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Active optics is a relatively new technology for reflecting telescopes which has 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.
Most modern telescopes are reflectors, with the primary element being a very large mirror. Historically, the mirrors had to be very thick to hold its shape to the required accuracy as the telescope travelled across the sky. This limited their maximum diameter to 5 or 6 metres (200 or 230 inches), such as in the Palomar Observatory's Hale telescope.
A new generation of telescopes uses instead very thin mirrors, which are too thin to keep themselves rigidly in the correct shape. Instead, an array of actuators behind the mirror keeps it in an optimal shape. The telescope may also be segmented into many small mirrors, preventing most of the gravitational distortion that occurs in large, thick mirrors.
The combination of actuators, a quality-of-image detector, and a real-time computer program to move the actuators to obtain the best possible image is termed "active optics".
The "activeness" in their name means that the system keeps the primary mirror in its optimal shape against all environmental factors such as gravity (at different telescope inclinations), wind, telescope axis deformation, etc. Active optics correct all factors that may affect image quality at timescales of one second or more. The telescope is therefore "actively" still, in its optimal shape.
Active optics should not be confused with even newer adaptive optics, which operates on a much shorter timescale to compensate for atmospheric effects, rather than for mirror or lens distortion. Factors that affect the image at faster timescales (1/100th seconds or even less) are usually caused by the atmosphere and are not easily corrected with primary mirrors. For these, the adaptive optics technology has been developed for use with small corrective mirrors and recently for secondary mirrors.
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External links
"Active optics"