Past Treasure of the Month – June 2011

Gregorian Telescope, ca. 1760

The Rosenberg Library was pleased to share an 18th century Gregorian telescope, one of the firsts of its kind, as the June 2011, Treasure of the Month. This telescope, purchased as part of the Mary Moody Northen Maritime History Collection, is of a design that was first published by Scottish mathematician James Gregory in 1663.


The Gregorian telescope is an early type of reflecting telescope which uses a combination of curved mirrors to reflect light to form an image. Reflecting telescopes were first produced in the mid 17th century as alternatives to refracting telescopes, the earliest types of optical telescopes that use lenses to form images. Refracting telescopes produce images that are subject to many types of optical distortion, including color distortions and imperfections in image shape. Because their lenses cannot be practically made larger than 1 meter in diameter, reflecting telescopes are used. In fact, all major telescopes used in astronomical research are reflecting telescopes.

James Gregory’s design, the first reflecting telescope plan, appeared in his 1663 publication Optica Promata. His early attempts to build the telescope were not successful because, according to his own confession, he had no practical skill nor could find a capable optician to construct his design. However, the telescope did gain the interest of the scientific community, including that of Robert Hooke, who successfully built the telescope in 1673. Unfortunately, by the time the Hooke produced the Gregorian telescope, Isaac Newton had constructed his own reflecting telescope, the Newtonian design.

Telescope assembled

Today, the Gregorian telescope is a rare specimen, and modern astronomers, seldom encounter the full design. Gregorian optics, however, are still used in large radio telescopes, or telescopes that operate on radio frequencies with dish antennas. One of the best examples of a radio telescope with Gregorian optics is the Arecibo Observatory in Puerto Rico. This telescope, built between 1960 and 1963, was the first in history to directly image an asteroid. It’s been used in experiments for extraterrestrial communication and in the discovery of neuron stars.

As part of the Mary Moody Northen Maritime History Collection, this particular telescope was acquired in 1980 along with a collection of maps, sea charts, books, manuscripts, and navigational instruments related to navigation in the Gulf of Mexico and Caribbean waters. Consisting of five components, the Gregorian telescope includes an oak tongue and groove case, a brass tripod and mount, an extending brass lens, and a black brass and pasteboard cover.