Telescopes are lens assemblies that are designed to magnify and view distant objects. Telescopes include small devices such as spotting scopes for viewing wildlife, and very large devices for astronomical study. Most telescopes are assemblies of lenses and mirrors that are used to gather light. Other telescopic devices are designed to detect objects by the emission or reflection of different types of radiation.
Telescopes come in three major optical types: refractor telescopes, reflector telescopes, and catadioptric telescopes. In a refracting telescope, light enters through the eyepiece lens and is bent as it passes through the objective lens in the body of the telescope. In refractor telescopes, the telescope lens is convex in shape, meaning that it is thicker in the middle than at the edges. In a reflecting telescope, light is reflected and focused by using a concave mirror. Typically, this mirror is a glass disk ground into a concave or parabolic shape. The mirror in reflecting telescopes is also coated with a thin film of silver or aluminum to make it highly reflective. By contrast, catadioptric telescopes use a combination of refracting and reflecting lenses and mirrors to focus the incoming light.
Telescopes are differentiated by their resolving and magnifying power. The magnification of a telescope is based on the eyepiece, and is equal to the ratio of the focal lengths of the eyepiece and the objective lens. The resolving power in telescopes is the ability of the telescope to produce clearly defined objects, and is the ratio of the wavelength of light observed to the diameter of the objective lens.
Telescopes use many types of accessories. Examples include finders or scopes, photographic accessories, equatorial mounts and tripods, and filters for viewing solar phenomenon. Specialized telescopes include products such as radio telescopes. A radio telescope uses a radio receiver and antenna mounted in a large reflector dish to gather radio waves. Because many celestial objects do not emit visible light, radio telescopes provide a way to detect them. Radio astronomy is used to view objects such as pulsars or quasars that produce radiation in the radio frequency (RF) region of the electromagnetic spectrum.
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