Gregorian Telescope

Gregorian Telescope is one type of reflecting instrument, the others being due to Newton, Cassegrain, and Herschel. In the Gregorian instrument rays of light are reflected back from the large concave mirror, and after passing through the focus are again reversed in direction by a small concave mirror that brings them to a focus a short distance behind the great mirror, in the centre of which a small hole is cut to let them pass. An eye-piece is adjusted to receive them from this second focus and transmit them to the eye of the observer. The general disadvantages of this type of instrument are referred to in the article on telescopes.