Low-order wavefront sensing in tomographic multi-beaconadaptive optics systems

摘要

We present a concept to perform low-order wavefront sensing in multi-laser guide star adaptive optics systemsoperating using a large format NIR detector with windowing capability with near diffraction limited or partiallycorrected NGS tip-tilt stars with time varying Strehls. Most contemporary adaptive optics systems in develop-ment for large telescopes, viz., the next VLT adaptive optics facility that serves as a pathfinder to the EuropeanELT, Gemini MCAO, W. M. Keck observatory's Next Generation Adaptive Optics (NGAO) System, The LargeBinocular Telescope and the Thirty Meter Telescope's NFIRAOS are multi-laser guide star systems that provideAO correction over a large field. In such systems even faint tip-tilt (TT) stars image are characterized by eithera well corrected (MOAO case) or at least a partially corrected (MCAO or GLAO case) diffraction limited coredue to high order sharpening by the LGS WFS. In such a regime of low-order sensing one could envisage usingpixels as field stops and choosing a appropriate plate scale to minimize the sky background.Simulations are used to predict the performance of such a sensor when guiding on point sources and onextended objects of varying brightness and for different levels of high order correction.The parameter space explored includes tip-tilt and tip-tilt, focus and astigmatism (TTFA) sensor performancefor various plate scales, TT sensor performance vs. level of high order correction (TT star Strehl) and TT sensorperformance vs. TT object size for a given detector noise, gain and a simple centroiding algorithm. Due to smallsky noise contribution at plate-scales le 100 mas/pixel, the optimum low-order wavefront sensor plate scale isfound to be 80-100 mas/pixel (3x-4x λ/d in J- and H- bands) for the Keck NGAO system.