History of the Newtonian Reflector

By Tim Trott, 6th April 2014 in Astronomy

The Newtonian Reflector was invented by Sir Isaac Newton in 1672 and was the first to feature the revolutionary 45° mirror.

When the telescope was introduced to England by Thomas Harriot in 1609 it transformed classical science. For the first time, people could see the lunar mountains, Jupiter's moons, Saturn's rings and countless wonders in the Milky Way.

Although these telescopes were a breakthrough, the limitations imposed by glass making at the time limited telescopes to single lens, no greater than 3 inches diameter with focal lengths around 18m. They had tiny fields of view, dim aberrated images, and low magnification.

Woodcut illustration of a 45 m (148 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.
Woodcut illustration of a 45 m (148 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.

For this reason, astronomers started experimenting with mirrors as an alternative. In 1614, Niccolo Zucchi experimented with concave mirrors and concave eyeglasses in search of better images than could be obtained with refractors. By the 1660s James Gregory in Scotland and Robert Hooke in London were also trying out mirrors, but it was Isaac Newton's homemade presented to the Royal Society in 1672 that constituted the real breakthrough.

 

A replica of Newton's second reflecting telescope, which he presented to the Royal Society in 1672.
A replica of Newton's second reflecting telescope, which he presented to the Royal Society in 1672.
Stimulated by the works of Gregory and Hooke, Newton set about his own research and produced a 1.5 inch tin and copper allow mirror in 1669 and devised a more efficient optical configuration to use it. Instead of tilting the mirror, or using secondary concave curved mirrors that required several optical surfaces to create an image, Newton had a brilliant idea. Why not intercept the light reflected from the concave mirror just short of focus with a secondary flat mirror set at 45°. Light could then exit the tube via an eyepiece at the side. Newton's primary mirror only had a focal length of 165mm, but with its impressive ball joint mounts and a delicate screw focuser, it performed brilliantly. The Newtonian reflector was born.

Newton's reflector had another enormous advantage, that being there were no chromatic aberrations. Without chromatic aberrations you get an image free from false colour and by brining all the rays of light into a single focus, it can give sharper results. Chromatic aberrations occur as different wavelengths of light travel at different speeds through glass - hence the rainbow from a prism.

By 1721, the design had not changed, it just got bigger. John Hadley demonstrated a Newtonian reflector with a 6-inch mirror and 6 ft. focal length and boasted a 200x magnification. By 1846, William Lassell discovers Neptune's moon Triton using a home-made editorially mounted 24-inch Newtonian reflector.

Patrick More and his 12.5
Patrick More and his 12.5" Newtonian Reflector

More recently, Sir Patrick Moore used his 12.5-inch Newtonian reflector to complete his iconic lunar cartographic work.

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