Minimal astigmatism surfaces.
It has been pointed out that the meridional object and image foci as well as the meridional center of curvature will lie on the same straight line if the condition Delta (n cos(2)i/t) = Delta (n/t) is satisfied. The surfaces fulfilling this condition do not introduce astigmatism; hence they have been called minimal astigmatism surfaces. The generalized aplanatic points are defined and discussed, and the conditions for the incidence angle, which make it possible to replace the aspheric surfaces by spherical ones, are considered. A general method for the calculation of the minimal astigmatism surfaces is described.
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