Gregorian telescope
An early type of reflecting telescope, designed by the Scottish mathematician and astronomer James Gregory (1638-1675), that uses two curved mirrors: a concave paraboloidal primary and a concave ellipsoidal secondary. The primary collects and brings light to a focus, while the secondary, positioned a little way beyond the primary's focal plane, reflects the beam, diverging from the focus, back through a hole in the center of the primary and out the bottom end of the instrument. Although the Gregorian is free from chromatic and spherical aberration, it requires a long telescope tube. It was rendered obsolete by the Cassegrain telescope.
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