Autofocus doesn't work when you're taking an astronomical image; you must focus the camera yourself. There are so many techniques for focusing a DSLR that I've devoted a whole chapter to it (Chapter 8, p. 89).
Obviously, you can focus a DSLR by looking into the eyepiece, just like a film SLR. Make sure the eyepiece diopter is adjusted so that you see the focusing screen clearly; your eyes may require a different setting at night than in the noonday sun.
But you can usually do better by confirming the focus electronically. The easiest way to do this is to take a short test exposure (such as 5 seconds for a star field) and view it enlarged on the camera's LCD screen. Repeat this procedure until the focus is as good as you can get it. If your camera offers magnified live focusing, that's even better.
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Although we usually tend to think of the digital camera as the best thing since sliced bread, there are both pros and cons with its use. Nothing is available on the market that does not have both a good and a bad side, but the key is to weigh the good against the bad in order to come up with the best of both worlds.