Hi Jason,
I haven't been on IIS for ages but I was a devotee of the 400mm f5.6 lens - great for astro and daytime work as stated.
links to a couple of astro images taken with this lens and a modded/cooled 40D...
http://s327.photobucket.com/user/dou...ml?sort=4&o=33
http://s327.photobucket.com/user/dou...ml?sort=4&o=62
Cheers
Doug