Everything I shot for several years was with catadioptrics.
2X TCs are generally shit. Your mileage will vary, and 1000mm lenses are relatively useless.
Front element diameter determines how dark the lens is to look through. If you use a split hemisphere focusing screen it will be useless on pretty much any catadioptric even in almost full sunlight.
They are VERY difficult to focus.
Lenses I own, and have used extensively:
Sigma 600mm/8 (120mm front element variety)
-fairly shit, but adequate, the Bower and other similar looking designs available as new on ebay are almost cecrtainly this design, chinese knocked-off.
Tamron Adaptall-2 SP 500mm f/8 Mirror (55BB)
-(relatively) Excellent IQ, dark to look through on APS-C with high contrast focusingscreen. Nice slick helicoid. Small. DO need one of the rear filters in for proper operation. Clear one is only useful filter.
ZM-6A 6.3/500 Mirror Zenit Tele Lens M42
-equally excellent if not slightly better than above. Larger diameter - brighter, much heavier. Stiffer helicoid.
May require simple correction of rear element array if cant focus :
http://oh-hi.info/tech/ZM-6A_before+after_MirrorRectification-y2048px.jpgSMC Pentax Reflex 1:11 1000mm
-Very good. Totally impractical. Distance haze always an issue with 1000mm and inbuilt rear UV filters of some but limited use. Not a bad lens. Huge, heavy.
MTO-11CA 10/1000
-equal or better than above. RUSSIAN = Much easier to disassemble. Stiff as fuck helicoid
http://oh-hi.info/2011/?from=2011-07-23.jpgand scroll *UP* for both 1000mm.. probably Pentax first and MTO some time later. exif says 800mm for these lenses until i started manually correcting it to 1000mm later
Tamron 350mm f5.6 (06B )
-best catadioptric I have ever used despite this:
http://oh-hi.info/tech/dust_tamron-350mmf5.6-06B.jpg Rare. Expensive. Unbeatable for tiny and light, but a modern non-reflex autofocus lens would always be better value and IQ.
