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active opticsThe techniques by which corrections may be made to the shape of a large mirror or radio dish to adjust for minute-long or hour-long drifts from its designed shape. These variations in shape arise as a telescope is subjected to slowly changing forces, including the effects of gravity on different telescope orientations, temperature changes within and exterior to the structure, and wind; they result in an imperfect image and are particularly severe in large telescopes. With active optics, the shape of the reflector is adjusted to give a highly accurate surface and so maintain the quality of the image; in a segmented primary mirror, for example, the position, spacing, and tilt of individual segments can be controlled, while in a thin monolithic meniscus mirror, forces are applied at numerous positions on the mirror back to counteract naturally occurring changes in shape. Active optics are used on the latest generation of giant telescopes, including the 3.5-m NTT of the European Southern Observatory and the 10-m Keck Telescopes. An analogous system, in which the panels making up the receiving surface of a radio telescope can be individually controlled by actuators in order to change the overall shape of the dish, is in use on such instruments as the 100-m Green Bank Telescope. See also adaptive optics.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006