I've shot terrestrial for fun with my 12inch f11 LX200 Meade telescope at prime focus. This equates to about 3000mm f11.
Rather disappointing if you shoot at distant objects due to atmospheric haze.
Yet what did the adapters and the telescope cost? $3-4K or so?
There's a point where stupid is as stupid does, you know... If you need such a big lens, there are easier ways to make it work.
http://www.telescope.com/control/product/~...roduct_id=09972Here's a roughly 1000mm equivalent f/5.3 for cheap. Very good optics for the price. Has a 2 inch focuser as well, so it'll fit a simple mount/adapter for a DSLR.
http://www.telescope.com/control/product/~...roduct_id=05270Yep, literally that simple. Not sure exactly how much the extension tubes add to the focal length, though, hence the "roughly" comment on my part.
It amazes me that people spend money on these old monster fixed focal length lenses when there are far superior options out there for a tiny fraction of the money. As camera makers stopped developing these super-long lenses, telescopes kept getting better. Most of the same companies, in fact, moved where the remaining money was - in astronomy.
Many decent telescopes are made to be just as precise if not moreso than any fixed zoom on a camera. Few people know that this sort of thing is an option, though. And, as a bonus, the manufacturers will almost always list the exact optical clarity right up front in easy to compare numbers.
The only advantage a Canon or similar zoom has, of course, is that it's a camera lens(refracting design) and so you can track objects easier. Most of the telescopes sold are reflectors of various types and the mount is usually on the side. But most telescopes have a decent enough spotting scope on them to get you in the general area quickly. The live view of the camera's LCD gives you instant feedback - really nice compared to doing this in the old days with film.
Not light, though.. none of these are, really.(22lbs for that one without the mounts or adapter - close to 30lbs ready to mount on a tripod.
(yes that enormous Meade is a *BIT* overkill - lol)