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.
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.
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.