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 of the largest 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 built since the 1980s 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.
A small part of the beam leaks through beam steering mirrors and a four-quadrant-diode is used to measure the position of a laser beam and another in the focal plane behind a lens is used to measure the direction. The system can be sped up or made more noise-immune by using a PID controller. For pulsed lasers the controller should be locked to the repetition rate. A continuous (non-pulsed) pilot beam can be used to allow for up to 10 kHz bandwidth of stabilization (against vibrations, air turbulence, and acoustic noise) for low repetition rate lasers.
Sometimes Fabry-Pérot interferometers have to be adjusted in length to pass a given wavelength. Therefore the reflected light is extracted by means of a Faraday rotator and a polarizer. Small changes of the incident wavelength generated by an acousto-optic modulator or interference with a fraction of the incoming radiation delivers the information whether the Fabry Perot is too long or to short. Long optical cavities are very sensitive to the mirror alignment. A control circuit can be used to peak power. One possibility is to perform small rotations with one end mirror. If this rotation is about the optimum position, no power oscillation occurs. Any beam pointing oscillation can be removed using the beam steering mechanism mentioned above.
X-ray active optics, using actively deformable grazing incidence mirrors, are also being investigated