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.