Progress toward developing the TMT adaptive optical systems and their components

Abstract

Atmospheric turbulence compensation via adaptive optics (AO) will be essential for achieving most objectives of the TMT science case. The performance requirements for the initial implementation of the observatory's facility AO system include diffraction-limited performance in the near IR with 50 per cent sky coverage at the galactic pole. This capability will be achieved via an order 60x60 multi-conjugate AO system (NFIRAOS) with two deformable mirrors optically conjugate to ranges of 0 and 12 km, six high-order wavefront sensors observing laser guide stars in the mesospheric sodium layer, and up to three low-order, IR, natural guide star wavefront sensors located within each client instrument. The associated laser guide star facility (LGSF) will consist of 3 50W class, solid state, sum frequency lasers, conventional beam transport optics, and a launch telescope located behind the TMT secondary mirror. In this paper, we report on the progress made in designing, modeling, and validating these systems and their components over the last two years. This includes work on the overall layout and detailed opto-mechanical designs of NFIRAOS and the LGSF; reliable wavefront sensing methods for use with elongated and time-varying sodium laser guide stars; developing and validating a robust tip/tilt control architecture and its components; computationally efficient algorithms for very high order wavefront control; detailed AO system modeling and performance optimization incorporating all of these effects; and a range of supporting lab/field tests and component prototyping activities at TMT partners. Further details may be found in the additional papers on each of the above topics.

Split atmospheric tomography using laser and natural guide stars

“…An overview of the TMT LGS MCAO system and its expected performance can be found in [18,19]. The reference design MCAO configuration includes order 63ϫ 63 and 75ϫ 75 wavefront correction at 800 Hz using a pair of conventional piezo-stack DMs conjugate to ranges of 0 and 12 km, six LGS WFSs of order 60ϫ 60, two near-infrared TT NGS WFSs, and one near-infrared TTFA NGS WFS.…”

High fidelity sky coverage analysis via time domain adaptive optics simulations

“…The Thirty Meter Telescope (TMT) narrow field infrared adaptive optics system (NFIRAOS) [4] is an MCAO system with two deformable mirrors (DMs) conjugated to the range of 0.0 and 11:3 km, respectively, an asterism of 6 sodium laser guide stars arranged in a pentagon with a 35 00 radius plus one more on axis, and up to three natural guide star low-order wavefront sensors (1 tip/tilt/focus/astigmatism (TTF) and up to two tip/tilts only). The ground-conjugate DM is mounted on a tip/tilt stage (TTS) to eliminate the need for a separate tip/tilt mirror.…”

“…Current [1][2][3] and next generation [4,5] ground based astronomical telescopes rely upon laser guide star adaptive optics (LGS AO) systems to achieve diffraction limited resolution. Because the laser is launched from the ground and reflected back from the sodium layer, it cannot reliably measure the tip/tilt wavefront aberration caused by the atmospheric turbulence or telescope vibration.…”

Analysis of the improvement in sky coverage for multiconjugate adaptive optics systems obtained using minimum variance split tomography

“…Laser guide star (LGS) adaptive optics (AO) systems have been either employed [1][2][3] or are planned [4,5] on most ground-based large-aperture astronomical telescopes to overcome the sky coverage limitations of natural guide star (NGS) AO systems. However, because the laser is projected from the ground and scattered back from the sodium layer, it cannot reliably measure the tip/tilt (TT) wavefront aberration caused by atmospheric turbulence or telescope vibration [6].…”