Thought the old MPCC II version might be a problem so borrowed a mark 3 but nothing changed. Began to think it was a combination of all three glass interfaces causing the problem then realised that I could place the filter in front of the coma corrector so tried it and it worked. There is still some kind of reflection going on but it is very faint and smooth enough to be handled during processing.
Pictures
1 Ha Flat reflection problem
2 over stretched image (tarantula) showing bright corners and darker center
3 Ha Flat with filter in front of coma corrector
Not sure why I got the reflection but maybe some optics engineers out there can.
Cheers
Stephen