# When was the first true Gregorian telescope built?

I know that James Gregory came up with the idea of using a parabolic mirror to eliminate spherical aberration in 1663.

The theory behind it had actually been around since classical antiquity I think.

However, no one could make a parabolic mirror at the time, so eventually, Newton made his own design and utilized a spherical one.

I am trying to figure out how long it took before a Gregorian telescope was actually made.

Most sources say that Hooke made one in 1673.

However, I am reading this souce which says:

The first person to actually create a parabolic mirror was another English mathematician, John Hadley. In 1721, he built a Gregorian reflector whose mirror had very little spherical aberration.

## 1 Answer

There is a confusion in your question. ALL reflecting telescopes ideally should have parabolic mirrors. The problem is that a parabolic mirror is difficult to make. So they were made with spherical or approximately parabolic mirrors.

All this has nothing to do with the difference between Newtonian, Gregorian and Cassegrain telescopes. The distinction between these three is in the optical scheme.

Newtonian uses one concave and one flat mirror (or just one concave).

Gregorian uses two concave mirrors, and

Cassegrain uses one concave and one convex mirror.

All these mirrors should be parabolic in an ideal situation. First Gregorian was made by Hooke in 1673.

This was 5 years before John Hadley was born. Hadley made a parabolic mirror in 1721, for a Newtonian telescope btw.

The principle on which the parabolic mirror works was indeed known in antiquity. However they did not know in antiquity how to make good mirrors, neither flat, not parabolic nor spherical (at least no evidence survived that they did know. The legend of Archimedes mirrors burning ships at a distance appears at a much later period. This is not possible even with modern mirrors). This was a problem even in Newton's time, what metal to use. Good mirrors with silver coating on glass were produced much later. They also did not know in antiquity that one can use this principle to make telescopes. (Or, if they knew, this knowledge did not survive).

EDIT. History of (optical) telescopes is explained in great detail in the book Telescopes, by Gerard P. Kuiper and Barbara M. Middlehurst, U. Chicago Press, 1961. Very interesting reading. Includes the history of glass making and mirror polishing technology.

• So when Hooke made a Gregorian, he used a spherical mirror?
– Luke
Mar 12, 2015 at 22:28
• Probably. Otherwise Hadley would not be credited with this 48 years later. As I understand, Hadley's and other mirrors of 18 and 19 centuries were only approximately parabolic. More or less close to parabolic. There is no simple mechanical process to make a mirror exactly parabolic. Mar 13, 2015 at 2:02
• That would explain why the Gregorian built by Hooke didn't seem to be very popular. The only real advantage it would have over a Newtonian at that point is that it let you stand behind the telescope, but I would imagine it would be much harder to build.
– Luke
Mar 13, 2015 at 13:13
• Please feel free to add an answer to In a reflecting telescope (Cassegrain), is there a difference between using a concave or convex secondary mirror? in Astronomy SE, or to comment on or correct mine, which is basically just snippets from Wikipedia.
– uhoh
May 19, 2021 at 3:38
• – uhoh
May 19, 2021 at 3:49