Gregorian telescope
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The Gregorian telescope is a type of reflecting telescope designed by Scottish mathematician and astronomer, James Gregory in the 17th century.
He described the design in his 1663 publication Optica Promota (The Advance of Optics). Early attempts to build a Gregorian telescope failed, and it wasn't until ten years later, aided by the interest of experimental scientist Robert Hooke, that a working instrument was actually constructed. Gregory's design pre-dates the familiar form of reflector which Sir Isaac Newton first designed and made around 1670.
The Gregorian telescope consists of two concave mirrors; the primary mirror (a concave paraboloid) collects the light and brings it to a focus at the secondary mirror (a concave ellipsoid) where it is reflected back through a hole in the centre of the primary, and thence out the bottom end of the instrument where it can be viewed with the aid of the eyepiece. This design of telescope renders an upright image, making it useful for terrestrial observations.
The Gregorian design has the advantage over Newton's design, in that it requires a shorter tube for the same focal length. The design was largely superseded by the Cassegrain telescope, and is not commonly in use today, other than for spotting scopes.
