# Coma

Also called: coma aberration.

A flaw of Newtonian mirrors that turns stars near the edge of the field into little comets; worse in fast telescopes.

A parabolic mirror is perfect at the centre of the field, but away from it stars smear into fan shapes pointing outward. The smear grows with distance from the centre and with the inverse square of the [focal ratio](https://astropilot.space/glossary/focal-ratio): at f/4.5 a star 10′ from the centre is smeared by about 6″; at f/8, less than 2″.

It's an optical property of the mirror (not a fault, and not to do with comets): a [coma corrector](https://astropilot.space/glossary/coma-corrector) removes it. Bad [collimation](https://astropilot.space/glossary/collimation) makes it worse and puts it off-centre.

**In AstroPilot:** With a coma corrector on a fast Dobsonian, object pages show how much coma it removes at the edge of your eyepiece's field.

**Related:** [Coma corrector](https://astropilot.space/glossary/coma-corrector.md), [Focal ratio](https://astropilot.space/glossary/focal-ratio.md), [Dobsonian](https://astropilot.space/glossary/dobsonian.md), [Collimation](https://astropilot.space/glossary/collimation.md)

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