Neuro, do you know what causes donut bokeh? Is it the primary mirror itself, or is it to do with the support mechanism for the secondary mirror, or poor coatings typical of cheap lenses, or...?
It's the fact that the entrance pupil (the image of the aperture itself) is an annulus. Actually, you can give that look to a regular lens by sticking a small, round sticker like one of these:
…on a cheap UV filter and putting it on the lens. If you like donuts, that is… Same principle as those bokeh effects kits.