Douglas M. Summers

The W. M. Keck Observatory Laser Guide Star Adaptive Optics System: Overview

The Keck Observatory began science observations with a laser guide star adaptive optics system, the first such system on an 8-10 m class telescope, in late 2004. This new capability greatly extends the scientific potential of the Keck II Telescope, allowing near-diffraction-limited observations in the near-infrared using natural guide stars as faint as 19th magnitude. This paper describes the conceptual approach and technical implementation followed for this system, including lessons learned, and provides an overview of the early science capabilities.

The W. M. Keck Observatory Laser Guide Star Adaptive Optics System: Performance Characterization

The Keck II Telescope is the first 8-10 m class telescope equipped with a laser guide star adaptive optics (LGS AO) system. Under normal seeing conditions, the LGS AO system produces K-band Strehl ratios between 30% and 40% using bright tip-tilt guide stars, and it works well with tip-tilt guide stars as faint as , with partial correction for stars up to a magnitude fainter. This paper presents the algorithms implemented m p 18 R in the LGS AO system, as well as experimental performance results. A detailed error budget shows excellent agreement between the measured and expected image quality for both bright and faint guide stars.

The first laser guide star adaptive optics observations of the galactic center : Sgr A*'S infrared color and the extended red emission in its vicinity

We present the first Laser Guide Star Adaptive Optics (LGS-AO ) observations of the Galactic center. LGSAO has dramatically improved the quality and robustness with which high angular resolution infrared images of the Galactic center can be obtained with the W. M. Keck II 10-meter telescope. Specifically, Strehl ratios of 0.7 and 0.3 at L’[3.8 µm] and K’[2.1 µm], respectively, are achieved in these LGS-AO images; these are at least a factor of two higher and a factor of four to five more stable ag ainst atmospheric fluctuations than the Strehl ratios delivered thus far with the Keck Natural Guide Star AO system on the Galactic center. Furthermore, these observations are the first that cover a large area (76 ′′ × 76 ′′ ) surrounding the central black hole at diffractionlimited resolution for an 8-10 meter class telescope. Durin g our observations, the infrared counterpart to the central supermassive black hole, Sgr A*-IR, showed signific ant infrared intensity variations, with observed L’ magnitudes ranging from 12.6 to 14.5 mag and a decrease in fl ux density of a factor of two over an 8 minute interval. The faintest end of our L’ detections, 1.3 m Jy (dereddened), is the lowest level of emission yet observed for this source by a factor of 3. No significant varia tion in the location of SgrA*-IR is detected as a function of either wavelength or intensity. Previous claim s of such positional variations are easily attributable to a nearby (0. 09 or 720 AU, projected), extended, very red source, which we suggest arises from a locally heated dust feature. Near a peak in its intensity, we obtaine d the first measurement of SgrA*-IR’s K’-L’ color; its K’-L’ of 3.0 ± 0.2 mag (observed) or 1.4 ± 0.2 (dereddened) corresponds to an intrinsic spectral index of � -0.5 ± 0.3 for F� � � � . This is significantly bluer than other recent infrared meas urements from the literature, which suggest � = -4 ± 1. Because our measurement was taken at a time when Sgr A* was�6 times brighter in the infrared than the other measurements, we posit that the spectral index of the emission arising from the vicinity of our Galaxy’s central black hole may depend on the strength of the flare, with stronger flares giving rise to a higher fraction of high energy electrons in the emit ting region. Subject headings:black hole physics ‐ Galaxy:center — infrared:stars ‐ techniques:high angular resolution

Keck Observatory Laser Guide Star Adaptive Optics Discovery and Characterization of a Satellite to the Large Kuiper Belt Object 2003 EL61

The newly commissioned laser guide star adaptive optics system at Keck Observatory has been used to discover and characterize the orbit of a satellite to the bright Kuiper Belt object 2003 EL_(61). Observations over a 6 month period show that the satellite has a semimajor axis of 49,500 ± 400 km, an orbital period of 49.12 ± 0.03 days, and an eccentricity of 0.050 ± 0.003. The inferred mass of the system is (4.2 ± 0.1) × 10^(21) kg, or ~32% of the mass of Pluto and 28.6% ± 0.7% of the mass of the Pluto-Charon system. Mutual occultations occurred in 1999 and will not occur again until 2138. The orbit is fully consistent neither with one tidally evolved from an earlier closer configuration nor with one evolved inward by dynamical friction from an earlier more distant configuration.

