  Active optics is defined as the control of the shape and the alignment of the
components of an optical system at low temporal frequencies. For modern large
telescopes with flexible monolithic or segmented primary mirrors and also
flexible structures this technique is indispensable to reach a performance
which is either diffraction limited for an operation in space or limited by the
atmosphere for an operation on the ground. This article first describes the
theory of active optics, both of the wavefront analysis and the correction
mechanisms, then the design of three representative active optics systems, two
in telescopes of the four and eight meter class with meniscus mirrors and one
in a telescope with a segmented primary mirror, and, finally, presents
practical experience with these active optics systems.
