I've just noticed that my 200mm lens has a rectangular opening at the back that is probably the same size as the sensor on a pentax body.
Problem is that my adapter is screw-on so the rectangle is rotated vis a vis my QHY8 CCD. It creates a wide slit for lack of a better word and probably a bit of vignetting as a result.
I have noticed bright stars become oval about 50% from the center of the field but there's no aberration what so ever on other stars to the edge. Is it possible that this restriction cause some diffration effect like bathinov slit?
I guess if it does then I have to stop the lens so the aperture is smaller than the width of that slit? Or can I unscrew that back plate safely and cut it out in a circular shape?
I'd carefully remove it, it think it was designed as a form of baffle.
By removing it carefully, you can at least return it to standard if necessary.
Gary
