@Geggo
I see you're using a mto-11ca.
I bought a used Samyang 500mm f6.3 this week, also looked for a mto-11ca first, but they're all on sale half the world away and shipping costs are huge with that weight

How is the handling of the mto-11ca, is it also a pain in the ... to get sharp focus ?
The Samyang is said to be really soft in focus, but I'm very happy with the results I got.
Biggest problem is the T-stop, it's almost 2 stops bigger then F-stop on these mirror lenses.
I've also read that some of these mirror lenses have a converter element at the back, not sure if the Samyang has one.
But I find it typical that the lens is about 120mm long, so if the light path is 3 times as long, as it should be for a mirror lens, then it gives about 360mm focal length, maybe add some 50mm more for the T-mount, then it could only reach about 400mm focal length

Some say, if it has a converter element at the back, removing it could give much sharper results.