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 its shadow can appear as a dark spot in daylight.