Point Spread Function Reconstruction for Laser Guide Star Multi Conjugate Adaptive Optics Systems on Extremely Large Telescopes


L. Gilles (1), B. L. Ellerbroek (1), L. Wang (1), J-P. Veran (2) and C.Correia (2)


(1) Thirty Meter Telescope Observatory Corporation, 1111 S. Arroyo Parkway, Suite 200, Pasadena, California 91105, USA (2) NRC Herzberg Institute of Astrophysics, 5171 W. Saanich Road, Victoria, BC, V9E 2E7, Canada


We discuss a point spread function (PSF) reconstruction strategy for laser guide star (LGS) multi conjugate adaptive optics (MCAO) systems on Extremely Large Telescopes (ELTs). The approach is based on system telemetry data, combined with model data obtained from a high-speed highly-parallelized Monte Carlo numerical simulation of the full system. Such a simulation is fed by critical system data, including the Cn2 turbulence profile estimated in real-time during LGS tomography, wavefront sensor (WFS) signal levels, control loop parameters, and calibration data to account for non-common-path (NCP) aberrations and sodium layer tracking errors. The long-exposure science optical transfer function (OTF) (Fourier transfrom of the PSF) is estimated as as a product of 3 terms: (i) the OTF of a point-source LGS, estimated from system telemetry using Veran’s method, (ii) a model anisoplanatism filter, computed from the numerical simulation to account for the difference between the OTFs for a point-source at the location of the LGS and the science target, and (iii) a tip/tilt/tilt anisoplanatism filter obtained from system and model telemetry, and expressed as a system-tomodel OTF ratio.A Strehl ratio error sensitivity analysis is given for the Thirty Meter Telescope (TMT) Narrow- Field Infrared AO System (NFIRAOS), for the case of uncertainties in the turbulence seeing parameter r0, and for the case of un-modeled tip/tilt disturbances simulating system vibrations.

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