active optics

The 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

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References in periodicals archive

The "adjustment," on a nearly continuous basis, of a fragile and flexible telescope is what constitutes active optics. Sensors on the telescope (such as the Shack-Hartmann wavefront sensor, which is nicely explained in the book) provide input to computers, which are programmed to drive mechanical actuators to produce counterdistortions in the optics resulting in a high-quality image regardless of temperature, wind, "local air" seeing, and pointing/tracking behavior of the telescope.

All Things Reflecting

The New Technology Telescope relies on a feedback system, called active optics, to achieve its resolving power.

Space telescope: a saga of setbacks

"At the very first trial of the active optics, while the telescope tracking and pointing were not fine tuned and the telescope was still misaligned and the secondary not properly supported, the RMS [root-mean-square] error of the uncorrected wave front was found to he 1350 nanometers [about 2 1/2 wavelengths of yellow light], mostly astigmatism, coma and triangular coma due to the poor supports.

A classic vs. high-tech telescope

The change comes largely from successes in recent years in using computer programs and active optics directed by computer-driven servomechanisms to compensate for atmospheric turbulence.

Taking the measure of the stars

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