[PRejto (Peter)]

Terry was on the money. Black matt paper seems to have resolved the issue.

New flat through filter wheel (with luminance filter) looks pretty good. Just a bit of vignetting, but I expected that.

Peter

Edit: I think I know what might have caused this. The FW carousel is in glassy black. Perhaps that directed light back to the threads on the extension tubes. Even removing the carousel wouldn't fix the isue because the back wall of the FW housing is also glossy black. Makes me think that no filter wheel ought to have anything other that a matt finish!