M42 / Orion Nebula

although this is a full-frame camera and everything could have fit into the x1 corrector.

Yes, this is a full-frame, and the corrector does not alter the focal length.

So full-frame is cool, but what about vignetting? The secondary mirror of our telescope can't fully illuminate that entire sensor. It's clear that flats were used, but still, there simply can't be any light in the corners.

Hmm, I didn't apply flats, by the way, because they turned out to be bad. There actually is signal there, and it's even decent. I just stretched the histogram in Siril right now, and it hardly goes into the black. Also, vignetting probably played a significant role, as it wasn't much larger than the frame overlap area, which gets cropped anyway.

Interesting. So with a 0.75 reducer, it will definitely work.

Indeed. Probably only a new secondary mirror will do, but we likely won't find out soon, because a new camera arrived for me today (QHY268M + CFW3M-US), and it's not full-frame, but APS-C. Now I need to switch to N.I.N.A., figure out plate solving and auto-focusing. APT is certainly good, but its manual nature, lagginess, etc. It's time to move to the stage of "start the sequence and go drink tea in the car"))