I do agree with Wavytone...
However, there are cheap alternatives - try some of old, M42 lenses.. for Zenit (russian made).
My Tair-11A (135mm, F2.8, stopped down to F4) is way better than any Canon lens I've had in my hands for testing.
Also, have a look at this post:
http://www.iceinspace.com.au/forum/s...4&postcount=13
There are some links to what I have done in the past, resulting in lenses that are good enough for astrophotography, but not too expensive.