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Three-mirror anastigmat is a class of reflector-telescope designs consisting of those with three mirrors chosen so as to reduce some common types of aberration beyond what two mirrors can do, the term pointing out that astigmatism is one of the aberrations addressed. The original such design has a concave parabolic primary mirror, a convex spherical secondary, and a concave spherical tertiary. It was developed in the early 20th century, and various three-mirror anastigmat designs have since been developed using other combinations of three mirrors, the design types including Korsch telescopes, Paul-Baker telescopes, and Eisenberg-Pearson Telescopes. Example three-mirror anastigmat telescopes: