Hmm, I am interested in this answer as well. Looking at the catadioptric design...



...I am guessing that it'd be very tough (=expensive) to change the sizes of the primary and secondary reflectors on the fly.

With this design I think you really want to avoid overfilling the primary and secondary reflectors, or else you'll get all kinds of stray light and kaleidoscopic entertainment. So it'd be tough to design a system that allows you to change the total amount of light in to the primary while also changing the secondary. You could probably swap out both mirrors but that'd be a pain.

Perhaps a limiting aperture could be placed at the node of the focuser....