Predicted sky coverage for the TMT MCAO system NFIRAOS


David Andersen, Lianqi Wang, Brent Ellerbroek, Glen Herriot




TMT has chosen the MCAO system NFIRAOS to be its first light AO system in part to provide astronomers with exceptional sky coverage. The TMT science requirements demand that its AO system provide wavefront errors of less than or equal to 191 nm at the galactic pole at least 50% of the time (under median atmospheric conditions when observing at zenith). This requirement drove many aspects of the NFIRAOS design from the size of the FOV, to the use of NIR MCAO-corrected NGSs, to the sensitivity of the WFSs (and corresponding limiting magnitude of the NGSs). In this paper, we build upon the sky coverage simulations of L. Wang et al. to produce smooth sky coverage maps generated for different hour angles (potential exposure time lengths), and different atmospheric conditions. We show that NFIRAOS should meet its sky coverage requirement at the North Galactic Pole, and that the sky coverage generally will be much higher than 50% at lower galactic latitudes.

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