This is by far my largest and longest focal-length lens. Rumour has it that the MTO 1000 mm catadioptric lens was “standard issue” to KGB surveillance operatives during the Cold War Era. MTO itself also has an interesting history. Seems it started out by making the huge glass reflectors for Russian anti-aircraft arc-lights. It made 250,000 of these reflectors apparently, which were used extensively to try to protect Leningrad (Petrograd/St Petersburg) from German air raids during WW2.
Anyway back to the present, my particular MTO lens was probably manufactured during the early 1970’s. I really do like this thing. Sadly however it has had very little use since COVID lockdown. Its minimum focal distance is about 8 metres. Great if I want to check the padlock on my shed door. lol 
But for any creative usage, I need to take it for a walk.
Problem with that is it’s a heavy beast: 2.433kg including lens caps, µ4/3 adaptor and Arca Swiss mounting plate. Means I also need to take my trusty but chunky Benbo #1 tripod, since everything else I have is probably too flimsy for that sort of mass.
It also absolutely dwarfs my µ4/3 cameras. So much so that the lens itself has to be tripod-mounted, and the camera mounted on the lens. Otherwise I fear it would probably tear the flange-mount off the front of the camera.
I always felt that catadioptrics look cool too. They appeal to what my wife rather uncharitably describes as my “inner six-year-old”. In any event, I think I may succumb to the temptation of a Tamron 500mm catadioptric at some point, even though I really don’t need one. 
Of course if one is shooting with a lens that effectively has a bloody great hole in one end, there are inevitably going to be limitations and artefacts. Though I note that of late, such artefacts have become regarded by some observers as a feature rather than a bug!
Some sample pictures
The results from the MTO are actually quite remarkable, especially considering its age, providing one doesn’t mind a but of “impromptu weight-training”. These were taken late one afternoon, from Portsdown Hill (elevation ~120 metres) overlooking Portsmouth and Portsea Island…
Portsmouth Historic Dockyard, roughly 4.5km to 5.5km away.
I also have a slightly earlier version of the Samyang 500mm . It’s branded “Opteka” and its fixed aperture is f/8. However, its IQ is significantly poorer than the MTO. Though it is also significantly lighter: 433 grams including adaptor and lens-cap and will mount safely on the camera without further support.
In any event, it’s good to see catadioptrics coming back into fashion. I have always been rather fascinated by mirror lenses and I’ve been particularly tempted by the Tamron Adaptall SP 500mm F/8 Catadioptic which turns up on eBay every now and then. This is said to be significantly sharper than the Samyang and its cousins. And I have a bit of a thing for vintage Tammies anyway!
- More information about this magnificent old lens…
https://lens-db.com/mto-11-1000mm-f10-mc/