Images through the eyepiece of my telescope look great. When I hook up my DSLR with a T-ring and T-adapter I can't get a clear picture. What’s wrong?

It's more than likely a focusing issue.

In-focus images take work, especially if you are conditioned by autofocus technology in conventional photography. 

Focusing is always a challenge through long lenses and even more so with most SLRs.  Digital SLRs or DSLRs - unlike many film SLRs - lack focusing aids, instead relying on autofocus, which is at its weakest with long focal lengths. So when used with a telescope in place of the lens, a typical DSLR is difficult to accurately focus.

There are tools available to make it a bit easier. Use a focusing magnifier.  Also consider getting another body with the live-focus feature, which allows you to see a magnified image direct from the imaging chip with the camera’s post viewer as you focus.

I
f you are doing astrophotography and letting a software package such as MaxDSLR  control your camera, you can analyze focus with the software and even use the program to control an electric focuser on your telescope.

You can also use techniques based on optical testing. Diffraction or apodizing masks will give your stars pointy diffraction spikes, which become more prominent as you get close to focus. A Scheiner-Hartmann mask is simply a tube cap with two round holes in it that gives a double image for out-of-focus stars. The images coalesce at focus, making it easier for some people to achieve focus. Finally, knife edge and Ronchi focusers use distinctive patterns near and at focus as focusing aids, but require removal of the camera to focus.

Updated 12/27/13