The Gregorian telescope is a type of reflecting telescope designed by Scottish mathematician and astronomer James Gregory in the 17th century, and first built in 1673 by Robert Hooke. James Gregory was a contemporary of Isaac Newton, both often worked simultaneously on similar projects, Gregorys design was published in 1663 and pre-dates the first practical reflecting telescope, the Newtonian telescope, built by Sir Isaac Newton in 1668. However, Gregorys design was only a description and he never actually constructed the telescope. It was not successfully built until five years after Newtons first reflecting telescope, the Gregorian telescope is named after the James Gregory design which appeared in his 1663 publication Optica Promota. Similar theoretical designs have found in the writings of Bonaventura Cavalieri. Gregorys early attempts to build the telescope failed, since he had no practical skill himself and it was not until ten years after Gregorys publication, aided by the interest of experimental scientist Robert Hooke, that a working instrument was created. The early Scottish optician and telescope maker James Short built Gregorian telescopes with mirrors made from the highly reflective speculum metal. The Gregorian design solved the problem of viewing the image in a reflector by allowing the observer to stand behind the primary mirror and this design of telescope renders an upright image, making it useful for terrestrial observations. It also works as a telephoto in that the tube is shorter than the systems actual focal length. The design was superseded by the Cassegrain telescope. It is still used for some spotting scopes because this design creates an image without the need for prisms. The Steward Observatory Mirror Lab has been making mirrors for large Gregorian telescopes at least since 1985, in the Gregorian design, the primary mirror creates an actual image before the secondary mirror. This allows for a stop to be placed at this location. This is an advantage for solar telescopes, where a field stop can reduce the amount of heat reaching the secondary mirror. The Solar Optical Telescope on the Hinode satellite is one example of this design, the MeerKAT, the Green Bank Telescope, the Arecibo Observatory, and the Allen Telescope Array are all radio telescopes employing off-axis Gregorian optics. The Vatican Advanced Technology Telescope, the Magellan telescopes, and the Large Binocular Telescope use Gregorian optics, the Giant Magellan Telescope will also use Gregorian optics. List of telescope types Gregor telescope at the Teide Observatory, James Gregory is the inventor of the Gregorian telescope. James Gregory Telescope at the university of St. Andrews, Large Binocular Telescope James Short Gregorian reflecting telescope, Who was James Gregory, National Museums Scotland
Image: Gregorian telescope circa 1735 in Putnam Gallery, 2009 11 24
Image: Gregorian telescope circa 1735 in Putnam Gallery 2, 2009 11 24
Image: Lanature 1873 telescope gregory
Diagram of the lightpath through a Gregorian telescope.