# Central obstruction

Also called: secondary mirror, obstruction.

The secondary mirror blocking the middle of a reflector's or SCT's aperture, which slightly lowers contrast.

Newtonians, SCTs and Maksutovs have a small mirror in front of the main one. It blocks some light (typically 15–35% of the diameter) and spreads a little more of a star's light into the diffraction rings, slightly softening fine planetary contrast compared with a refractor of the same aperture. At low power with a big [exit pupil](https://astropilot.space/glossary/exit-pupil) its shadow can appear as a dark spot in daylight.

**Related:** [Reflector](https://astropilot.space/glossary/reflector.md), [Schmidt–Cassegrain](https://astropilot.space/glossary/schmidt-cassegrain.md), [Airy disk](https://astropilot.space/glossary/airy-disk.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/central-obstruction