# Aperture

The diameter of a telescope's main lens or mirror — the single most important number.

Aperture decides how much light a telescope collects ([light grasp](https://astropilot.space/glossary/light-grasp)) and how fine a detail it can resolve ([resolution](https://astropilot.space/glossary/resolution)). Doubling it collects four times the light and resolves details half the size. An 8-inch (203 mm) telescope shows hundreds of galaxies; a 16-inch shows spiral arms in dozens of them.

**In AstroPilot:** Every visibility rating in AstroPilot depends on your instrument's aperture.

**Related:** [Light grasp](https://astropilot.space/glossary/light-grasp.md), [Resolution](https://astropilot.space/glossary/resolution.md), [Focal ratio](https://astropilot.space/glossary/focal-ratio.md), [Limiting magnitude](https://astropilot.space/glossary/limiting-magnitude.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/aperture