I'm looking to buy a wide-angle lens for astrophotography, specifically on a Canon 450D (Rebel XSi). Which characteristics should I be looking for in a lens?
1) The faster aperture the better. You probably want 1.4 if possible, especially on a crop camera. However, the widest 1.4 I know of is a 24, which isn't all that wide on a crop. 2.8 can work, but not as well.
2) It should have low coma. Distant bright spots should remain that - spots. Most lenses add a bit of a "tail" to them.
3) Ideally it should have a hard focusing stop at infinity. Usually you have to use live view to focus stars. I'm not absolutely certain but think some Zeiss and Rokinon lenses have this. (I have the rokinon 24 f/1.4 and 14 f/2.8 but haven't had a chance to use them for astrophotography yet. They are supposed to be good though.)
Really though this is one aspect where full frame will get you a real advantage. Then you can use a 24 f/1.4 with appropriate wideness, plus the high ISO shooting will be infinitely better than on the Rebel.