Active optics is usually understood to refer to a low frequency control system applied to the primary mirror of large reflecting telescopes. It corrects for optical aberrations like spherical aberration and astigmatism either detected in the initial set-up of the telescope or caused by flexure of the mirror as the telescope's orientation is changed or its temperature varies. To make the corrections an active optics system detects errors in the mirror's surface shape either directly or via its effect on astronomical images and corrects them using a motorised support system under the mirror. The mirror can be either monolithic, in which case the flexure function has no discontinuities, or segmented where the flexure function is discontinuous.
The first significant step on the road to designing an active optics system appears to have been taken by André Couder in 1931 when he suggested that the astigmatism of inadequately supported mirrors could be corrected by applying appropriate forces to the back of the mirror. Astigmatism left over after mirror manufacture could also be corrected in this way. But he could only measure the astigmatism qualitatively off-line making such corrections cumbersome and slow. As a result his suggested procedure was limited to the initial setting up of the telescope, rather than being applied continuously as in a modern active optics system. Independently Dmitri Maksutov appears to have come up with a broadly similar proposal in 1948, but it had the same limitations.
Technological developments in computer and sensor systems after the Second World War began to make it possible to design much more sophisticated active optics systems. In the event the first major telescope to use a form of active optics was the Multiple Mirror Telescope (MMT) which consisted of six identical 1.8 m telescopes on a common mount. The MMT was designed to bring all six images to a common focus on its central axis. It was planned to continuously monitor the flexure of the structures of the six individual telescopes using an internal laser system, and to correct them in real time to ensure that all six images coincided.