My ONAG went back to the USA for adjustment so my setup was not my usual. It was quite simple consisting of only a filterwheel and camera. I went back and looked at flats from the ONAG setup and see only the smallest hint of central darkening. But, that was with different filters. When I get a chance Iwill take more flats with the new filter but my usual setup. This will provide a better comparison.
I will need to process out the errors in the lousy looking blue combined stack as I've broken down the setup I had capturing that data. Live and learn! I took flats, looked at them and thought they were OK. Greg,your advice to check when going along is very good advice.
But, if it's IR stray light why isn't blue blocking this? So perhaps it isn't IR but blue light reflecting off something? But then I would expect my LED light box to transmit enough blue to see the refection.
I wonder if it's possible to take flats using just the night sky? i.e., long exposures using light pollution as the light source and then process out the stars? Is that too far fetched? I imagine gradients would be a quite large issue.
Peter
EDIT: I see this is not an original idea:
http://www.cloudynights.com/topic/46...the-night-sky/