Why do astronomers use telescopes? As a child, the idea behind telescopes was strictly magnification. However astronomers have a different purpose in mind. Indeed magnification is a consideration for astronomers, but the main idea behind telescopes is their light gathering capability. Many of the objects in the sky are not visible to the naked eye. Not because they are too far away and therefore too small, but because they are too faint. Consider a whisper. In order to hear a sound which may be too faint to be heard you normally cup your ear. The reason for this is to make the ear larger. In this fashion the ear is capable of receiving more of the sound wave and therefore able to hear noises which would normally be inaudible.

The telescope operates under the same principle. The large aperture of the telescope acts as a cup for the eyes. In this way, objects which would normally be too dim (not necessarily too small) to be seen can be viewed. Astronomers are therefore interested in telescopes which are as large as possible.

There are two basic designs for telescopes. The first design involves lenses and the concept of refraction. The second design uses mirrors and reflection.

If you were to stand in the water of the swimming pool, you might notice that your legs seem shorter than normal. Or place a stick in the water at an angle, it may look as if the straight stick is bent. These are both examples of refraction. Refraction occurs because the speed of light changes when it moves from air to water. The speed of light in water is approximately 30 percent slower than the speed of light in air. Because of this changing speed, the path of light changes direction at the air-water boundary. The same occurrence takes place whenever light encounters a new medium in which it has a different speed.

The diagram below shows the path of light through a glass prism. When the white light strikes the prism on the left it changes direction at the air glass boundary. Since red light and blue light have different speeds in glass, the angle of their directions is different. This is why white light is broken into the spectrum when it passes through glass.

Ideally, when glass is ground into the shape of a lens all of the rays converge on the same point. This is called a focal point. The distance between the lens and the focal point is the focal length. Since red light and blue light bend at different angles through glass they converge at different points. This problem in lenses is referred to as chromatic aberration.

In a similar fashion, rays of light bouncing off a curved mirror also converge at the same point. In reality, all the rays do not converge to the same spot. The rays near the central axis focus at one place, and rays far from the central axis focus at a different location. This is why a magnifying glass will never focus light to a tiny point. The diagrams below shows this for lenses and mirrors. This problem is referred to as spherical aberration. Since mirrors suffer from spherical aberration and lenses suffer from chromatic and spherical aberration, mirrors are preferred. In addition, mirrors are easier to make and cheaper to build in large sizes. Today, most major telescopes are built using mirrors.

Below are simplified diagrams for refracting and reflecting telescopes.

Besides visible light telescopes, astronomers use telescopes which are sensitive in all portions of the electromagnetic spectrum. On the surface of the Earth, most telescopes are made for visible, infrared, and radio radiation. This is because the atmosphere of the Earth blocks out too much radiation in other portions of the spectrum.

For many years pictures of the sky were taken with film. But film is difficult to store, takes a long time to process, and is very expensive. Today, most pictures are taken with a CCD. A CCD is a computer chip capable of recording light information electronically. This information is then processed by a computer. The information can be stored on a disk, and the CCD can be used again. The CCD is a major component in most of today's video cameras.

The diagrams and pictures below show the fundamental workings of reflecting and refracting telescopes. You will also see examples of both of these types of telescopes. Note that for most reflecting telescopes the length of the scope is small compared to the diameter. In the case of refracting telescopes the length of the telescope is long compared to the diameter of the lens.

Refractors

Keck Reflectors (currently the largest in the world with a 10m diameter each).

The University of Wyoming Infrared Telescope

Why do astronomers build telescopes and observatories on mountain tops? At high altitudes the atmosphere of the Earth is less likely to interfere with the light waves coming from outer space. There are two major problems caused by the atmosphere of the Earth. The first is the distortion of the light from outer space. As the atmosphere shakes, it causes the stars to twinkle or blur. This problem is referred to as "seeing." This second problem is that the atmosphere blocks certain wavelengths of light. Some wavelengths are blocked more than others. As we climb in altitude the atmosphere thins. Thinner atmosphere blocks less light. Of course clouds block even visible light. Therefore mountain tops above the clouds are preferred.

The best way to avoid atmospheric difficulties is to build the observatory outside the atmosphere of the Earth. To this end, astronomers and engineers design satellites to act as their observatories. The images from these satellites are many times better than the images from the best Earth-based telescopes. Images from the Hubble space telescope are leaps and bounds above what we had initially expected. These images are all over the World Wide Web.

The Hubble Space Telescope

This page was last updated on 06/13/01.