I use a few of them. Takahashi FSQ106N with the dedicated 1.6x extender. Two of the elements are crystalline fluorite. The other two are the mating glasses. Extender is 5 element. Superb, and I mean SUPERB contrast and color correction, beating out all of the Canon glass that I use or have access to - that includes the 300 2.8L EF IS, 400 2.8L EF IS, 500 4L EF IS, 600 4 EF IS, 800 5.6L EF IS - and I'm a HUGE Canon fan.
But they don't make that one anymore. Seems as if they're trying to get rid of the lead in the glass that they use so they've revamped it to the newer version listed below.
My other scopes that I use are from Astro Physics. Contrast is a TOUCH lower than the Takahashi offerings. ED Glass. But the color correction and precision of the optics are superb.
The Takahashi FSQ106N with the extender is 850mm at f/8. Or without the extender it would be 530mm at f/5. Manual focus only. Astro Physics, they have two types in the 5 inch class (front diameter). There's the f/6 version which is 780mm at f/6 and there is the f/8 version which is 1040mm at f/8. The f/6 version is very popular due to the size and weight. 14 pounds, but they have to be on a tripod obviously. Once when you get used to the focusing (rack and pinion), the images that come from these optics will put the 12 grand 800 f/5.6L EF IS to SHAME. That's if you don't rely on the AF or the IS of the 12 grand Canon lens.
Expect to pay about 3 grand for a used Takahashi. And about 6500 for a used Astro Physics. Still far cheaper than the 800 5.6 lens and the optical quality is incredible. You can find them on astromart.com every once in a while. The extra 5500 that you save can go to a 1ds3 body! Or maybe a nice Gitzo Carbon fiber tripod with a Wimberly head...!
You might also want to check out the magazine Nature Photographer, winter issue. I have a published article in there dealing with these optics for nature photography/terrestrial usage. You can find these at a Borders or Barnes and Noble bookstore, or other large bookstores. Worth a read.