I tried attaching my camera to a telescope and it worked but the center part of the picture was always blacked out. I quite doing this and just tried to get as large a lense for the camera as I could afford.
Telephoto lenses can do some amazing photography of the sky. The problem is the sky moving as you expose. The rule is 600 divided by the focal length in mm of the lens is the maximum amount of exposure time you can expose before the stars start to trail and become little lines instead of pinpoint stars. 600 divided by a 50 mm lens is 12. So 12 seconds is the maximum length of your exposure with that 50 mm lens. Camera on a tripod naturally. Good luck.