Telescopes are instruments made to help us see remote objects up close. Some types are also able to see electromagnetic radiation. This name was first given to an instrument invented by Galileo, used to view the sky. There are several different types of telescopes, and the name can refer to instruments that are used to view many different areas of the electromagnetic spectrum. The major types of telescope are optical, radio, and X- and gamma-ray telescopes.
Optical Telescopes
These are what we usually think of when someone mentions telescopes. They gather light from the visible part of the spectrum, and focus it to make distant objects look closer and brighter. Most of them use one or more than one curved optical pieces. These can be lenses or mirrors, and their purpose is to gather and focus light or (in some cases) infrared or ultraviolet radiation. Once the light has been gathered and focused, we either view it through an eyepiece, photograph it, or send it to a computer for analysis. Optical telescopes are common in astronomy, and are part of many other non-astronomical devices. For instance, camera lenses are a type of telescope, binoculars are essentially a pair of telescopes, and hunting spotting scopes are another variation.
Optical telescopes can be divided into three classes. Refracting telescopes use only lenses. Reflecting telescopes use entirely an arrangement of mirrors to collect and focus light, excepting the lens in the eyepiece. A catadioptric telescope, on the other hand, uses both, and has a corrector plate to fix any distortions.
Radio Telescopes
Radio telescopes don't look like what we'd think of as a telescope at all. They're actually a type of directional radio antenna. They're often attached to a parabola, or dish, that looks a lot like an old style satellite dish. Some radio telescope are actually made up of groups of dishes to create the effect of a much larger one. These telescopes collect microwave radiation, which can be used to study the effects of the Big Bang. They also are sometimes used to collect radiation from objects which don't emit much visible light. Quasars are one of these types of objects. SETI and other organizations also use radio telescopes to help look for life in space.
X-Ray and Gamma-Ray Telescopes
Since these types of radiation will pass through most types of glass and metal, it's necessary to use a different method to collect them. Some X-ray telescopes do so by using ring shaped mirrors to reflect the rays. Gamma-ray telescopes are more likely to keep from completely focusing, and instead use masks. The shadows from the mask are then interpreted to create an image. These kinds of telescopes are found on high balloons and on satellites, but not on the ground, because our atmosphere doesn't allow the radiation they collect to come through.
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