Coma Corrector

In a reflecting telescope, coma makes stars at the edge of the field look like blurry comets whose tails point out radially from the center of the field. All parabolic mirrors used in Newtonian reflecting telescopes are limited in field sharpness due to coma. The faster your telescope's focal ratio, the worse the effect. Even a perfectly made 13" f/4.5 parabola has a diffraction limited field that’s only about 0.1° across.

Coma correctors are auxiliary lens systems that reduce the coma at the edges of the field to provide images that are sharp across the entire eyepiece or photographic field. You no longer have to constantly shift your scope to keep objects centered for sharp viewing.