Methods for correcting tilt anisoplanatism in laser-guide-star-based multiconjugate adaptive optics

Multiconjugate adaptive optics (MCAO) is a technique for correcting turbule nce-induced phase distortions in three dimensions instead of two, thereby g reatly expanding the corrected field of view of an adaptive optics system. This is accomplished with use of multiple deformable mirrors conjugate to d istinct ranges in the atmosphere, with actuator commands computed from wave -front sensor (WFS) measurements from multiple guide stars. Laser guide sta rs (LGSs) must be used (at least for the forseeable future) to achieve a us eful degree of sky coverage in an astronomical MCAO system. Much as a singl e LGS cannot be used to measure overall wave-front tilt, a constellation of multiple LGSs at a common range cannot detect tilt anisoplanatism. This er ror alone will significantly degrade the performance of a MCAO system based on a single tilt-only natural guide star (NGS) and multiple tilt-removed L GSs at a common altitude. We present a heuristic, low-order model for the p rincipal source of tilt anisoplanatisin that suggests four possible approac hes to eliminating this defect in LGS MCAO: (i) tip/tilt measurements from multiple NGS, (ii) a solution to the LGS tilt uncertainty problem, (iii) ad ditional higher-order WFS measurements from a single NGS, or (iv) higher-or der WFS measurements from both sodium and Rayleigh LGSs at different ranges . Sample numerical results for one particular MCAO system configuration ind icate that approach (ii), if feasible, would provide the highest degree of tilt anisoplanatism. compensation. Approaches (i) and (iv) also provide ver y useful levels of performance and do not require unrealistically low level s of WFS measurement noise. For a representative set of parameters for an 8 -m telescope, the additional laser power required for approach (iv) is on t he order of 2 W per Rayleigh LGS. (C) 2001 Optical Society of America.