Sky coverage estimates for the natural guide star mode of the TMT facility AO system NFIRAOS

Although the TMT AO system NFIRAOS will operate primarily in a laser guidestar multi-conjugate AO mode, it will also provide a conventional natural guide star (NGS) mode for use on very narrow science fields containing a bright star and/or when laser propagation is prevented by thin cirrus clouds or other circumstances. The number of bright stars suitable for use with a high order AO system is limited, so we have performed a sky coverage analysis to determine the likelyhood of achieving a given Strehl ratio when observing a randomly selected science field. The results obtained are significantly better than for existing NGS AO systems, largely due to (i) the anticipated availability of large, high-speed detector arrays with sub-electron read noise, and (ii) the benign telescope windshake disturbances predicted for TMT. Order 60×60 wavefront sensing and correction is preferred to lower order AO compensation, and an H-band Strehl of 0.25 [0.50] is obtained with sky coverage of about 1.0 [0.1] per cent at the Galactic pole in median seeing. This level of performance will provide an important capability for TMT well into the life of the observatory.