If you visualize the light cone from a star coming out of the rear of your telescope you want to have the film plane at the point where the light comes together to a point. So find a bright star and if you take a spacer that is exactly the depth of your camera from where it attaches to the telescope to were the film is (celestron sells one of these I think they call it a focal tester) and put a sharp knife edge there when you move the star image across the knife the image will dissapear all at once when it is in focus, if you are not in focus the image will appear to go out from one side or the other slowly because the knife edge is too far up or down the light cone rather than at the point where the cone is at a point focus. I myself bought one of the tools from Celestron that I mentioned earlier but you can do the same thing by opening the rear of your camera and using a rubber band or something to hold a razor blade on the rails that the film runs accross. If I can help anymore feel free to E-mail me at starman12@hotmail.com : How do you do knife-edge focusing I can not take proper pictures with my telescope. : help please