I have used my TeleVue Genesis (4" refractor, 500mm fl f5) as a telephoto lens and it's as good as any of the long Canon telephoto lenses. The disadvantage is that it's larger and heavier than a 500mm lens, plus it's manual focus only. However the image quality is very high indeed and I used it a lot before I bought a 500mm lens!

All you need is the appropriate "T" adapter, plus you may well need an extension tube which fits into the focuser if you want to use it without the diagonal in order to get the camera at the correct location. You may need a few inches of extension if you want to focus close. Most of the astrophotography components will assume you want to focus at infinity. To focus at 10ft, focus shifts back about 4" compared to infinity. I made my own 2" diameter extension from brass tubing and a modified "T" mount.

BTW for those thinking this may be a "cheap" route to a long lens, it's not. I think the current price of the 101NP is around $3700. It weighs 10lbs and it's 29" long. That's 4lb heavier and over a foot longer (and 2/3 stop slower) than a Canon 500/4.5L.