Interesting info on Canon lenses can be found here:
http://www.canon-europe.com/Support/...ork_iii_en.asp
However, my personal approach to the "problem" of ideal astro lens would be quite different..... simply because I can not possibly afford what Cannon is offering (apart from 400D or whatever body of course). Besides, for astro work AF, IS etc etc... are not needed.
The only essential things the astro lens must have are good, flat MTF curve (that means sharpness at reasonable aperture, to have shorter exposure times), not too much geometrical distortions (however this is not important for "pretty picture" at all, it may be important only if someone wants to do positional astrometry but I doubt anyone on this forum really wants to do this) and reasonable CA, which usually shows most severely at the image corners.
There are many alternatives out there for 1/10 of a Canon price or even lower, while results are the same (or almost the same), so all those Canon k$ are impossible to justify to my ministry of finance :-)