How Telescopes Reveal the Distant Universe

For almost the whole of human history, the study of the heavens was limited to what the unaided eye could see. A few thousand stars, the wandering planets, the Moon and the occasional comet made up the entire visible universe. The invention of the telescope changed this situation completely. By gathering and focusing light, a telescope allows astronomers to see objects far too faint and too distant for the eye alone, and in doing so it has repeatedly transformed our picture of the cosmos. Understanding how a telescope works explains both its power and the reasons astronomers keep building ever larger ones.

The most important task of a telescope is not, as many people assume, to magnify. Its chief purpose is to collect light. The eye has only a small opening through which light can enter, so it captures very little of the faint glow arriving from a distant star. A telescope uses a large lens or mirror to gather far more of that light and bring it to a single focus, producing a bright image where the eye would see nothing at all. The larger the lens or mirror, the more light it can collect, and this is the single most important measure of a telescope's usefulness.

There are two main ways of gathering light, and they divide telescopes into two families. A refracting telescope uses a lens at the front to bend the incoming light and bring it to a focus, much as a magnifying glass does. A reflecting telescope, by contrast, uses a curved mirror at the back to catch the light and reflect it to a focus. Large lenses are heavy, sag under their own weight and are difficult to make flawless, so almost all of the great research telescopes built in modern times are reflectors, whose mirrors can be made far larger and supported from behind.

Besides collecting more light, a large telescope offers a second advantage: it can reveal finer detail. The ability to distinguish two objects that lie close together in the sky is limited by the size of the opening through which the light passes. A bigger mirror can, in principle, separate objects that a smaller one would blur into a single point. This is another reason astronomers are always seeking greater size, since a larger instrument promises both a brighter image and a sharper one.

In practice, however, telescopes on the ground face a serious obstacle: the atmosphere itself. The same moving air that makes stars appear to twinkle also blurs and unsteadies the images seen through a telescope. To reduce this problem, major observatories are built on high mountains, above much of the atmosphere, and in dry places where the air is calm and clear. Even so, the restless air sets a limit on what any ground-based instrument can achieve, however large and well made it may be.

The most powerful solution to this problem has been to place telescopes above the atmosphere altogether. A telescope carried into orbit looks out at the universe through no air at all, and so can produce images of extraordinary sharpness. Freed from the blurring and the glow of the atmosphere, space telescopes have returned views of distant galaxies and faint, remote objects that no instrument on the ground could match. Their cost and complexity are great, but the clarity they offer has made them among the most valuable tools in all of science.

Astronomers have also learned that visible light is only a narrow slice of what the universe sends out. Many objects give off radiation the eye cannot detect, such as radio waves, infrared and X-rays, each carrying its own kind of information. Special telescopes have been built to capture these invisible signals, revealing cold clouds where stars are forming, the fierce surroundings of collapsed stars, and other phenomena that would remain entirely hidden to ordinary sight. By opening these new windows, astronomers have found that the familiar starlit sky is only a small part of a far richer whole.

Every advance in telescope design has widened the horizon of the known universe, from the first small instruments that revealed the moons of distant planets to the giant mirrors and orbiting observatories of today. Each improvement in the ability to gather light and resolve detail has brought fainter and more distant objects into view, and with them new questions. The telescope remains, four centuries after its invention, the single instrument most responsible for our expanding knowledge of what lies beyond the Earth.