Think of looking from a point on the film plane toward the lens. Vignetting will not occur as long as the entire rear exit pupil of the lens is visible. If the rays that form the image on the film plane came from a great distance, the aperture would cast a shadow that cut off the corners of the image. The imaging rays from a telescope come from the secondary mirror through the center tube which forms the limiting stop. For total freedom from vignetting, an adapter should be used which opens up the constrictions described above.
Several manufacturers supply such adapters.
Lumicon being one. I inspected the real image at the back of the telescope with an eyepiece just as one normally does when viewing but without the adapter plate. When inspecting the exit pupil it is clear that slight vignetting starts to take place with any deviation from the optical axis at all. This vignetting is very small. I estimated it to be a half stop to a whole stop at a distance of 25 mm from the optical axis.