Evaluating sky coverage for the NFIRAOS tip/tilt control architecture

The Narrow Field Infrared Adaptive Optics System (NFIRAOS) is the first light Laser Guide Star (LGS) Multi-Conjugate Adaptive Optics (MCAO) system for TMT. NFIRAOS needs to correct 2-axis tip/tilt jitter disturbances, including both telescope vibration and atmospheric tip/tilt, to a residual of 2 milli-arcsecond (mas) RMS with 50% sky coverage at the Galactic pole. NFIRAOS will utilize multiple infrared tip/tilt sensors, as sky coverage benefits greatly from wavefront sensing in the near IR where guide star densities are greater and the NFIRAOS AO system "sharpens" the guide star images. NFIRAOS will also utilize type II woofer-tweeter control to correct tip/tilt jitter. High amplitude, low bandwidth errors are corrected by a tip/tilt platform (woofer), whereas the low amplitude, high bandwidth disturbances are corrected by the deformable mirrors. A prototype development effort for the relatively large, massive DM tip/tilt stage is now underway. Detailed Monte Carlo simulations of the complete architecture indicate that the sky coverage and tip/tilt control requirement for NFIRAOS can be met, with some margin available for stronger input disturbances or shortfalls in component performance.