@Marcel is correct, use manual mode, I have tried using the camera and telescope together, I found the moon very bright, so try it at f11 or higher.
Telescopes are fixed aperture lenses, so are mirror lenses. So all that you have to adjust the light is ISO and the exposure.
Yes, you are right. My slip.Not to put words into aroy's post, but I believe that he means "shutter speed" instead of "exposure".
WM