Low-order wavefront sensing in tomographic multi-beacon adaptive optics systems

We present a concept to perform low-order wavefront sensing in multi-laser guide star adaptive optics systems operating using a large format NIR detector with windowing capability with near diffraction limited or partially corrected NGS tip-tilt stars with time varying Strehls. Most contemporary adaptive optics systems in development for large telescopes, viz., the next VLT adaptive optics facility that serves as a pathfinder to the European ELT, Gemini MCAO, W. M. Keck observatory's Next Generation Adaptive Optics (NGAO) System, The Large Binocular Telescope and the Thirty Meter Telescope's NFIRAOS are multi-laser guide star systems that provide AO correction over a large field. In such systems even faint tip-tilt (TT) stars image are characterized by either a well corrected (MOAO case) or at least a partially corrected (MCAO or GLAO case) diffraction limited core due to high order sharpening by the LGS WFS. In such a regime of low-order sensing one could envisage using pixels 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 on extended 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 performance for various plate scales, TT sensor performance vs. level of high order correction (TT star Strehl) and TT sensor performance vs. TT object size for a given detector noise, gain and a simple centroiding algorithm. Due to small sky noise contribution at plate-scales le 100 mas/pixel, the optimum low-order wavefront sensor plate scale is found to be 80-100 mas/pixel (3×-4× λ/d in J- and H- bands) for the Keck NGAO system