I don't know about this particular Canon lens .. but I did some study of other (older and today much cheaper) Canon lenses and some results of those investigations are posted below:
http://www.iceinspace.com.au/forum/s...ad.php?t=58629
http://www.iceinspace.com.au/forum/s...ad.php?t=39293
http://www.iceinspace.com.au/forum/s...ad.php?t=51866
Neither of those lenses work for astrophotography at full aperture (and for 200mm external aperture mask must be used). The 100mm one works best with it's own iris, but shows significant CA at corners - however this is not a problem as it is asily correctable by suitable software ( PT lens for example).
Christian Buil has done a lot of tests on Canon lenses, have a look here:
http://astrosurf.com/buil/
Also, you may consider Tair-11A, (Zenit lens) it is very sharp (but stepped down to f3.5 ~ f4) especially in the centre (at the corners it shows a bit of coma)
The other good one may be Telezenitar APO 135 F2.8.
All lenses above could be found for less that $250.. 10x cheaper but not THAT much worse
