A mirror has substantially less spherical aberration than a lens of equivalent focal length. In addition, the spherical aberration of a concave mirror is opposite in sign to that of a positive lens. Therefore, several researchers (Maksutov 1944; Bowers 1950) have proposed the use of a weak negative lens in conjunction with a concave mirror. Schmidt (Hodges 1953) proposed a weak aspheric corrector at the center of curvature of a spherical mirror.