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Gregorian telescope() The first compound reflecting telescope to be devised, designed by the 17th-century Scottish mathematician James Gregory. It has a small concave secondary mirror that is mounted beyond the focal plane and reflects the light back through a central hole in the paraboloid primary mirror (see illustration). The design, published in 1663 in Gregory's book Optica promota , requires a secondary mirror of ellipsoid figure. It has a small field limited mainly by coma. In the aplanatic Gregorian, which uses an ellipsoidal primary, coma and spherical aberration are both eliminated but the field is limited by astigmatism. Because of the imperfect grinding techniques available to him during the early 1660s, Gregory was unable to build the telescope he designed before Newton and Cassegarin produced their reflectors in the following decade. The more compact Cassegrain configuration, similar to the Gregorian design, is usually preferred.
a reflecting system consisting of a concave paraboloidal primary mirror and a concave ellipsoidal secondary mirror. One focus of the latter is made coincident with the focus of the parabaloid, and the image of the celestial body is constructed in the second. The Gregorian telescope produces an erect image that is free of aberration on the optical axis. The design of the telescope was proposed by J. Gregory in 1663.