The Gregorian Telescope

Fig 1: James Gregory

The Gregorian Telescope is basically a reflector telescope invented and designed by well known 17th century mathematician and astronomer James Gregory (1638-1675). He was born in Scotland and was a contemporary of Sir Isaac Newton. Both the scientists worked together on different projects. However, Gregory had proposed the design of his reflector telescope in 1663 in a publication called ‘Optica Promota’ before Newton built his first reflector telescope in 1668. But Gregory did not build his telescope practically; it was just a hypothesis then. Finally, in 1673, after 5 years of the building of Newton’s Telescope, this Gregorian Telescope got its physical appearance. However, it was the first practical telescope design in history and as a credit, the design of the scope is named after the inventor, James Gregory.

Fig 2: Gregorian Telescope

After publishing his theoretical design, Gregory tried multiple times to build it. As he did not have practical skills and did not find any optician to help him, he ultimately failed to build one. Later, Robert Hooke, an experimental scientist showed his interest to work on it and finally successfully built it in 1673. Moreover, Scottish optician James Short made Gregorian Telescope with highly reflective parabolic mirrors.

The Gregorian design was a very successful optical instrument to view the night sky and was widely used at that time due to its high performance for more than a century. Though by time, the design was improved by several scientists later. Nowadays the use of this telescope is rare for some reasons, yet there are some special fields, where it is used still now.

The Gregorian design used basically two mirrors. The primary mirror was used as the main light-gathering mirror. The other mirror was the secondary mirror which was located beyond the focal point of the primary mirror. The light reflected from the primary mirror to the secondary mirror and then went out of the telescope along a small hole in the primary mirror.

The pathways of lights passing through the Gregorian telescope

Fig 3: The pathways of lights passing through the Gregorian telescope.

In the figure, we can see that, as light travels through the telescope, it at first enters the tube at A and then reflects by the primary mirror (D). The primary mirror used here is a concave parabloid. So the light is reflected by the primary mirror and the light travels to the secondary mirror (B). This mirror B is a concave ellipsoid and it is located at the focal point (C) of the primary Mirror (D). This arrangement allows the image of the telescope located on the right side up which is advantageous for sky observation. This is an advanced version of Newton’s Telescope as the Newtonian telescope made the inverted images. 

Limitations and Disadvantages of Gregorian Telescope

  1. James Gregory used two concave mirrors in his reflector telescope which is more complicated to make compared to the Newtonian Telescope. Newton used only one concave mirror and a flat mirror.
  2. According to the Gregorian design, the primary mirror must have a hole at the center. But making a hole in a mirror and exactly at the center is a tough job.
  3. The Gregorian design makes the telescope larger because the secondary mirror is beyond the focal point of the primary mirror.

Advantages of Gregorian Telescope

  1. According to the Gregorian design, the image is focused before the secondary mirror, there, we can place a stop in the tube, which will block the extra light from reaching the secondary mirror. This system makes the views of the objects that are seen by the telescope clearer. Besides, this also restricts the heat transmitted. This is very useful in solar observations and experiments. Even now, this design and setup are used in many modern telescopes.
  2. We know that the refractor telescope has some optical problems like chromatic and spherical aberration. The Gregorian telescopes do not cause these problems.
  3. The Gregorian design offers a unique upright image which is why still, in some secondary or smaller sighting telescopes, this design is used.

However, nowadays, the Gregorian design is replaced by Cassegrain Telescopes, which provide higher performance, though it has a lot of similarities.

Modern Observatories that use the Gregorian Design

You will find the Gregorian telescopes in Mount Graham International Observatory In Arizona if you look through the Vatican Advanced Technology Telescope and the Large Binocular Telescope. This design is used also in many radio telescopes like Green Bank Telescope which is kept at the NRO in West virginia. In Puerto Rico, the well-known Arecibo Radio Telescope is also a Gregorian telescope.

Modern Day Improvement on the Gregorian

The optical system of the Gregorian design has been advanced a lot nowadays to overcome the limitations. Instead of the previous system, experimental scientists implemented the Compact Gregorian Reflector. This is an advanced optics system that significantly reduces the telescope’s size.