First post here. I've read tons of posts though! I'm fairly new to astrophotography and started with a Skywatcher 100ED refractor and an EQM-35 pro mount. I've since stumbled across a used Losmandy Titan mount and a Meade LX850 ACF 16" OTA. It's huge! And the set up is very heavy. With a field flattener/reducer, I'm able to get my Canon EOS Ra to work perfectly with my 100ED refractor. I've finally gotten the Titan mount and Gemini 2 working perfectly, but the pictures I take with the Ra when it's attached to the Meade has quite a lot of curvature and vignetting. I've attached a single quick sub from when I was focusing the telescope to show what I'm talking about. It was also shot from my light polluted backyard (Bortle 5), which may have something to do with that as well.
I don't have anything connected to the visual back of the Meade besides my camera. I don't quite know enough about back focus to know if that's an issue. I've also tried looking for field flatteners for the Meade, but I'm not sure how well they would work, as I believe the meade has a reducer built in already. Do any of you have any suggestions for what I could do to correct this? I've been toying with the idea of buying a dedicated astronomy camera with a smaller sensor, but I'd like to be able to get this working with the Canon. I've been searching through tons of forum posts, as well as astro bin to see what other people have done to get a full frame camera working with their scopes, but I haven't found any posts or pictures with the same scope as mine, using a full frame DSLR. I did find one picture on Astrobin of someone using a Canon 5D Mark IV on the LX200 16" telescope. I'm thinking of posting a comment asking what his imaging train looks like to get a good image without the issues I'm seeing.