TMT-AGE: wide field of regard multi-object adaptive optics for TMT
ABSTRACT
We introduce current status of the feasibility study on a wide field of regard (FoR) Multi-Object Adaptive Optics (MOAO) system for TMT (TMT-AGE: TMT-Analyzer for Galaxies in the Early universe). MOAO is a system which realize high spatial-resolution observations of multiple objects scattered in a wide FoR. In this study, we put emphasise on the FoR as wide as 10′ diameter. The wide FoR is crucial to effectively observe very high-redshift galaxies, which have low surface number density. Simulations of an MOAO system with 8 LGSs show close-to-diffraction-limited correction can be achieved within 5′ diameter FoR and moderate AO correction can be achieved within 10′ diameter FoR. We discuss overall system design of the wide FoR MOAO system considering the constraint from the stroke of small-size deformable mirror (DM). We also introduce current status of developments of key components of an MOAO system; high-dynamic range wavefront sensor (WFS) and large-stroke small-size DM, and real time computer (RTC) with fast tomographic reconstruction.
1. INTRODUCTION
Multi-Object Adaptive Optics (MOAO) is an AO system that delivers close-to diffraction-limited images of multiple objects scattered in a wide Field of Regard (FoR) simultaneously.1 The 3D volume structure of the atmospheric turbulence is estimated by tomographic reconstruction with wavefront measurements of multiple laser guide stars (LGSs). The estimated 3D structure of the atmospheric turbulence is integrated along the lineof-sight of each target and the optimised correction for each target is determined. Multiple targets are picked up by multiple science paths. The optimised correction for each science path is applied to a deformable mirror (DM) in the science path. The control of the DM is made in open-loop manner. Because Tip-Tilt (TT) and focus components cannot be sensed with the LGSs, natural guide stars (NGSs) are necessary to measure the components. Because TT component also has anisoplanatism, we need multiple NGSs within the wide FoR to estimate TT component of each science path. Bright NGSs within the wide FoR can also be used for high-order wavefront measurements.2
Multiple deployable integral field units (IFUs) assisted with the MOAO system is a powerful instrument to obtain high-resolution 3D (two spatial and one wavelength dimension) information of statistical number of galaxies. For Thirty Meter Telescope (TMT), two feasibility studies have been done for MOAO-assisted multiple-IFUs NIR spectrograph under the name of Infrared Multi-Object Spectrograph (IRMOS).3 One is done by members of University of Florida (UF) and Herzberg Institute of Astrophysics (HIA), IRMOS-UF/HIA concept4–6 and the other one done by members of Caltech and Laboratoire d’Astrophysique de Marseille (LAM), IRMOS-Tipi concept.7 Both of these feasibility studies conducted following the IRMOS specification with near diffraction-limited PSF within 5′ diameter FoR.
Following the feasibility studies on IRMOS, we started a feasibility study of an MOAO system covering even wider FoR up to 10′ diameter under the name of TMT-AGE (TMT-Analyzer for Galaxies in the Early universe). The wide FoR is crucial to effectively observe very high-redshift galaxies at z > 5, whose surface number density is rather low. In this paper, we describe the science drivers for a wide FoR MOAO system and MOAO performance simulations in the wide FoR. Then, we describe overall system considerations emphasising requirements for DMs. Finally we mention on-going developments related to components of the MOAO system; high dynamic range wavefront sensors (WFS), large stroke DMs, and fast tomography real time controller.
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