Active optics

Active optics is a telescope technique that slowly adjusts optical elements to compensate for structural or thermal deformation. It preserves image quality without requiring the elements to resist every changing load unaided.

Primary mirror: The main light-collecting mirror in a reflecting telescope. Its shape is adjusted to correct deformations that blur the telescope’s image.
Adaptive optics: A technique that rapidly corrects changing wavefront distortions, especially those caused by Earth’s atmosphere. It corrects fast atmospheric turbulence, whereas active optics corrects slower structural and thermal changes.
Large Binocular Telescope: An optical telescope with two 8.4-meter primary mirrors mounted side by side. Its large mirrors illustrate active support in modern telescope construction.
Very Large Telescope: A group of four 8.2-meter optical telescopes operated by ESO in Chile. Its Unit Telescopes use active optics to maintain the shape of their primary mirrors.
Mirror support system: A structure that holds and supports a telescope mirror. Actuators in the support distribute forces across the mirror’s back surface.
Passive optics: An optical system that relies on fixed design and materials rather than powered adjustments during operation. Passive designs avoid active correction but must withstand deformation through stiffness and stability.
New Technology Telescope: A 3.58-meter telescope at ESO’s La Silla Observatory, inaugurated in 1989. Its thin mirror and active support helped demonstrate a new approach to large telescope design.
Keck Observatory: An astronomical observatory in Hawaii with two telescopes whose primary mirrors are segmented. Its segmented mirrors use active control to maintain alignment and optical figure.
Wavefront sensing: Measurement of the shape and phase of an optical wavefront. Its measurements reveal image errors that guide mirror adjustments.
Tip-tilt correction: Correction of image motion by changing an optical element’s pointing angle. It removes image shifts rather than correcting the mirror’s changing surface figure.

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