NFIRAOS - first light adaptive optics system for TMT. In Adaptative Optics for Extremely Large Telescopes

Abstract. NFIRAOS, the TMT Observatory’s initial facility AO system will correct atmospheric turbu-lence with 50 per cent sky coverage at the galactic pole. It is a multi-conjugate AO system feeding science light from 0.8 to 2.5 microns wavelength to three near-IR client instruments. NFIRAOS is an order 60x60 system with two deformable mirrors optically conjugated to 0 and 11.2 km. Very low background is an important design driver: one DM is mounted on a tip/tilt platform to reduce surface count; the optics are cooled to – 30 C. NFIRAOS ’ real time control uses six sodium laser wavefront sensors and up to three IR natural guide star tip/tilt and/or tip/tilt/focus sensors located within each client instrument. To compensate errors arising from the variability of the sodium layer, to which extremely large telescopes are more sensi-tive, NFIRAOS uses innovative algorithms coupled with a pair of Truth wavefront sensors that monitors a natural star at low bandwidth. For calibration, NFIRAOS includes simulators of both natural stars at infin-ity and laser guide stars at varying range distance. It also includes an IR acquisition camera, and a high-speed NGS WFS for operation without lasers.