active optics

Active optics is a relatively new technology for reflecting telescopes developed in the 1980s, which has more recently enabled the construction of a generation of telescopes with 8 metre primary mirrors. Active optics works by "actively" adjusting the telescope's mirrors. This method is used by, among others, the Nordic Optical Telescope, the New Technology Telescope and the Keck telescopes, as well as all large telescopes built in the last decade.
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A system of electronic supports which deform the mirror inside a telescope into the most precise possible shape to maximise its performance. This allows much larger, lighter mirrors to be constructed since they no longer need to support their own weight. See also adaptive optics.