[toyos]

I put 4.5" off-axis aperture stops on a 12" dob and a 12" SCT (I know a 12" f/10 SCT with a mask is not ideal) and compared them against 4" & 5" Apo's, and to be fair I brought the budget Chinese 127mm Apo too. Even the cheap 127mm Chinese Apo blew both reflectors away by far when used with the 4.5" aperture stops. The reflectors showed much darker backgrounds & tighter stars with the aperture stops, but the images were greatly dimmed too; while the refractors showed brighter & crisper images with dark backgrounds, fitting the definition of contrast better.

As mentioned before, direct aperture comparisons between lenses and mirrors will not be accurate. In general, 6" FMC lenses will transmit significantly more light than 6" mirrors can reflect. In my humble opinion, if the mirror of a 10" f/5 dob is still spotlessly clean and freshly re-surfaced, it should perform more or less on par with a 6"-6.5" Apo in terms of light-gathering ability ONLY, the image quality will not be in the same league. Most of my observing is done from my fairly light-polluted backyard.

Don't get me wrong, I like the big dobs too. I'm even considering getting one of those Obsession dobs (can someone tell me how easy/painful it is to set up an 18"/20" dob by yourself?). Just my 2c.