My first guess was based on a previous thread nearly a year ago about the primary baffle tube in these scopes not being long enough. After 3D printing an extension, the halo was still present on test flats. I would also suspect internal reflections, although, everything in my system is pretty dark. Train: Moonlight 2.5" focuser->RCFLAT3->Celestron OAG->STL. The connections for the OAG are from PreciseParts and are much larger than M48 (which even so would only exhibit vignetting). Maybe it is a filter issue? At this point I have no clue. Any suggestions would be appreciated. Perhaps my extension tube was still not long enough for the primary baffle. Anyways, clear skies!