Why adaptive secondaries?

Adaptive optics (AO) combines technologies that enable the correction in real time of the
wavefront distortion caused by the terrestrial atmospheric turbulence. An adaptive secondary mirror (ASM),
unlike conventional adaptive optics systems, does not add any polarization, reÑective losses, and emissivity.
Following successful implementation of tip/tilt secondary mirrors, most recent large telescope projects
have considered the possibility of incorporating ASMs. This paper brieÑy reviews the development of ASMs
and examines the issues which have arisen and also presents the predicted performance of an ASM system. It
is concluded that adaptive secondary approach is an equally satisfactory or preferred solution to conventional
AO systems.