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 was not 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.

Diagram of the lightpath through a Gregorian telescope.

The Gregorian telescope consists of two concave mirrors; the primary mirror (a concave paraboloid) collects the light and brings it to a focus before 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, which brings the light to a focus after the secondary mirror, in that it requires a shorter tube for the same focal length. The design was largely superseded by the Cassegrain telescope. It is still used for some spotting scopes because this design creates an erect image without the need for prisms.

In the Gregorian design, the primary mirror creates an actual image before the secondary mirror. This allows for a field stop to be placed at this location, so that the light from outside the field of view does not reach the secondary mirror. This is a major advantage for solar telescopes, where a field stop (Gregorian stop) can reduce the amount of heat reaching the secondary mirror and subsequent optical components. The Solar Optical Telescope on the Hinode satellite is one example of this design.

For amateur telescope makers the Gregorian can be less difficult to fabricate than a Cassegrain because the concave secondary is Foucault testable like the primary, which is not the case with the Cassegrain's convex secondary.