Sigma lenses are very good, but you might consider that a Canon lens with image stablising will be a better lens in the long run. I find that my best lens which is a 80-400mm VR Nikon gives me the best images. A mate of mine who does great terrestrial photography has all Canon lens for his 20D. The images that are produced are simply breath taking. You can find his stuff at this link.
http://timtam.deviantart.com/gallery/
That aside the lens you have highlighted is a macro lens and not designed for what you want. You might want something line this: Canon EF 28-135mm f3.5-5.6 IS USM. Which is around $821 Australian and designed for what you want. The 28mm is reasonably wide although something like this might also suit: Canon EF-S 17-85 f4-5.6 IS USM. The 17mm is wider still. In case you don't know the smaller the number in mm the wider the angle that you get.
Anyway don't get the one you asked about unless you want to do macro shots. I have a macro 105mm and it does not focus well past 600mm. So that is not what you want for astrophotography.
Hope this helps.