# Airy disk

Also called: diffraction, diffraction rings.

The tiny disk, surrounded by faint rings, that a perfect telescope makes of a star — set by its aperture.

No telescope makes a star a true point: diffraction spreads its light into a small disk with faint rings, which you can see at high power on a steady night. The bigger the aperture, the smaller the disk. Defocusing a star slightly and comparing both sides of focus is a classic test of optics and [collimation](https://astropilot.space/glossary/collimation).

**Related:** [Resolution](https://astropilot.space/glossary/resolution.md), [Collimation](https://astropilot.space/glossary/collimation.md), [Seeing](https://astropilot.space/glossary/seeing.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/airy-disk