The Gregorian telescope is a type of

reflecting telescope A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...

astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...

James Gregory in the 17th century, and first built in 1673 by Robert Hooke. James Gregory was a contemporary of

Isaac Newton Sir Isaac Newton () was an English polymath active as a mathematician, physicist, astronomer, alchemist, theologian, and author. Newton was a key figure in the Scientific Revolution and the Age of Enlightenment, Enlightenment that followed ...

, and both often worked simultaneously on similar projects. Gregory's design was published in 1663 and pre-dates the first practical reflecting telescope, the Newtonian telescope, built by Sir Isaac Newton in 1668."Isaac Newton: adventurer in thought", by Alfred Rupert Hall
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However, Gregory's design was only a theoretical description, and he never actually constructed the telescope. It was not successfully built until five years after Newton's first reflecting telescope.

History

The Gregorian telescope is named after the James Gregory design, which appeared in his 1663 publication (The Advance of Optics). Similar theoretical designs have been found in the writings of Bonaventura Cavalieri ( (On Burning Mirrors), 1632) and

Marin Mersenne Marin Mersenne, OM (also known as Marinus Mersennus or ''le Père'' Mersenne; ; 8 September 1588 – 1 September 1648) was a French polymath whose works touched a wide variety of fields. He is perhaps best known today among mathematicians for ...

(, 1636). Gregory's early attempts to build the telescope failed, since he had no practical skill himself and could find no optician capable of actually constructing one. It was not until ten years after Gregory's 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 parabolic mirrors made from the highly reflective speculum metal.

Design

The Gregorian telescope consists of two concave

mirror A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...

s: the primary mirror (a concave

paraboloid In geometry, a paraboloid is a quadric surface that has exactly one axial symmetry, axis of symmetry and no central symmetry, center of symmetry. The term "paraboloid" is derived from parabola, which refers to a conic section that has a similar p ...

) collects the light and brings it to a focus ''before'' the secondary mirror (a concave

ellipsoid An ellipsoid is a surface that can be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface; that is, a Surface (mathemat ...

), 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 An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as Optical telescope, telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks thro ...

. The Gregorian design solved the problem of viewing the image in a reflector by allowing the observer to stand behind the primary mirror. This design of telescope renders an erect image, making it useful for terrestrial observations. It also works as a telephoto lens with its tube much shorter than the system's actual

focal length The focal length of an Optics, optical system is a measure of how strongly the system converges or diverges light; it is the Multiplicative inverse, inverse of the system's optical power. A positive focal length indicates that a system Converge ...

. 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. 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 a real image before the secondary mirror. This allows for a

field stop Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the infrastructure of an airport * Battlefield * Lawn, an area of mowed grass * Meadow, a gras ...

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.

Gallery

File:Gregorian telescope circa 1735 in Putnam Gallery, 2009-11-24.jpg, A Gregorian telescope circa 1735 File:Gregorian telescope circa 1735 in Putnam Gallery 2, 2009-11-24.jpg, Side view Image:Lanature1873 telescope gregory.png, 1873 diagram of a Gregorian telescope

Examples

*The

MeerKAT The meerkat (''Suricata suricatta'') or suricate is a small mongoose found in southern Africa. It is characterised by a broad head, large eyes, a pointed snout, long legs, a thin tapering tail, and a brindled coat pattern. The head-and-body ...

, the Green Bank Telescope, the

Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...

, 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. *The NSF's Daniel K. Inouye Solar Telescope

References

External links