All About Glass

You are here

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 real glimpse 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, turned out to be limited.

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, he ushered in the age of modern astronomy.

Pushing the limits

200-inch Pyrex® mirror blank, Palomar Observatory

Some astronomers thought the universe was one grand galaxy. Others thought it contained 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. In astronomical refractors, both the large objective lens and the eyepiece lens are convex. The objective lens focuses light from a distant object, forming an inverted image. The eyepiece lens then magnifies the image. Astronomers see the images upside down, but the orientation doesn’t matter to them.

Telescopes that reflect light

Telescopes that reflect light

A reflecting telescope uses a curved mirror to form an image. The mirror acts as a light bucket to collect light from a distant object. In some reflectors, like the Hale, a smaller mirror captures the image and inverts it, sending it to a photographic plate or to an eyepiece lens that magnifies it.




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

Published on October 25, 2011