I recently purchased the Canon 200mm EF 2.8 L series lens based on some very positive reviews. Not only is an excellent telephoto for daylight photography being sharp and possessing fast silent autofocus, it also makes an excellent astrograph. Because of rain and cloud I haven't been able to use it much, but here are my first tests:
http://www.pbase.com/terrylovejoy/gallery/200mm
From my observations of the Eta Carina image the lens displays excellent resolution across the field wide open, with some minor distortion of stars at the edges (from what I have seen even the best lenses show this to some extent). There is also some slight decentering from left to right (although this could be my 300D).
I currently use the 300D + 200mm lens and the 350D + 100mm lens to search for comets (and soon nova). With the 100mm lens I can reach stars around mag 15 or even fainter when conditions are good, with the the 200mm lens mag 17 might be possible so perhaps supernova hunting is feasible.
Terry