Why adaptive secondaries?

Adaptive optics (AO) combines technologies that enable the correction in re al time of the wavefront distortion caused by the terrestrial atmospheric t urbulence. An adaptive secondary mirror (ASM), unlike conventional adaptive optics systems, does not add any polarization, reflective losses, and emis sivity. Following successful implementation of tip/tilt secondary mirrors, most rec ent large telescope projects have considered the possibility of incorporati ng ASMs. This paper briefly reviews the development of ASMs and examines th e issues which have arisen and also presents the predicted performance of a n ASM system. It is concluded that adaptive secondary approach is an equall y satisfactory or preferred solution to conventional AO systems.