Gregorian telescope

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Ray path in Gregorian telescopeclick for a larger image
Ray path in Gregorian telescope

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.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Gregorian Telescope


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.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

Gregorian telescope

[grə′gȯr·ē·ən ′tel·ə‚skōp]
A reflecting telescope having a paraboloidal mirror with a hole in the center and a small secondary (concave ellipsoidal) mirror placed beyond the focus of the primary mirror; light is reflected to the secondary mirror and back to an eyepiece at the hole; the telescope produces an erect image but a small field of view.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.