Coma (optics)

Coma of a single lens. Each cone of light focuses on different planes along the optical axis.
Coma of a single lens. Each cone of light focuses on different planes along the optical axis.

In optics (especially telescopes), the coma (/ˈkoʊmə/), or comatic aberration, in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components that results in off-axis point sources such as stars appearing distorted, appearing to have a tail (coma) like a comet. Specifically, coma is defined as a variation in magnification over the entrance pupil. In refractive or diffractive optical systems, especially those imaging a wide spectral range, coma can be a function of wavelength, in which case it is a form of chromatic aberration.