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.
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Related terms
- Light-gathering power
How much light an instrument collects compared with the eye; it grows as the square of the aperture. Against a 7 mm pupil, a 200 mm telescope gathers (200 ÷ 7)² ≈ 800 times more light. That is what brings galaxies invisible to the naked eye into view.
- Resolving power
The smallest separation between two stars that an instrument can show as distinct, in arcseconds. It depends only on the aperture: about 1.2″ for 100 mm and 0.6″ for 200 mm. Seeing often prevents you from reaching it.
- Focal ratio
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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