Gregorian telescope

Lore & Background

The Gregorian telescope is named after the James Gregory design, which appeared in his 1663 publication Optica Promota (The Advance of Optics). Similar theoretical designs have been found in the writings of Bonaventura Cavalieri (Lo Specchio Ustorio (On Burning Mirrors), 1632) and Marin Mersenne (L'harmonie universalle, 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.

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. 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The design was largely superseded by the Cassegrain telescope as equivalent focal length is greater than the physical focal length which provides better illumination uniformity. 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 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.

Reader's Guide

The Gregorian telescope holds significance as an early reflecting telescope design that pre-dates the Newtonian telescope, though it was not successfully built until five years after Newton's first reflecting telescope. Its design solved the problem of viewing the image in a reflector by allowing the observer to stand behind the primary mirror, and it renders an erect image, making it useful for terrestrial observations. The design also works as a telephoto lens with its tube much shorter than the system's actual focal length. Although largely superseded by the Cassegrain telescope due to better illumination uniformity, the Gregorian is still used for some spotting scopes because it creates an erect image without prisms. Its legacy includes continued use in large telescopes: the Steward Observatory Mirror Lab has been making mirrors for large Gregorian telescopes at least since 1985. Examples of radio telescopes employing off-axis Gregorian optics include MeerKAT, the Green Bank Telescope, the Arecibo Observatory, and the Allen Telescope Array. Optical telescopes using Gregorian optics include the Vatican Advanced Technology Telescope, the Magellan telescopes, and the Large Binocular Telescope. The Giant Magellan Telescope will also use Gregorian optics, as does the NSF's Daniel K. Inouye Solar Telescope. 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.

Did You Know?

• James Gregory never actually constructed the telescope; it was first built by Robert Hooke in 1673.
• The Gregorian design pre-dates the Newtonian telescope, published in 1663, five years before Newton's first reflecting telescope.
• The Solar Optical Telescope on the Hinode satellite uses a Gregorian design with a field stop to reduce heat.

Origins and the Shadow of Newton

The concept of the Gregorian reflecting telescope traces back to a 1663 treatise called Optica Promota, authored by the Scottish mathematician and astronomer James Gregory. Though Gregory's description was not the very first of its kind—earlier theoretical sketches appeared in the work of Bonaventura Cavalieri in 1632 and Marin Mersenne in 1636—his formulation became the one that would eventually be realized in metal and glass. The critical weakness of Gregory's contribution was purely practical: he lacked the hands-on skill to build his own instrument and could not locate an optician willing or able to do it for him. His design therefore remained on paper while his contemporary Isaac Newton, working in parallel on similar optical problems, produced a functioning reflecting telescope in 1668. It was not until 1673, when the experimental scientist Robert Hooke took up the challenge, that a working Gregorian instrument finally existed. Later, the Scottish optician James Short crafted Gregorian telescopes featuring parabolic mirrors fashioned from speculum metal, bringing the design into the realm of everyday astronomical observation.

Anatomy of the Optical Path

At its core, the Gregorian telescope relies on a pair of concave mirrors working in concert. The primary mirror, shaped as a paraboloid, gathers incoming light and drives it toward a focal point that falls in front of the secondary mirror, which is shaped as an ellipsoid. Rather than sending the light out the side of the tube as a Newtonian design would, the secondary bounces the converging beam back through a small aperture at the center of the primary. The observer then views the image through an eyepiece positioned at the rear of the instrument, standing directly behind the primary mirror. This arrangement produces an erect image, a feature that makes the design particularly well suited to terrestrial observation. Additionally, the folded light path gives the telescope a telephoto character: the physical tube is considerably shorter than the system's true focal length. The primary also forms a real image at a point before the secondary, creating a natural location where a field stop can be inserted to block stray light from outside the intended field of view.

Practical Strengths and Niche Applications

Although the Cassegrain configuration has largely overtaken the Gregorian for many professional applications—partly because its longer effective focal length relative to tube length yields superior illumination uniformity—the Gregorian retains several distinct practical advantages. Its ability to deliver an upright image without the addition of prisms makes it a natural choice for spotting scopes and other terrestrial instruments. In solar astronomy, the real-image point formed by the primary before the secondary provides an ideal spot for a Gregorian stop, a field stop that intercepts excess heat before it can reach the secondary mirror and downstream optics. The Solar Optical Telescope aboard the Hinode satellite exploits precisely this arrangement. For amateur builders, the Gregorian offers a fabrication advantage over the Cassegrain: because its secondary mirror is concave, it can be tested using the same Foucault knife-edge method applied to the primary, whereas the Cassegrain's convex secondary resists that straightforward approach. The Steward Observatory Mirror Lab has been producing mirrors for large Gregorian instruments at least since 1985, attesting to the design's continued relevance.

From Radio Dishes to Giant Magellan

The Gregorian optical layout has found a remarkably broad range of modern implementations, spanning both optical and radio astronomy. Among the most prominent radio telescopes employing off-axis Gregorian optics are the MeerKAT array, the Green Bank Telescope, the Arecibo Observatory, and the Allen Telescope Array. In the optical domain, the Vatican Advanced Technology Telescope, the twin Magellan telescopes, and the Large Binocular Telescope all rely on Gregorian configurations. The upcoming Giant Magellan Telescope will likewise adopt Gregorian optics, ensuring the design's presence in the next generation of extremely large ground-based observatories. The NSF's Daniel K. Inouye Solar Telescope also utilizes this architecture. Smaller but historically significant examples include the James Gregory Telescope housed at the University of St. Andrews and the Gregor telescope at the Teide Observatory. Together, these instruments demonstrate that a design first sketched in a 17th-century Scottish treatise continues to underpin some of the most powerful observational tools in modern science.