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Register a new userDissecting Galaxies with Adaptive Optics
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We describe several projects addressing the growth of galaxies and massive black holes, for which adaptive optics is mandatory to reach high spatial resolution but is also a challenge due to the lack of guide stars and long integrations. In each case kinematics of the stars and gas, derived from integral field spectroscopy, plays a key role. We explain why deconvolution is not an option, and that instead the PSF is used to convolve a physical model to the required resolution. We discuss the level of detail with which the PSF needs to be known, and the ways available to derive it. We explain how signal-to-noise can limit the resolution achievable and show there are many science cases that require high, but not necessarily diffraction limited, resolution. Finally, we consider what requirements astrometry and photometry place on adaptive optics performance and design.
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We overview the current status of photometric analyses of images collected with Multi Conjugate Adaptive Optics (MCAO) at 8-10m class telescopes that operated, or are operating, on sky. Particular attention will be payed to resolved stellar population studies. Stars in crowded stellar systems, such as globular clusters or in nearby galaxies, are ideal test particles to test AO performance. We will focus the discussion on photometric precision and accuracy reached nowadays. We briefly describe our project on stellar photometry and astrometry of Galactic globular clusters using images taken with GeMS at the Gemini South telescope. We also present the photometry performed with DAOPHOT suite of programs into the crowded regions of these globulars reaching very faint limiting magnitudes Ks ~21.5 mag on moderately large fields of view (~1.5 arcmin squared). We highlight the need for new algorithms to improve the modeling of the complex variation of the Point Spread Function across the field of view. Finally, we outline the role that large samples of stellar standards plays in providing a detailed description of the MCAO performance and in precise and accurate colour{magnitude diagrams.
Since the year 2000, adaptive optics (AO) has seen the emergence of a variety of new concepts addressing particular science needs; multiconjugate adaptive optics (MCAO) is one of them. By correcting the atmospheric turbulence in 3D using several wavefront sensors and a tomographic phase reconstruction approach, MCAO aims to provide uniform diffraction limited images in the near-infrared over fields of view larger than 1 arcmin square, i.e., 10 to 20 times larger in area than classical single conjugated AO. In this review, we give a brief reminder of the AO principles and limitations, and then focus on aspects particular to MCAO, such as tomography and specific MCAO error sources. We present examples and results from past or current systems: MAD (Multiconjugate Adaptive Optics Demonstrator) and GeMS (Gemini MCAO System) for nighttime astronomy and the AO system, at Big Bear for solar astronomy. We examine MCAO performance (Strehl ratio up to 40percent in H band and full width at half maximum down to 52 mas in the case of MCAO), with a particular focus on photometric and astrometric accuracy, and conclude with considerations on the future of MCAO in the Extremely Large Telescope and post-HST era.
ERIS is the new AO instrument for VLT-UT4 led by a Consortium of Max-Planck Institut fuer Extraterrestrische Physik, UK-ATC, ETH-Zurich, ESO and INAF. The ERIS AO system provides NGS mode to deliver high contrast correction and LGS mode to extend high Strehl performance to large sky coverage. The AO module includes NGS and LGS wavefront sensors and, with VLT-AOF Deformable Secondary Mirror and Laser Facility, will provide AO correction to the high resolution imager NIX (1-5um) and the IFU spectrograph SPIFFIER (1-2.5um). In this paper we present the preliminary design of the ERIS AO system and the estimated correction performance.
This paper reviews atoms and ions in the upper atmosphere, including the mesospheric metals Na, Fe, Mg$^+$, Si$^+$, Ca$^+$, K and also non-metallic species N, N$^+$, O, H, considering their potential for astronomical adaptive optics. Na and Fe are the best candidates for the creation of polychromatic laser guide stars, with the strongest returns coming from transitions that can be reached by excitation at two wavelengths. Ca$^+$ and Si$^+$ have strong visible-light transitions, but require short wavelengths, beyond the atmospheric cutoff, for excitation from the ground state. Atomic O, N and N$^+$ have strong transitions and high abundances in the mesosphere. The product of column density and cross section for these species can be as high as $10^5$ for O and several hundred for N and N$^+$, making them potential candidates for amplified spontaneous emission. However they require vacuum-ultraviolet wavelengths for excitation.
It is widely believed that adaptive optics only has a role in correcting turbulent wavefronts on large telescopes using very bright reference stars. Unfortunately these are very scarce and many astronomical targets require wavefront correction to work over much of the sky. We therefore need to be able to use very much fainter reference objects. Laser guide stars in principle can allow 0.1 arcsecond resolution but have a number of severe technical problems that limit their application. Our aims are to provide imaging at even higher resolution than Hubble. Lucky Imaging completely eliminates the tip-tilt errors in astronomical wavefront detection. Most of the power that remains is in low order, large scale structures. These may be detected with high sensitivity using photon-counting EMCCD detectors working at high frame rate, up to ~100Hz. With a new design of curvature wavefront sensor, wavefront errors may be measured and corrected to give near diffraction-limited performance on large groundbased telescopes in the visible. Reference stars (and reference compact galaxies) fainter than I~17.5 mag may be used routinely. This paper will describe how these work, what detector and other hardware is needed and what software should be used to measure the wavefront errors and drive deformable mirror hardware. The software techniques that are used are those routinely applied for MRI and CT imaging. They are fast and relatively easy to implement. The net effect is that imaging systems can be constructed that improve substantially over Hubble resolution from the ground for a relatively modest sum of money.
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