Refractors
Reflectors

How a Telescope Works

Into the Looking Glass

HET mirrors

The segmented primary mirror of the Hobby-Eberly Telescope reflects the daytime sky in this aerial view. [Martin Harris/McDonald Observatory]

At its most basic, a telescope is a few pieces of glass held together with a metal tube or framework. It gathers more light than the eye alone and brings it to a focus.

Yet this simple concept has a profound impact on our knowledge of the universe around us. It allows us to see deep into the universe, glimpsing the bits of cosmic flotsam that formed the first galaxies. It shows us the surfaces of the other worlds in our solar system, and allows us to detect worlds orbiting other stars. It helps us analyze the composition of the stars and decipher their life stories.

These magic glasses come in two basic varieties: refractors, which gather light with a series of lenses, and reflectors, which use mirrors. The largest telescopes can detect objects that are millions of times fainter than the eye can see. They produce stunning images of the universe, or direct their light to scientific instruments that measure the light’s intensity or split it into its component colors.

These pages will tell you more about how telescopes work, and provide details on three instruments that play key roles in Fritz Benedict’s studies of stars and planets: the Hobby-Eberly Telescope and Harlan J. Smith Telescope at McDonald Observatory, and the space-based Hubble Space Telescope.