OK, first things first, make sure that "original" below right is active so that you see the images at the size I uploaded. The above are reduced full-frames to demonstrate that even though there has been little complaint as to the 20D being "noisy", the later two cameras, the 40D and the 500D are even less noisy. The 20D even demonstrates some kind of glow from the lower edge of the frame, something that I have never even noticed in actual use. One should keep in mind the extreme nature of these dark frames. We normally shoot subexposures to get the skyfog to be somewhere between 10% and 30% of the back-of-camera histogram. Even at a very dark site one hardly ever needs to shoot 20-minute subs, except if one is using an OTA with a very slow focal ratio, say, f10 or slower. Or else one is shooting narrowband, also at a very dark site, with an OTA slower than, say, f7. The images above do demonstrate that if you wish to shoot such loooong subs with a 20D, you'd better wish that the ambient temperature is at least some 10degC cooler than used for these dark frames :-) My previous experience had led me to be happy doing astrophotography with a 20D at temperatures below 25deg C. The 40D is able to do a decent job with typical subs a few minutes long even at a temperature of 30deg C. It does look like the 500D will be even more tolerant of higher ambient temperatures. Probably more so than myself ;-)

NB IMPORTANT! In order to keep the mirror up throughout an astro series of long exposures, i.e. to minimse mirror bounce, it has often been the practice to switch recent Canon DSLRs to LiveView before initiating the subexposure series. This way the mirror simply stays up and out of the way for the next couple of hours or however long you have programmed the series. Unfortunately if you do so with the 500D you will see a glow on the left side of the frames, similar to Amp Glow in older Canons. Solution? Do NOT use LiveView before initiating your subs. Normally the mirror bounce is small enough that in a long exposure you can ignore it completely. Nevertheless problems may arise if you are shooting with a very long focal length OTA, like an SCT, at a focal length of 2000 to 4000mm. In such a situation simply activate mirror lock-up and the self-timer, like we all used to do before LiveView hit the market. This works well with the timer/controller TC-80N3 or similar shutter activation. Your frames will then have no glow on the right side, like the clean frame shown above.

Note the megapixel climb showing itself in the display magnifications for the above cameras, 1:9 for the 20D and 1:12 for the 500D.