In a word: don't!
This question has been asked a number of times and been replied to by jjjnettie who has had one for quite some time and she has really persevered with it. I have seen the results and they are dismal to say the least. One of, if not the noisiest DSLR for astrowork that Canon has brought out. Personally I don't think Canon have brought out a consumer or prosumer DSLR that is really suitable for astro work since the 40D. Even thought the pixel size is a bit small (5.7 micron) for deepsky its a 14 bit camera which is a big bonus. Alternatively consider a 30D with a slightly bigger pixel (6.4) though only 12 bit as previous Canons
But that's just my opinion.