Hi jjj,
Like Jason, I'd recommend the EF35mm f/2 lens. My copy is fairly sharp even at f/2, and I use it both for astro and daytime use (e.g. kids portraits, for which it gives great results). I owned a nifty 50 briefly, but the 35 f/2 is a far better lens in my opinion. It's currently $372 delivered from DWI:
http://www.dwidigitalcameras.com.au/...p?idProduct=57
Hope this is useful.
Cheers,
Stephen