Comatic aberration

Comatic aberration (or coma), in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components in which rays of light, striking the objective away from the optical axis are not brought to focus in the same image plane. For example, stars appearing distorted, appearing to have a tail (coma) like a comet.

Specifically, the comatic aberration 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.