The Gregorian Telescope is an interesting design of reflector and it was the first practical design for a telescope that used a mirror. It is still used in modern times but only in very limited circumstances.
Credit for the first reflector goes to an Italian professor named Niccolo Zucchi. He made his first scope in 1616 but the design of his scopes was not very practical. James Gregory was a Scottish Mathematician and Astronomer of the 17th Century and he is credited with having designed the first practical reflector telescope. In 1663 he published this design in his book titled Optica Promota. But he didn't build the first working model until ten years later with the help of scientist Robert Hooke. It was in this ten year period that Sir Isaac Newton built his famous Newtonian telescope (1670). So the Gregorian telescope predates the Newtonian in design but Newton's was the first to be built.
How the Gregorian Telescope works
To understand how the Gregorian reflector works we will first take a look at the Newtonian telescope. In the Newtonian a parabolic mirror placed at the bottom of a tube and it focuses light back up the tube to a flat mirror that directs it out the side of the tube to an eyepiece. The eyepiece is on the side of the tube. In a Gregorian design the parabolic mirror is at the bottom of the tube and it focuses light back up the tube but the second mirror is an ellipsoid and it redirects the light back down the tube to the eyepiece through a hole in the center of the large primary mirror. This type of telescope has the eyepiece at the bottom.
The benefits and shortcomings
Newton's telescope has one parabolic mirror and one flat mirror while Gregory's has a parabolic as its primary mirror and an ellipsoid as its secondary. Additionally, Gregory's primary mirror needed a hole cut in the center of it. These things mean it is a much more optically complex instrument and much more difficult to make. Newton's design is much easier to make. This is probably why it took Gregory ten years to make his first working model.
The Gregorian does have some benefits over the Newtonian style scope. The ellipsoid secondary mirror is placed at a point after the focus of the light. This configuration, in conjunction with an eyepiece, provides an erect or right side up image while the Newtonian telescope gives an upside down image. This is a big benefit if you are using the telescope for terrestrial viewing. And because of this inversion of image after the secondary mirror it allows a baffle to be placed inside the tube. This baffle prevents unwanted light and heat from reaching the primary mirror. This is a very useful tool when a telescope is meant to be used for solar observations where heat is a big concern. For this reason the Gregorian design is sometimes still used for telescopes that will be used for solar observations.
The Gregorian design for a telescope is not used very often in modern times. It has been changed and much improved by the Cassegrain telescope. But the Gregorian does find limited use occasionally as a finder scope that accompanies a larger scope. This is because of the nature of it displaying properly oriented erect images. And because of its ability to have a baffle inside it is also sometimes used for telescopes built for solar observations.
Gregory's telescope design is not much used in modern times but it was the forerunner and inspiration for many of today's telescope designs.
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