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 performanc e, in this case a high Strehl ratio in the J, H, or K band. Some of th e factors considered in this study include optimization of the AO wave front reconstruction algorithm and control bandwidth, the effect of wi ndshake-induced tip/tilt jitter, and the conjugation of the AO system' s deformable mirror (DM) and wavefront sensor (WFS) to the dominant se eing 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 la yer typically improves sky coverage by about an order of magnitude rel ative to NGS AO, principally because a very dim natural guide star is sufficient for tip/tilt sensing when its image is sharpened by LGS hig her order compensation. The values assumed for windshake, seeing, zeni th 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 a nd the LGS AO systems. The first three of these factors will need to b e considered by scheduling algorithms for queue-based observing.