What is a fast telescope?

BBC Sky at Night Magazine's Scope Doctor Steve Richards solves your practical astronomy ailments.

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What is a fast telescope?

Steve says: “The expression ’fast’ has its roots in photography. To explain it, imagine two identical cameras fitted with lenses of the same focal length, but one lens has a narrower aperture than the other.

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The camera with the wider aperture lens needs shorter exposures to capture the same amount of light, so the lens is ‘faster’. The same is true for telescopes.

This ‘speed’ attribute is called focal ratio, and is determined by dividing the telescope’s focal length by its aperture.

For example, a scope with a 1,000mm focal length and 200mm aperture has a focal ratio of 5, normally written as f/5.

Telescopes with focal ratios below f/7 are generally known as fast, while those above f/9 are slow.”

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Email your astronomy queries to Steve at scopedoctor@skyatnightmagazine.com and it could get answered in a future issue of BBC Sky at Night Magazine.