Mattia's advice was "right on"; the "lure" of being able to photograph things that are far away with big long telephoto lenses that are "cheap" has been going on for many years now. Unfortunately, there is much more to making far away objects look close, than just having a lot of focal length; as Matia pointed out, you need a lot of light, and a lot of resolving power, and the only way to get both, is with a lot of aperture; the bigger the aperture, the more light, and the more resolving power. But that's only the beginning............even if you have a big telescope, you also need a big mounting; (which tend to be even more expensive than big telescopes ! ) It's kind of a "vicious circle" ! A few years back, a very clever fellow named John Dobson came up with a scheme to build a reflecting telescope, using what's known as the "Newtonian" principle, and the "big mounting" is completely made from plywood ! Dobson even ground his own mirrors, as have countless other amateur telescope makers, but I can tell you from experience........."it aint easy" ! A lot of companies are making and selling "Dobsonian" telescopes these days, and yes, they really DO
WORK ! And as big telescopes go, they are dirt cheap ! That would be the only practical way to getting into very long range photography, without spending "big bucks" ! But as for the big 600 - 1200mm "lens" you mentioned.........save your money !
If you end up buying one of those things, the only practical "use" i can think of for it, would be to use it to beat the guy who sold it to you over the head !
Stupidity: Doing the same thing over and over again and expecting a different result; Albert Einstein