A Short History Of Reflector Telescopes

Reflector telescopes, that is those that use mirrors rather than glass lenses to gather and focus light, have been around since the latter half of the 17th Century.

The idea behind this type of telescope actually had its origins in the first half of the 17th century when Marin Mersenne, a French mathematician and philosopher, theorized in 1636 that the aberration of colours that occurs in refracting telescopes could be avoided by the use of two paraboloidal mirrors. He believed that such a telescope would deliver much more accurate colour representation since mirrors, unlike glass lenses, focus all levels of the colour spectrum at the same point. Persuaded by fellow philosopher Rene Descartes that the idea would never work, Mersenne never put his theory into practice.

In 1663, Scottish mathematician James Gregory built a telescope that used two concave mirrors--the primary one slightly hyperboloid in configuration and the secondary one more ellipsoidal in configuration. Light is gathered by the primary mirror, which reflects it onto the second mirror and then back again through a tiny hole in the primary mirror to the eyepiece. Although Gregory's design is not unlike that used today for many reflector scopes, the one he built delivered a relatively disappointing image because of the low quality mirrors available at that time.

Robert Hooke in 1668 used multiple mirrors to build a significantly shorter telescope than had previously been the norm, and four years later Laurent Cassegrain improved on Gregory's design by substituting a convex mirror as his secondary.

The first truly successful reflector telescope was developed by Isaac Newton in 1668. Its three key components were a concave spherical primary mirror two inches in diameter; a flat, angled secondary mirror; and a convex eyepiece lens. Although Newton's design was by far the best that had been offered by anyone up to that date, the quality of the mirrors available in the late 17th century still failed to deliver images as high in quality as its designer had envisioned.

Reflector telescope technology took a major step forward in 1730 with the invention of the first parabolic and elliptic mirror by Scotsman James Short. Short's mirror virtually eliminated all distortions, making it ideal for use in the reflector. The popularity of reflector telescopes grow sharply in the late 18th century when William Herschel developed a 40-foot scope using a mirror that was four feet in diameter. The use of such large-scale mirrors enable the gathering of vast quantities of light, far more than could possibly be gathered using a telescope of refractor design.

Although the quality of reflector telescopes has steadily improved with the availability of higher quality mirrors, the basic design of these scopes is not significantly different today than it was in the 18th century.