Andrew,
I guess three methods spring to mind (for me) to help focus.
If you have a laptop PC and the right cables etc use DSLRFocus.
If you don't, maybe look into getting a focus magnifier for your camera. I have an Olympus Vari-magni Finder which magnifies the view finder by 1.25x or 2.5x. It is designed for the OM-1 but I gather it fits Canon 300D and 350D viewfinders as well... it might fit a 400D? I'm sure there would be similar gadjets made specifically for the 400D.
The last suggestion would be to use focus lock. I assume the 400D has focus lock? Use auto focus to focus on a bright star, set the focus lock, and hopefully it will be accurate enough as long as you don't change camera settings or zoom.
I'm sure there are others with better advice
. I don;t have a DSLR so I am
not speaking from experience here!
Al.