Telescope
telescope , traditionally, a system of lenses, mirrors, or both, used to gather light from a distant object and form an image of it. Traditional optical telescopes, which are the subject of this article, also are used to magnify objects on earth and in astronomy; other types of astronomical telescopes gather radio waves. Types of Optical Telescopes There are three major types of optical telescopes, classified according to the element that gathers and focuses the incoming light. In the refracting telescope, or refractor, light is bent, or refracted, as it passes through an objective lens. The objective lens is convex, i.e., thicker at the middle than the edges. Parallel light passing through the lens is refracted so that it converges to a point behind the lens, called the focus. The distance from the lens to the focus is called the focal length. In a reflecting telescope, or reflector, light is reflected by a concave mirror and brought to a focus in front of the mirror. If paral...