Merging Galaxies in GOODS-S: First Extragalactic Results from Keck Laser Adaptive Optics

The Center for Adaptive Optics Treasury Survey aims to combine deep Hubble Space Telescope (HST) images in the optical with deep Keck adaptive optics (AO) data in the near-infrared (NIR) to study distant galaxies, active galactic nuclei, and supernovae. We recently achieved an important new milestone by securing the first Keck laser guide star AO image of faint galaxies. Six galaxies with redshifts ranging from 0.3 to 1.0 were targeted in one pointing in the GOODS-S field. Two are Chandra Deep Field-South sources, XID-56 and XID-536, with complex morphologies suggestive of recent merger activity. Substructures seen in the NIR AO image (FWHM ~01), including multiple tight knots in XID-56 and a double nucleus in XID-536, are confirmed in the optical HST images. These structures are unresolved in the best seeing-limited (~05 FWHM) NIR images. Stellar population synthesis models of the substructures indicate that XID-56 is a gas-rich merger with a recent burst of star formation and significant amounts of dust. XID-536 appears to be a merger of two evolved stellar populations.

Implementation of a laser traffic control system supporting laser guide star adaptive optics on Mauna Kea

The idea of achieving Adaptive Optics over the majority of the sky using sodium laser guide stars is reaching maturity on Mauna Kea. However, Mauna Kea is a shared astronomical site with 13 institutions and 11 telescopes. Coordination between observatories with laser guide stars and facilities without laser guide stars must be accomplished to prevent sodium light (Rayleigh scatter and the laser guide star itself) from interfering with science observations at the non-laser facilities. To achieve this goal, a technical working group was organized with participation from several Mauna Kea observatories to discuss and agree upon an automated system for avoiding laser “beam” collisions with other telescopes. This paper discussed the implementation of a Laser Traffic Control System (LTCS) for Mauna Kea including a brief history of the coordination effort, technical requirements and details surrounding implementation of laser beam avoidance software, critical configuration parameters, algorithmic approaches, test strategies used during deployment, and recommendations based upon experiences to date for others intending to implement similar systems.

Laser guide star adaptive optics at the Keck Observatory

This paper describes the upgrades to the Keck II Adaptive Optics (K2 AO) system needed for laser guide star observing. The upgrade, including integration with the laser, is scheduled for completion in the winter of 2003. This upgrade includes the addition of a Low Bandwidth Wavefront Sensor (LBWFS) measuring focus and higher order terms, and a Lawrence Livermore National Lab quad-lens avalanche photodiode detector which monitors tip/tilt. Both observe a dim natural guide star. LBWFS corrections are applied as corrections to the high bandwidth wavefront sensor, which is observing the laser beacon. These subsystems drive focus stages, a deformable mirror, a tip/tilt mirror for the incoming starlight, and a tip/tilt mirror for pointing the propagating laser beam. Taken together, and in concert with the rest of the components of the K2 AO system, they provide the tools and the means to observe the universe as never before.

Keck laser guide star adaptive optics: science verification results

In this paper we describe the operational strategy and performance of the Keck Observatory laser guidestar adaptive optics system, and showcase some early science verification images and results. Being the first laser guidestar system on an 8-10 m class telescope, the Keck laser guidestar adaptive optics system serves as a testbed for observing techniques and control algorithms. We highlight the techniques used for controlling the telescope focus and wavefront sensor reference centroids, and a wavefront reconstructor optimized for use with an elongated guidestar. We also present the current error budget and performance of the system on tip-tilt stars to magnitude R=17. The capability of the system to perform astronomical observations is finally demonstrated through multi-wavelength imaging of the Egg proto-planetary nebula (CRL 2688).

Operational concept of the VLT's adaptive optics facility and its instruments

The ESO Adaptive Optics Facility (AOF) will transform UT4 of the VLT into a laser driven adaptive telescope in which the corrective optics, specifically the deformable secondary mirror, and the four Laser Guide Star units are integrated. Three instruments, with their own AO modules to provide field selection capabilities and wavefront sensing, will make use of this system to provide a variety of observing modes that span from large field IR imaging with GLAO, to integral field visible spectroscopy with both GLAO and LTAO, to SCAO high Strehl imaging and spectroscopy. Each of these observing modes carries its specific demands on observing conditions. Optimal use of telescope night-time, with such a high in demand and versatile instruments suite, is mandatory to maintain and even improve upon the scientific output of the facility. This implies that the standard VLT model for operations must be updated to cover these partly new demands. In particular, we discuss three key aspects: (1) the need for an upgrade of the site monitoring facilities to provide the operators with real-time information on the environmental conditions, including the ground layer strength, and their evolution throughout the night; (2) a set of tools and procedures to effectively use these data to optimize the short-term scheduling (i.e. with granularity of one night) of the telescope and (3) the upgrade of the current laser beam avoidance software to better cope with the AOF operational scheme, where the four laser units are continuously operated as long as the atmospheric conditions allow.

Angular anisoplanatism in laser guide star adaptive optics

The image quality obtained using laser guide star adaptive optics (LGS AO) is degraded by the fact that the wavefront aberrations experienced by light from the LGS and from the science object differ. In this paper we derive an analytic expression for the variance of the difference between the two wavefronts as a function of angular distance between the LGS and the science object. This error is a combination of focal anisoplanatism and angular anisoplanatism. We show that the wavefront error introduced by observing a science object displaced from the guide star is smaller for LGS AO systems than for natural guide star AO systems.