Many years ago, I bought a Tokina 500-mm f/8 telephoto lens for a Canon film camera. Now that film is obsolete, I wanted to try to couple this lens to my SBIG ST8300M imager, so that I could take wider views of the sky than I can get with my 12-inch Meade telescope.
The trick was findig a way to connect the telephoto lens to the SBIG imager securely. I removed some of the coupling rings that attached the lens to the old Canon camera. I replaced them with a spare adapter ring that has threads that fit into the SBIG camera. With a little bit of careful work, I was able to make a suitable coupling. Now all that I need to do when I want to use the lens is to screw it into the SBIG imager, as shown in the bottom picture.
This arrangement gives images that cover about 2.2° x 1.6° of sky, which is more than 12 times the area that I can get through the 12-inch Meade telescope with a focal reducer.
For an old telephoto lens that was just lying in a drawer, the images are quite good. This picture of the Andromeda Galaxy is one example of what it can do.