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Glossary > Telescopes > Optics > Aperture ratio

Glossary | Telescopes | Optics | Aperture ratio

Is the ratio of the aperture to the focal length of a telescope.

The focal ratio

The focal ratio of a telescope is the relationship between the telescope aperture and the focal length. A telescope with a 100 mm aperture and a focal length of 1000 mm has a focal ratio of f/10. The focal length is 10 times greater than the aperture.