Definition: Optical system of relatively large field and very high speed. * It is based on a concave spherical mirror with an aspheric plate (Schmidt plate) located at the mirror's centre of curvature. * Coma and astigmatism are highly corrected because the aspherical plate correcting spherical aberration and the aperture stop are at the centre of curvature of the mirror. * Widely used in astronomy: hence ~ camera, used in astrophotography, from 1931.

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