Gregorian Telescope

Gregorian telescope [grə′gȯr·ē·ən ′tel·ə‚skōp]

(optics)

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.

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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.