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f/ratio is given by dividing the focal length of the telescope by its apeture.
So as an example, for a 203mm apeture scope with a focal length of 1000mm, the result is:
1000 / 203 = f/4.9 (rounded off to f/5).
It is exactlly the same calculation that gives the f/ratio in a camera. Make the diaphram smaller ( the apeture ) you increase the f/number.
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