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Reflections on Glass: Telescope Mirrors

All About Glass

I contrived heretofore, a perspective by Reflexion.
      — Sir Isaac Newton, c. 1668

The refracting telescope gave astronomers their first up-close glimpses of the heavens. Then, it began to frustrate them. At higher magnifications, the instrument’s glass lenses produced distorted images. Pioneering British scientist Isaac Newton solved the problem by using a metal mirror to gather light. He built his reflecting telescope in 1668.

The more astronomers saw, the more they wanted to see. Reflecting telescopes became larger—until metal mirrors, too, reached their limitations.

Astronomy might have reached a dead end if glass mirrors hadn’t entered the picture. A glass mirror holds its focus better than a metal one because glass is less distorted by temperature changes. Glass doesn’t corrode and it’s easier to form into a precise shape. When French physicist Léon Foucault built the first large reflecting telescope with a glass mirror in 1864, pioneered the strategy for designing the major telescopes of the 20th century and beyond.

Pushing the limits

200-inch Pyrex® mirror blank, Palomar Observatory

A century ago, some astronomers thought the universe is just one big galaxy, the Milky Way. Others thought it contains many. In 1906, astronomer George Hale set out to build a telescope that could gather enough light to settle the debate.

Hale had already built the world’s biggest reflecting telescope. Its 60-inch (1.5-meter) mirror was made from the largest piece of glass that had ever been cast. Still, it wasn’t big enough. Making a larger one would be difficult, but that didn’t stop Hale. After four painful years of pushing glassmakers to their limit, he had a mirror blank nearly double in size. When astronomer Edwin Hubble used Hale’s new 100-inch (2.5-meter) telescope to study the heavens, the debate was settled. Our galaxy is just one among many.

Telescopes that refract light

Telescopes that refract light

A refracting telescope uses lenses to form an image. The objective lens gathers and focuses light from a distant object, forming an inverted image. The eyepiece lens then magnifies the image. Some astronomical telescopes give upside down images, but this orientation doesn’t matter for observing stars or planets.

Telescopes that reflect light

Telescopes that reflect light

A reflecting telescope uses a curved mirror as a light bucket to collect light from a distant object. In almost all reflectors, like the Hale, a second, smaller mirror then sends the light to a photographic plate or to an eyepiece lens to create an image for viewing.



View astronomical images from the Palomar Observatory, all photographed with the 200-inch Hale Telescope.

The Corning Museum of Glass
This article was originally published in Innovations in Glass, 1999, pp. 30–31.

Published on October 25, 2011