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View from a Scope | October 20, 2017

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Focal Ratio (f)

October 7, 2012 |

The focal ratio is the relationship between the diameter of the main optic (aperture) and the focal length. Lets use a Skywatcher Explorer 130 for the next example, the 130 signifies the aperture of the telescope and the length of this telescope is 650 so our focal ratio would be 650/130 = 5, so our f number would be f/5.

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