If you consider the maximum acceptable ISO on the A200 to be one stop lower(IF
) than the 400d(ISO400 vs 800) that would mean that the 400d can gather double the light, all other things being equal, without a drop in quality. I suppose it depends on how much noise you consider to be ok in your shots.
You could always lower the ISO and increase the length of the exposure, but from my limited experience in astrophotography, stars move during the exposure although not by much. How obvious the movement is, is dependent on your focal length and shutter speed.
Unless you have one of the awesome motorised mounts that Bob has!