Re zoom lenses for astrophotography - Have a look at this review on the 70-200 f2.8 IS by Jerry Lodriguss:
http://www.astropix.com/HTML/I_ASTRO...TS/C70_200.HTM
This is meant to be one of the best zooms you can buy. Keep in mind this with a Canon 1D mk2 with relatively large pixels so the artifacts would be more prominent with a 20D/30D/350D and especially a 400D.
What is worrying is that although Jerry has more weight with Canon than most of us he was told the lens was within spec
by Canon. Having said that I am sure I have seen better results with that lens elsewhere (maybe it was the non-IS version).
Anyway, if you want fast optics that are truly sharp corner to corner check out the Takahashi E180 ( 500mm f2.8 - although its really about f3.2-3.5 with secondary mirror obstruction):
http://www.galaxies.com/darrellccd.aspx
Terry