# Collimation

Aligning a telescope's mirrors so they share one axis — essential for sharp views in reflectors.

Bumps and temperature changes nudge a reflector's mirrors out of line, making stars flare into comet shapes at high power. Adjusting the secondary and primary mirror screws, with a collimation cap, Cheshire eyepiece or laser, brings them back. Fast mirrors (f/4–f/5) need it most precisely.

**At the eyepiece:** Check collimation every session on a Dobsonian; it takes a minute once you've learned it.

**Related:** [Reflector](https://astropilot.space/glossary/reflector.md), [Airy disk](https://astropilot.space/glossary/airy-disk.md), [Coma](https://astropilot.space/glossary/coma.md), [Focal ratio](https://astropilot.space/glossary/focal-ratio.md)

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