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RE: Chooseing The Proper Focal Length

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Posted by Jack McQuaide on September 5, 2000 03:09:25 UTC


There really is no "proper" focal length. You can make the focal ratio (f/5=40", f/6=48", f/7=56" etc) to just about whatever you desire. Take into consideration, that the higher ratios will make the length longer, and a larger scope. The lower ratios will make a shorter scope, but are more difficult to finish the mirror. The most common would be an F/6 to F/8 mirror. The longer focal ratios lend themselves towards planetary scopes, while shorter ratios are for deep space objects.


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