I recently bought (and right after that resold) the Falcon 800mm f/8 mirror lens. I didn't find much information about them, just that these are called "russen-tonnen"
(Russian barrels) and the are not very good. You can find them on ebay:
But this is definitely not a recommendation to buy them. Though they are relatively small and lightweight for 800mm (wich makes 1200mm effectively on my Nikon D300) they lack the necessary optical qualities. If you use a good 300mm lens and just crop out the interesting part (like the moon) – you wouldn't get much worse results. The build quality is nice but the mirror produces a shine and blur around every light reflection and the bokeh looks like condoms. The really annoying part is the focusing – normally you wouldn't expect the problem of having much of a trouble focusing to infinity, but if you are using a mirror "lens" "sharpness" - if we could call that – is a matter of moving the focusing ring a couple of millimeters - even when taking pictures of a object relatively far away – like the moon. If you would like to shoot birds – forget it, I doubt you would be quick enough. You can make nice pictures of it, but it just feels like a cheap telescope for kids which can be mounted on a camera – let's put it that way – for 180€ it's a fair prize, but you can't expect getting anything even average. The most fascinating part is how close 800mm gets you – but that's it.
I uploaded a few pictures to flickr: