A few suggestions on achiving focus while in prime focus mode I found at: http://www.covingtoninnovations.com/dslr/EOS300Dastro.html Try adding diffraction by putting a piece of window screen in front of the lens. It will produce bright spikes on all the stars, and you can focus the stars so that the spikes are brightest. Try a Hartmann mask (a lenscap with two big holes in it) in front of the lens while focusing. It will make out-of-focus stars appear double. The holes need to be big to admit enough light. You may be able to get some help from focus confirmation if you are using a Canon lens that is f/6 or faster. This is a way of getting the autofocus mechanism of the camera to tell you whether it thinks the picture is in focus. To try it, bring a bright star into the very center of the frame (the autofocus box). Hold the shutter button halfway down and adjust the focus. There will be a red blink, just like the autofocus signal, when the focus is correct. Hope this helps. Jim