Gary,
I have heard the 135 f2.8 SF has a lot CA (colour fringing).
I am a bit weary of zooms as there seems to be more variability in quality (problably due to complexity) and the often show more CA. I have seem very nice results with the 70-200 lenses (2.8 and 4, and the Sigma 70-200 2.8). If you can test first then that would be ideal. You should consider the following fixed focals for astronomical use:
50 f1.8 or 50 f1.4 (I would go for the 1.4 as although its a lot more expensive has a distance scale and is better built).
85f1.8, 100 f2 and 100 f2.8 USM Macro - All these are great lenses for astrophotography.
200 f2.8 L - A great lens that can be got new for 950$ from the states. Excellent even wide open. My favourite lens.
The zooms are often not good, but the 70-200 2.8 and 4 lenses are if you can get a good copy. You may have to close down a stop, although the f4 lens may be OK.
Here are some tests you might be interested in:
http://www.pbase.com/terrylovejoy/lens_tests
http://www.pbase.com/lumodomo/lens_tests (some of the images are processed too dark at the corners making it hard to tell ).
Terry