NASA - National Aeronautics and Space Administration

Telescopes help us see things that are too far away to be seen with our eyes alone. They also give us a closer view of distant objects. A common telescope looks like a long tube, with one end smaller than the other. You put the small end up to one eye and look through the tube. The things you see look closer than they really are.

Image to left: A large reflecting telescope like this one helps astronomers study the planets, stars, and other things in space. Credit: World Book illustration

Two small telescopes can be joined side by side to make binoculars. You can look through binoculars with both eyes. People at a sports contest sometimes use binoculars to get a closer look at the action.

Scientists called astronomers use large telescopes to study the planets, stars, and other things in space. Without telescopes, we would know little about such things.

The telescopes we see most often are called optical telescopes. These telescopes can "see" light, just as our eyes do. Other kinds of telescopes can "see" things that our eyes cannot see, such as radio waves and X rays. Astronomers use such telescopes to study objects in space that give off invisible rays. For example, they use radio telescopes to study objects that give off radio waves.

Telescopes have different sizes and shapes. Some telescopes are small enough to hold in one hand. Huge bowl-shaped radio telescopes may measure up to 1,000 feet (305 meters) across. This is almost as long as three football fields.

Some telescopes are parts of spacecraft. The Hubble Space Telescope, launched in 1990, explores the sky while it circles the Earth in space. Scientists on Earth control this telescope with radio signals. The signals point the telescope at objects the scientists want to study. This telescope gets a clear view of things in the sky because it travels above the air that surrounds the Earth. Although air seems clear, it makes stars and other sky objects look blurry. For example, stars seem to twinkle because their light comes to our eyes through moving layers of air. So, while telescopes on Earth's surface get a blurry image, the Hubble Space Telescope gets a sharp image.

How Optical Telescopes Work

We see objects because light given off or reflected by them enters our eyes. Optical telescopes work by bending this light. One kind of optical telescope has a curved piece of glass called a lens at its large end. This lens is the objective lens. It bends the light from the object in a way that forms an image -- a picture of the object -- inside the telescope. The light from this image then goes through another lens, called the eyepiece, at the small end of the telescope. The eyepiece bends the light again and makes the object look big. The bending of light by a lens is called refracting, and telescopes with such lenses are called refracting telescopes.

Another kind of optical telescope uses a mirror instead of a lens. The mirror is bowl-shaped. Light from an object bounces off the mirror in a way that forms an image of the object inside the telescope. Light from the image then goes through an eyepiece that makes the object look big. A mirror is said to reflect light, so these telescopes are called reflecting telescopes.

Astronomers can make a telescope work like a giant camera to take a picture of a faraway object. The astronomers can then study the picture.

Objects in space often look dim, even through a telescope. To make them look bright, astronomers use a telescope with a very large lens or mirror. The large size increases the amount of light that enters the telescope. So objects seen through the telescope look much brighter. The largest telescopes use mirrors instead of lenses. The tremendous weight of huge lenses would cause them to bend and warp.

How Radio Telescopes Work

Radio telescopes pick up faint radio waves that come from objects in space. Most radio telescopes capture these waves with a large, bowl-shaped part called a dish antenna, or a dish. This dish reflects radio waves the same way a mirror reflects light waves. But radio waves are much longer than light waves, so a radio telescope's dish has to be much larger than the mirror of an optical telescope.

The dish sends the radio waves to another part of the telescope that changes the waves into electric signals. A radio receiver strengthens the signals. It also keeps track of the point in space that the signals came from. This information goes to a computer that may draw a diagram of the source of the radio waves or give other information about the source.

History

The Dutch scientist Hans Lippershey probably made the first telescope in 1608. It was made of two glass lenses in a narrow tube. Within a year, the Italian astronomer Galileo built a telescope like Lippershey's. Galileo was the first person to use a telescope to study the sky. Galileo discovered some surprising things with his telescope. For example, he found that there were moons going around the planet Jupiter. In 1668, the English astronomer Isaac Newton built a telescope that used a mirror.

In the late 1930s, Grote Reber, an American engineer, built the first radio telescope and operated it in his backyard. Scientists who used early radio telescopes found strong radio waves coming from the sun. Later, astronomers found that radio waves also come from what was left of exploded stars in some dark parts of the sky. Later, scientists found rapidly spinning stars called pulsars that send out radio waves.

How to cite this article: To cite this article, World Book recommends the following format: "Telescope." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.

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