W. M. Keck Observatory (WMKO) is currently engaged in the design of a powerful new Adaptive Optics (AO) science
capability providing precision correction in the near-IR, good correction in the visible, and faint object multiplexed
integral field spectroscopy. Improved sensitivity will result from significantly higher Strehl ratios over narrow fields (<
30" diameter) and from lower backgrounds. Quantitative astronomy will benefit from improved PSF stability and
knowledge. Strehl ratios of 15 to 25% are expected at wavelengths as short as 750 nm. A multi-object AO approach
will be taken for the correction of multiple science targets over modest fields of regard (< 2' diameter) and to achieve
high sky coverage using AO compensated near-IR tip/tilt sensing. In this paper we present the conceptual design for this
system including discussion of the requirements, system architecture, key design features, performance predictions and
We present a concept to perform low-order wavefront sensing in multi-laser guide star adaptive optics systems
operating using a large format NIR detector with windowing capability with near diffraction limited or partially
corrected NGS tip-tilt stars with time varying Strehls. Most contemporary adaptive optics systems in development
for large telescopes, viz., the next VLT adaptive optics facility that serves as a pathfinder to the European
ELT, Gemini MCAO, W. M. Keck observatory's Next Generation Adaptive Optics (NGAO) System, The Large
Binocular Telescope and the Thirty Meter Telescope's NFIRAOS are multi-laser guide star systems that provide
AO correction over a large field. In such systems even faint tip-tilt (TT) stars image are characterized by either
a well corrected (MOAO case) or at least a partially corrected (MCAO or GLAO case) diffraction limited core
due to high order sharpening by the LGS WFS. In such a regime of low-order sensing one could envisage using
pixels as field stops and choosing a appropriate plate scale to minimize the sky background.
Simulations are used to predict the performance of such a sensor when guiding on point sources and on
extended objects of varying brightness and for different levels of high order correction.
The parameter space explored includes tip-tilt and tip-tilt, focus and astigmatism (TTFA) sensor performance
for various plate scales, TT sensor performance vs. level of high order correction (TT star Strehl) and TT sensor
performance vs. TT object size for a given detector noise, gain and a simple centroiding algorithm. Due to small
sky noise contribution at plate-scales le 100 mas/pixel, the optimum low-order wavefront sensor plate scale is
found to be 80-100 mas/pixel (3×-4× λ/d in J- and H- bands) for the Keck NGAO system.
A laser guide star (LGS) adaptive optics (AO) system generally requires additional tip/tilt information derived using a
natural guide star (NGS), while multi-LGS systems will benefit from measurement of additional low-order wavefront
modes using one or more NGS's. If we use AO sharpened NGS's, we can improve both the measurement accuracy and
accessible sky fraction while also minimizing the observational overhead of faint NGS acquisition. Multi-object
adaptive optics (MOAO) sharpening of NGS is possible where a good estimate of the NGS wavefront can be made, for
example where tomographic wavefront information is available. We describe a new approach for high Strehl ratio
sharpening, based on additional patrolling laser beacons, to eliminate generalized anisoplanatism and minimize
tomography error in the NGS direction.
We have built and field tested a multiple guide star tomograph with four Shack-Hartmann wavefront sensors. We predict the wavefront on the fourth sensor channel estimated using wavefront information from the other three channels using synchronously recorded data. This system helps in the design of wavefront sensors for future extremely large telescopes that will use multi conjugate adaptive optics and multi object adaptive optics. Different wavefront prediction algorithms are being tested with the data obtained. We describe the system, its current capabilities and some preliminary results.
In this paper, we provide an overview of the adaptive optics (AO) program for the Thirty Meter Telescope (TMT) project, including an update on requirements; the philosophical approach to developing an overall AO system architecture; the recently completed conceptual designs for facility and instrument AO systems; anticipated first light capabilities and upgrade options; and the hardware, software, and controls interfaces with the remainder of the observatory. Supporting work in AO component development, lab and field tests, and simulation and analysis is also discussed. Further detail on all of these subjects may be found in additional papers in this conference.
