Adaptive Optics Sky Coverage Calculations for the Gemini-North Telescope

Sky coverage results are presented for several natural guide star (NGS) and laser guide star (LGS) adaptive optics (AO) configurations using scenario, telescope, and AO design parameters derived from the Gemini-North 8 m telescope. "Sky coverage" is defined as that fraction of the sky over which the AO system provides a useful level of performance, in this case a high Strehl ratio in the J, H, or K band. Some of the factors considered in this study include optimization of the AO wavefront reconstruction algorithm and control bandwidth, the effect of windshake-induced tip/tilt jitter, and the conjugation of the AO system's deformable mirror (DM) and wavefront sensor (WFS) to the dominant seeing layer at the Gemini-North site. The degree of compensation of the tip/tilt guide star image by the higher order adaptive optics is also considered. LGS AO using a single beacon in the mesospheric sodium layer typically improves sky coverage by about an order of magnitude relative to NGS AO, principally because a very dim natural guide star is sufficient for tip/tilt sensing when its image is sharpened by LGS higher order compensation. The values assumed for windshake, seeing, zenith angle, and the conjugate range of the DM and WFS also have a highly significant effect upon the estimated sky coverage for both the NGS and the LGS AO systems. The first three of these factors will need to be considered by scheduling algorithms for queue-based observing.