I _did_ say it wasn't field rotation, for a couple of reasons. (Some people were fixated on that.) Since the spacing was apparently correct (for a Celestron FR - that _is_ what you have, right?) and changing it didn't change the artifact, there isn't much left other than a poor FR.
Yes you did. A Facebook group I'm in also said it was field rotation, but I was thinking "then why have a GEM mount". As for a bad focal reducer I can't do anything about it since I bought it on cloudy nights and I have no way to prove it to be bad. I can't see anything wrong with it by just looking at it either. From what I can see there's no missing glass or scratches. It's a celestron focal reducer too, just to reiterate what it is. Maybe I can just crop out the center and call it even. But it doesn't change my bortle 25 skies.