I've attached an example image that shows the problem. The first is unaltered, the second I forced the levels to emphasis the issue.
Ignore the black on the right - that's the OAG prism holder poking in.
What I'm interested in is the concentric rings, that are particularly concentrated in the centre. Are these gradients something I can fix by using flat frames (I haven't got a setup for this yet), or do I have some other flaw in my system?
I'm using a field flattener for this setup.
Thoughts?