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Focal ratio
Not sure if I understand this correctly so I thought I'd check... is the focal ratio just the focal length divided by the diameter of the primary mirror or lens, i.e. if my dob has a focal length of 1200mm and the primary mirror's diameter is 200mm does that mean that the f-ratio is f/6, or is there something I'm missing? Similarly, would the f-ratio of my 70mm, 900mm focal length refractor be f/12 (which doesn't seem right to me)?
Cheers
Wade
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