• Any device for concentrating electromagnetic radiation for human study. Optical telescopes do this by presenting a larger light-collecting area to incoming light than the human eye can, and by using photographic methods to collect light over longer periods than the near-instantaneous period over which the eye forms an image. In the last four centuries, massive ingenuity has been devoted to refining telescopes, with many types produced to resolve objects close together in the sky, give distortion-free images of large areas of the sky, and carry out other tasks. Making telescopes is an art and science at the forefront of modern technology. A telescope’s optics must be accurate to much less than the wavelength of light if they are to produce usable images. Telescope makers use advanced materials to produce large areas of high-quality optics capable of coping with temperature variations and other problems. Modern telescopes also involve computer controls and telecommunications links, often allowing astronomers to use them from thousands of kilometres away. Telescopes have also been developed for use in space and on the Earth across virtually the whole of the electromagnetic spectrum, including radio, microwave, X-ray, infra-red and ultraviolet frequencies. The term is also applied to machines for detecting gravity waves, neutrinos, cosmic rays and other astronomically rewarding phenomena – a usage which is technically dubious but which is true to the word’s Greek origins from the words for ‘seeing at a distance,’ even if the seeing takes an unanticipated form.

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