Rose and Wan showed that to reduce forms of this phenomenon, a quadrupole-octupole corrector must be spherically symmetric. Coefficients named for this phenomenon must always be positive in spherically symmetric, static electromagnetic fields, according to Scherzer’s theorem. A large amount (10[1])of this phenomenon is indicated by a Strehl ratio that is close to zero. George Ritchey and Henri Chrétien (“cray-tee-YAHN’s”) improved on a design by Laurent Cassegrain (“cahss-GRAN”) to reduce the amount of this phenomenon caused by mirrors. The resolution of electron microscopes is effectively limited by a form of this phenomenon, which occurs when the paraxial approximation no longer holds. A form of this phenomenon occurs when rays do not meet at the focal point of a lens. For 10 points, what phenomenon has “optical” and “spherical” versions, and causes images to appear distorted? ■END■