Hi Roger. Canon have a number of 50,, lenses. The standard one is f1.8 (~ $135) and stopped down a bit it makes a great astrolens for the price. They then have the f1.4 (~$600-$800) which is brilliant apparently and then the f1.2 ( ~
) which is also very good as well
I concur about the zooms though the 17-40 is f/4 and by all acounts is a cracker of a lens. My biggest issue with zoom lenses is preventing the zoom ring from moving accidentally when focusing. Personally of the few lenses I've used I much prefer a fixed length lens.
Oh and Mick, the canon 85mm f/1.2 is supposed to be the pick of the crop but at around $2500 -$3000 you'd hope it would be. A comparison review of the f/1.2 and f/1.8 (not astro though) can be found here
http://www.wlcastleman.com/equip/reviews/85mm/index.htm