The focal length divided by the aperture, written f/5: a 200 mm telescope with a 1,000 mm focal length works at f/5. A “fast” instrument (f/4 to f/6) gives wide fields and short photo exposures; a “slow” one (f/10 to f/15) suits the planets. For the eye, at the same magnification, it does not change how bright the image looks.
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Related terms
- Aperture
The clear diameter of the main lens or mirror, in millimetres: the single most important figure of a telescope. The larger it is, the more light it gathers and the finer the detail it can show. A 200 mm telescope collects four times as much light as a 100 mm one.
- Focal length
The distance between the main lens (or mirror) and the point where it brings a star to focus, in millimetres. A long focal length gives a larger image and a narrower field. A 1,200 mm telescope magnifies 60 times with a 20 mm eyepiece.
- Coma
A flaw of parabolic mirrors: away from the centre of the field, stars take the shape of a little comet, tail pointing outwards. It grows quickly as the focal ratio drops: faint at f/8, obvious at f/4. Poor collimation shifts it and makes it visible in the centre.
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