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Full Physical Optics Sky Coverage Simulation for MCAO and MOAO Systems on ELT’s

Abstract 188

Submitted by Lianqi WANG


L. Wang and B. Ellerbroek


Thirty Meter Telescope Project


The "sky coverage" of an AO system is conventionally expressed as the cumulative probability of obtaining a given level of wavefront correction for an arbitrarily selected science object. For multi-conjugate and multi-object AO (MCAO and MOAO) systems, the evaluation of sky coverage is challenging because asterisms of three or more natural guide stars (NGS’s) must be considered. Our previous Monte Carlo sky coverage analyses of such systems have been performed using simplified, linear and Zernike-based models, and were consequently not able to model physical optics effects or the full dimensionality of the higher-order wavefront control loop. In this paper, we described a full rank, physical optics, integrated sky coverage simulation tool for modeling both the NGS and LGS AO control loops of MCAO and MOAO systems on ELT’s. Sufficient computational efficiency to enable practical Monte Carlo simulations over a large number of natual guide star asterisms can be obtained using the so-called "split tomography" control architecture, in which the higher-order wavefront correction computed from the LGS WFS measurements is not affected by the lower-order NGS control loop. We first compute and store time histories of 1) the atmospheric modes that are blind to LGS WFS and 2) natual guide star point spread functions for a full ensemble of several hundred of NGS over the course of a single AO simulation run. The behavior of the NGS loop may then be evaluated separately for each NGS asterism in a post-processing step to derive sky coverage statistics. This post-processing analysis also helps us find the best approach to WFS centroiding to maximize skycoverage.

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