In Astro photograhy it is better to use a manual SLR than the more modern electronic ones. This is simply that during long exposures, the shutter is held open, and in an electronic SLR this is done using batteries. You will go through a lot of batteries in a very short space of time.

Manual SLR's are becoming harder to come by, and I only know of the rebadged Pentax, the Centon K100 available from Jessops as the only manual SLR you can buy new. I use a secondhand Pentax ME Super

This has a built in light meter, with fully automatic expoure control, the ability to overide via manual exposure, 125x for flash photography and the ever important Bulb setting for extended exposure.

The shutter can be fired with a lockable cable release. I use the stock 50mm Pentax lens as well as a Sigma 70mm - 210mm zoom lens for piggyback photography.