Wait, there is something terribly wrong with the assembly of your camera.
Did it come from the factory that way, or had you at some point reassembed the camera window after removing the window glass for cleaning?
Notice that your window's long axis is aligned to the sensor's short axis. The window assembly (held by the four cross headed screws) should be rotated by 90 degrees.
That being said, your original problem is not caused by this, but this would cause vignetting, and especially severe if you use a fast OTA/lens.
Chen
