Relationships have been obtained that determine the position of the entrance pupil of an optical system made from two reflective spherical surfaces for which the third-order coma equals zero, depending on the central screening over the pupil diameter. A basis is thus provided for achieving aplanatic correction of the aberrations by bringing one of the surfaces of a plane-parallel plate into coincidence with the plane of the entrance pupil and imparting a quasi-planoidal shape to it, and the sequence is given for designing it. It is shown that planastigmatic correction of the aberrations is possible.
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