For night sky imagery under a clear sky, manually focus on a bright star using "live view". This definitely works with a 450D and any lens below F5.6 - I do this quite often, being an astrophotographer and using a 450D.

Put the camera on 800 ISO and an exposure time set (in M) of at least 10 seconds. Put the lens on 'Manual". Put the focus ring near the infinity mark, locate a bright star in teh sky, aim and put the star on the LCD screen, center it and and then press the screen magnification to 10x. Then focus more accurately untill the star is pinpoint. Try to find a star that is not too bright (and not too faint).

For night sky imagery under a clear sky, manually focus on a bright star using "live view". This definitely works with a 450D and any lens below F5.6 — I do this quite often, being an astrophotographer and using a 450D.

Put the camera on 800 ISO and an exposure time set (in M) of at least 10 seconds. Put the lens on 'Manual". Put the focus ring near the infinity mark, locate a bright star in the sky, aim and put the star on the LCD screen, center it, and then press the screen magnification to 10x. Then focus more accurately until the star is a pinpoint. Try to find a star that is not too bright (and not too faint).