There is a lens is available from several brands. Mine came from samyang and since it says “Made in Korea” on it they may be the people who make the lens. Samyang have developed a fairly positive reputation of late making cheap but good manual lenses. The Samyang 800mm mirror however comes from a previous generation.
Mirror lenses never really had the best of reputation and to be honest the Samyang 800mm is towards the bottom of the pile. But still 800mm is 800mm right? And at £155 its fairly affordable.
So is it worth it for wikipedians? To be honest probably not. There simply isn’t that much that needs shooting at 800mm. Most of the time its simply a matter of getting closer. Of the stuff there is it generally isn’t too good at that either. If it wasn’t too slow for sports at f8 its fiddly focusing system (¼ of a turn between 3.5m and ∞) makes it pretty much unusable in that area. Wildlife would be a more classic use but the money would probably be better spent joining a birdwatching society. Unless you live outside north America and western Europe your local wildlife has already been well documented by people with more serious kit. Photos taken in zoos and the like fill up a lot of the gaps. That’s not to say that there isn’t still work to do but it will involving going after less common species and given the time and effort required to do that in most cases better kit can be justified. As for astrophotography thanks to NASA and wikipedians with serious gear (yes technically the samayang lens is a Schmidt–Cassegrain telescope however some wikipedians have far better ones and tracker mounts) that area is pretty much saturated. About the only other use is taking photos of things like castles that are on private land making it impossible to get close to them (taking local laws into account obviously).
While the lens isn’t that heavy its fairly large making it unlikely that it is something you would carry around on the off-chance it would come in handy.
Outside of wikipedia’s however things are a bit different. 800mm may not be particularly useful but it is fun particularly once you get past the fiddly focusing system. The build quality is solid enough that you don’t need to worry about it falling apart and the image quality is certainly usable (although the fixed f8 does limit you to bright light only). In fact if you take images beyond 100 meters or so the limiting factor is likely to be the clarity of the atmosphere rather than the lens. If 800mm isn’t enough the T2 mounting system that the lens uses can be used to attached the lens to a micro four thirds system giving you 1600mm equivalent. Why you would want to do this I’m not sure but its not like you have to justify it to anyone. Samyang also sell a x2 extender for the lens although I can’t imagine the results are much good. I did try sticking it on the canon x2 extender but since the extender expects there to be some lens electronics to talk to this doesn’t work.
As with most mirror lenses its worth getting the lens hood with it. Samyang do make a lens hood for the lens (nice solid metal) but I had to phone up the supplier I was using in order to get hold of one. One problem with the lens hood is that since it attaches inside the barrel it is impossible to attach the lens cap while it is in place and the thread on the hood tends to stick making it hard to remove.
Overall the lens scores not so well on the image quality stakes but high on the fun stakes. For wikipedains though there are more useful lenses although not at the same price.