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F/15, F/9, F/6, F/4

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Posted by Kip Crawford on April 6, 2001 06:20:11 UTC

Basically any telescope with a fast focus like f/4.5 to about f/6 is desireable for shorter exposure. Over all focus for anything is f/6 to f/9. Refractors (or SCT's) that have longer focals lengths from f/9 to f/15 are better suited for long term exposures of deep sky and planetary work. Depending on what you really are going to use the scope for may help you decide which focal length is more suited for you. Basically from my experiences a refractor or reflector that has a focal ratio of f/6, f/7 and f/8 is the best all around ratio's for both deep sky and planetary astrophotograpy or not. Refractors tend to have a sharper image on planets, however a good collimated scope with good optics can do the same.

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