![]() ![]() ![]() Thus, the simplest form of refractor consists of an objective and an eyepiece, as illustrated in the diagram. A second lens, referred to as the eyepiece lens, is placed behind the focal plane and enables the observer to view the enlarged, or magnified, image. ![]() It should be noted that the light will be inverted at the focal plane. In a refractor the first lens through which light from a celestial object passes is called the objective lens. The focus is the point, or plane, at which light rays from infinity converge after passing through a lens and traveling a distance of one focal length. This diagram illustrates the principle of refraction and the term focal length. The physical shape of the components may be convex, concave, or plane-parallel. The glass is referred to as a lens and may have one or more components. The name refractor is derived from the term refraction, which is the bending of light when it passes from one medium to another of different density-e.g., from air to glass. Refracting telescopesĬommonly known as refractors, telescopes of this kind are typically used to examine the Moon, other objects of the solar system such as Jupiter and Mars, and binary stars. For explanation of instruments that operate in other portions of the electromagnetic spectrum, see radio telescope X-ray telescope and gamma-ray telescope. This article describes the operating principles and historical development of optical telescopes. These developments have contributed dramatically to advances in scientific knowledge about the solar system, the Milky Way Galaxy, and the universe as a whole. Observational capability has been further enhanced by the invention of various kinds of auxiliary instruments (e.g., the camera, spectrograph, and charge-coupled device) and by the use of electronic computers, rockets, and spacecraft in conjunction with telescope systems. Since Galileo’s pioneering work, increasingly more powerful optical telescopes have been developed, as has a wide array of instruments capable of detecting and measuring radiation in every region of the electromagnetic spectrum. ![]() Until then, magnification instruments had never been used for this purpose. Galileo revolutionized astronomy when he applied the telescope to the study of extraterrestrial bodies in the early 17th century.
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