I frind it interesting that Michael finds the 500mm/8 not very sharp. Basically, mirror lenses can beat any APO refractor design hands down, on-axis. Problems arise when the designer tries to produce a flat image field to cover a certain frame size, in the Sony/Minolta case it is full 35mm format. Recently there has been a very real breakthrough with astro usage of mirror configurations, where a dead-flat field can now be produced over an APS sized frame, e.g. Canon's 1.6x crop DSLRs. Sorry, no 35mm format frame coverage yet. Here is my review of one mirror scope (a Celestron C14, that's 14" diameter aperture) being used as a 675mm focal length, f/1.9 "lens"
http://www.pbase.com/samirkharusi/c14hyperstar It is, frankly, superb and for astro usage it has completely replaced my Canon 600mm/4.0L IS lens.
Now, if only one could make a similar aberration-correction gismo for the C14 to end up with a flat image field at 4000mm/11 and 2000mm/5.6. I'd be very happy to use it with an APS-sized Canon 50D or similar. Apparently it is theoretically possible to make a diffraction-limited field corrector for the thing at f11, but those who can, feel that the market is too small
