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.
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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.
- Limiting magnitude
The brightness of the faintest stars you can see. With the naked eye it ranges from 3 or 4 in a city to 6.5 or more under a very dark sky. A telescope of aperture D (in mm) pushes it further by about 5 log₁₀(D ÷ 7) magnitudes: a 200 mm telescope reaches roughly magnitude 14.
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