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Andy
According to info on a few DSLR Astrophotography sites ( I like Jerry Lodriguss Astropix ) the Canon 450D has an optimal dynamic range at ISO 400 or ISO 800
So try both settings with same exposure time but capture plenty of data and try not to be tempted to push subs any longer than 3 minutes
Together with dark calibration you will reduce your read noise
Good luck with it all !!
Martin
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