A reflecting telescope with a concave paraboloidal primary mirror and a concave ellipsoidal secondary mirror. The secondary mirror reflects light to a focus behind the primary, which must therefore have a hole in it. The advantage of the system compared with the Cassegrain is that the image is upright, but the drawback is that the Gregorian has a longer tube. Gregorians, proposed in 1663 by J. Gregory, were popular in the 18th century but are now rarely seen.
Subjects: Astronomy and Astrophysics.