What is the size of the aperture on the front of the coma corrector? Is the opening very big or is the element retained in a cell with a wide ring at the front? It is possible that the vignetting source is the coma corrector, or as Oberon mentioned, it is possible that a lot of in-focus is causing the front of the focuser tube to intercept the off axis rays.
While I agree that your secondary mirror may be a bit small, this would not cause you to see a diffraction artifact (which I of course think this is). It would cause illumination falloff but that would not produce the ring.
The clear aperture of the coma corrector is important at these speeds. If it has a retaining ring that is rather wide, this could be an issue.
I think you are in the right spot.
And the picture thought the focuser... This view needs to be done with the same attachments or devices that you use to image minus the camara. If you rack out to the same position that you would use for imaging, then hold your eyes behind that to the point where your camera focal plane would be (Flange to Focal Plane distance) and move your eye to the edge of the field, you should be able to see the entire primary mirror in the view. If you cannot see the entire primary because some part of your adatper setup, focuser tube, or your Coma Corrector blocks the edge of the primary mirror, that is your vignetting source.
Again, I am not 100% sure that this is your problem but having see it many times over the decades, I would say that it is highly likely to be your problem.
It is though I assure you nothing to do with your collimation or your spider. Neither of these have anything to do with it because they are in no way capable of producing a curved diffraction pattern.
Also, don't dismiss glint. I mentioned this earlier and should have given it more emphasis, but this should be easy to check. Any hard anodized surface that is not matte or is not threaded can cause glint, but this looks more like diffraction to me. When you sight though the setup though, pay careful attention to all of the surfaces inside your focuser and attachments to ensure that all of the surfaces are very flat black.
Edited by Eddgie, 24 February 2017 - 09:32 AM.