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Tony, the 20D has very low noise and in fact the noise performance, at even the highest ISO, is probably far better than that of a prosumer digicam, so the shorter you can make your exposures (less tracking error), or the more light you can gather in your current 30 second exposure the better. If you're doing really long guided exposures then the lower the ISO the better. Unless the noise is noticeable don't worry about it. BTW many sharpening algorithms actually add noise to achieve their effect.
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