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Hi Gareth,
The maximum aperture you have available is specific to the lens. In general, the more you pay for a lens the wider it's maximum aperture. However, for astro work it often pays to close the aperture a few stops from wide open. You collect less light, but it more than pays off in image quality.
Also, higher iso will give you greater sensitivity so more light collected per unit time, but at the cost of higher noise in the image. The best iso to use is usually either 400 or 800. But it will depend on the temperature of your cameras sensor at the time.
Adam
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