An efficient adaptive optics (AO) simulation code was developed, which enables first-order simulations of extremely high-order systems. The Monte-Carlo-type code employs a sparse conjugate gradient algorithm for wavefront reconstruction, and a separation of spatial frequencies into two domains to economize on the number crunching. High-order multi-conjugate adaptive optics (MCAO) systems are thereby readily simulated on a single standard PC. The code is presently being applied to MCAO design studies for the Euro50 extremely large telescope (ELT), addressing a number of issues not previously subjected to realistic simulation due to the excessive computational load. We report in this paper on the latest results obtained from simulating two specific aspects of the Euro50 AO system: turbulence prediction and laser guide star (LGS) beacon sythesis. The two studies presented here are represetative examples of a number of technology studies being enabled by the new fast simulation codes.
Among the science challenges of the Extremely Large Telescopes (ELTs), four object types are studied for performance with a 50 m ELT with adaptive optics (AO), Euro50. Emphasis is on planetary systems and very distant objects. For planetary systems and their evolution, we examine high resolution imaging of the nuclei of comets and high-resolution imaging, photometry and low and intermediate resolution spectroscopy of Kuiper-Belt objects. Imaging of Earth-like planets is discussed. The very high contrast imaging necessary for these purposes is discussed together with the relevant error sources. Finally, photometry and classification of supernovae is discussed and examined. The performance of a 50 m AO ELT is compared to corresponding data obtainable with current VLTs equipped with AO.
In the near future several astronomical observatories in Chile are planning to use sodium laser guide stars to increase the sky coverage provided by their adaptive optics facilities. Knowledge of the mesospheric sodium layer behavior is crucial to predict the performance of future laser guide star adaptive optics systems. Whereas the sodium layer has been observed quite extensively at several locations, many of them in the Northern Hemisphere, very little measurements have been made in Chile. The Gemini Observatory therefore initiated a year-long sodium monitoring campaign at the Cerro Tololo Inter-American Observatory located only a few kilometers away from the Gemini South telescope where a conventional laser guide star facility will be offered to the community in 2005, soon to be upgraded to a multi-conjugate adaptive optics system with five laser guide stars. This paper reports on the laser-based sodium monitoring experimental set up and data reduction techniques, and presents some preliminary results on the sodium column density and layer altitude variations observed from February 2001 to February 2002. Implications for the Gemini South Adaptive Optics system expected performance are presented as well.
Multi-Conjugate Adaptive Optics (MCAO) holds the promise of moderate to large adaptively compensated field of view with uniform image quality. This paper is a first effort to analyze the fundamental limitations of such systems, and that are mainly related to the finite number of deformable mirrors and guide stars. We demonstrate that the ultimate limitation is due to the vertical discretization of the correction. This effect becomes more severe quite rapidly with increasing compensated field of view or decreasing wavelength, but does not depend at first order on the telescope aperture. We also discuss limitations associated with the use of laser guide stars and ELT related issues.
This paper focuses on two main categories of the multiconjugate adaptive optics (MCAO) parameter space for performance optimization: the geometrical configuration of guide stars and deformable mirrors (DMs), and the wavefront reconstructors. From the first category it is shown how, for a fixed reconstructor and imaging wavelength, the performance metrics with a few important exceptions improve with an increasing number of (1) DMs, (2) actuators per DM and (3) guide stars. The metrics are seen to degrade with (1) an increasing field of compensation and (2) DM conjugation altitude mismatch with the significant turbulent atmospheric layers. In the second category, this study also compares the performance with a fixed MCAO configuration using the least- square estimator (LSE) and the maximum a posteriori estimator (MAP) for wavefront reconstruction. The MAP is shown to perform significantly better than the LSE at low or intermediate signal-to-noise ratios (SNRs), and somewhat better even in the absence of noise due to its a priori knowledge of the phase statistics.