Think you know who invented the reflecting telescope?

By Mel Bartels, January 2014

The problem with Isaac Newton’s 1672 telescope is that he got the design wrong, didn’t build a good model and didn’t disclose his technique for decades. He wrote that his design of a telescope with a spherical primary and elliptical plane mirror was fully adequate. But it was not, as we now know.

A telescope of his design of four foot focus was brought to the Royal Society’s attention shortly after and was found wanting. The truth is that the spherical aberration present in his design rendered it unusable. He did not know how to make parabolic mirrors. Isaac Newton’s model received much attention but ended up sitting on the shelf making not the slightest impact for half a century.

Prior to Newton’s design, John Gregory devised an all mirror design in 1663 with a perforated parabolic mirror and a smaller curved mirror at the front of the tube sending light back down the tube. But the parabolic curves were far too difficult for opticians of his era and attempts to build the telescope failed.

In the same year as Newton’s telescope, 1672, Sier Guillaume Cassegrain, a sculptor (note the connection between his skill in casting metal sculptures and casting speculum mirrors), wrote of his design for a reflecting telescope, where Gregory’s concave secondary mirror is replaced with a convex mirror. Today we judge Newton’s invention as likely earlier that Cassegrain’s, though it was hypocritical of Newton to criticize Cassegrain for using spherical mirrors. Regardless, the inventions of Newton and Cassegrain went nowhere.

These designs languished partly because neither Newton nor Cassegrain gave information on how to cast the mirror or how to grind and polish the mirror, though Newton did describe his polishing technique thirty years later.

Fifty years later, in 1722, John Hadley unveiled the first reflecting telescope worthy of the name. Hadley had figured out how to parabolize his speculum mirrors. His telescope was a six inch f/10 on a solid altazimuth mount with slow motion controls. He had eyepieces that gave powers up to 230x.

His telescope was tested against a refractor of 123 feet focal length and found to be its equal in resolution though somewhat dimmer. With John Hadley’s telescope, observers could see the five moons of Saturn, Cassini’s Division and even Saturn’s ring shadow on the planet itself.  Unlike his predecessors, Hadley shared his polishing and parabolizing technique. His method of testing has been used ever since, particularly in modern times by John Dobson.

Thomas Edison is considered the inventor of the light bulb not because he was first to demonstrate an incandescent light but because he developed the first practical and commercially viable electric light bulb. Karl Benz invented the automobile because he created the first true car though Leonardo da Vinci created the first designs. Galileo invented the telescope because he was first to grasp its astronomical potential and develop telescopes of increasing power even though Lippershey was first to publish and is accorded prime inventor status.

Similarly, John Hadley should be considered the inventor of the reflecting telescope. It is more proper to speak of  looking through a Hadleyian telescope than a Newtonian.

The result of Hadley’s invention cannot be understated. Once he showed his telescope and shared how to build it with others, opticians began making parabolic primary mirrors and the era of the reflector truly began.


Louis Bell’s The Telescope, McGraw-Hill, 1922

Henry King’s The History of the Telescope, Charles Griffin and Co, 1955

John Hadley biography               

Hadley’s Reflector

Scott Berkun’s The Myths of Innovation, O’Reilly, 2007