Proceedings Volume 4007

Adaptive Optical Systems Technology

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Proceedings Volume 4007

Adaptive Optical Systems Technology

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Volume Details

Date Published: 7 July 2000
Contents: 14 Sessions, 117 Papers, 0 Presentations
Conference: Astronomical Telescopes and Instrumentation 2000
Volume Number: 4007

Table of Contents

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Table of Contents

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  • Wavefront Compensation
  • Wavefront Sensing
  • Lasers: Technology, Techniques, and Monitoring
  • Systems Under Development
  • Multiconjugate
  • Lasers: Technology, Techniques, and Monitoring
  • Systems Under Development
  • Lasers: Technology, Techniques, and Monitoring
  • Systems Under Development
  • Wavefront Compensation Optimization
  • Multiconjugate
  • Wavefront Compensation Optimization
  • Wavefront Sensing
  • Wavefront Compensation Optimization
  • Atmospheric Characterization and Modeling
  • Wavefront Compensation Optimization
  • Systems Under Development
  • Wavefront Compensation Optimization
  • Atmospheric Characterization and Modeling
  • Systems on the Sky
  • Prospects
  • Astronomical Science
  • Image Analysis and Deconvolution
  • Atmospheric Characterization and Modeling
  • Image Analysis and Deconvolution
  • Astronomical Science
  • Image Analysis and Deconvolution
  • Astronomical Science
  • Image Analysis and Deconvolution
  • Astronomical Science
  • Atmospheric Characterization and Modeling
  • Image Analysis and Deconvolution
  • Instrumentation for Astronomy with AO
  • Systems on the Sky
  • Systems Under Development
  • Lasers: Technology, Techniques, and Monitoring
  • Systems Under Development
  • Systems on the Sky
  • Systems Under Development
  • Solar
  • Multiconjugate
  • Systems Under Development
  • Systems on the Sky
  • Systems Under Development
  • Solar
  • Systems Under Development
  • Solar
  • Systems on the Sky
  • Systems Under Development
  • Astronomical Science
  • Multiconjugate
  • Extremely Large Telescopes
  • Atmospheric Characterization and Modeling
  • Lasers: Technology, Techniques, and Monitoring
  • Wavefront Compensation Optimization
Wavefront Compensation
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Adaptive secondary mirror for the 6.5-m conversion of the Multiple Mirror Telescope: latest laboratory test results of the P36 prototype
Armando Riccardi, Guido Brusa-Zappellini, Valdemaro Biliotti, et al.
The 336-actuator adaptive secondary unit (MMT336) for the new MMT is being assembled in Italy and will be delivered in June 2000 for the acceptance test at Steward Observatory (Tucson, AZ). The latest results obtained on a reduced-size (36 actuators) prototype called P36 are reported, confirming a settling time less than 1 ms measured in previous tests. The flattening procedure has been successfully tested on the P36 unit, reducing the initial surface error of 1.1 micrometer down to 43 nm rms. Moreover the dynamical tests on the P36 unit show that the system is able to attenuate the atmospheric-induced error from 466 nm to 31 nm rms. This in the case of median seeing condition at MMT (0.75 arcsec) a high wind speed (48 m/s) and a 1 kHz command rate per actuator. Finally, in the same conditions the atmospheric error is effectively attenuated up to a frequency of 100 Hz (OdB attenuation level).
Adaptive secondary mirror developments at UCL
The Optical Science Laboratory (OSL) has been involved in the development of large adaptive mirrors that takes a novel approach using metal mirror technology. This paper details the latest developments and preliminary test results of a prototype technology demonstrator.
Novel spatial light modulators for active and adaptive optics
We present here results of laboratory and field experiments using two novel active optic elements, an electro-static membrane mirror, and a dual frequency nematic liquid crystal. These devices have the advantage of low cost, low power consumption, and compact size. Possible applications of the devices are astronomical adaptive optics, laser beam control, laser cavity mode control, and real time holography.
Wavefront conjugation using electron-gun-controlled piezoelectric materials
George C. Nelson, John A. Main, Jeffrey W. Martin
Current adaptive optics designs often rely upon multiple actuators to mechanically deform mirrored surfaces. The spatial resolution of control is dependent upon the physical size of these actuators and the number of actuators present. Piezoelectric materials may be used for actuation however the classical control methods of these materials also rely on discrete areas of actuation and require lead wiring for each electrode, patch, or stack utilized. Electron gun control of piezoelectric materials eliminates the need for discrete, segmented electrodes and their associated lead wiring. This method also holds the potential for much finer control resolution since the restraining parameter is beam size. Lightweight piezoelectric ceramics may possibly be used as wavefront conjugating mirrors. Piezo-ceramic plates can be actuated with an electron gun and a single distributed electrode of optical quality. The electron gun functions as a pointing device while varying the potential, referred to as backpressure, of the single electrode controls the magnitude of actuation. By using this method, future corrective optics may significantly surpass current design performance without significantly increasing system complexity.
Microdeformable mirror for adaptive optics systems on extremely large telescopes
Different research groups around the world are currently involved in the design of extremely large optical telescopes, ranging from 30 to 100 m. In order to restore diffraction limited images, this next generation of telescopes highly relies on upgraded adaptive optics (AO) systems. For an efficient AO system, high-order perturbation correction is needed. This can be achieved by the development of new technologies based on micro-opto-electro-mechanical systems (MOEMS). We describe this new technology and analyze the abilities of the main candidates to reach the specifications required. By listing the key parameters, we establish advantages and disadvantages of different MOEMS architectures. In order to characterize these micro-optical components, we are developing a bench set-up for electrical and optical tests, including a new surface characterization method based on the knife-edge test.
Optical fabrication of the MMT adaptive secondary mirror
Hubert M. Martin, James H. Burge, Ciro Del Vecchio, et al.
We describe the optical fabrication of the adaptive secondary mirror for the MMT. The 640 mm f/15 secondary consists of a flexible glass shell, 1.8 mm thick, whose shape is controlled by 336 electromagnetic actuators. It is designed to give diffraction-limited images at a wavelength of 1 micron. For generating and polishing, the shell was supported by attaching it to a rigid glass blocking body with a thin layer of pitch. It could then be figured and measured using techniques developed for rigid secondaries. The highly aspheric surface was polished with a 30 cm stressed lap and small passive tools, and measured using a swing-arm profilometer and a holographic test plate. The goal for fabrication was to produce diffraction-limited images in the visible, after simulated adaptive correction using only a small fraction of the typical actuator forces. This translates into a surface accuracy of less than 19 nm rms with correction forces of less than 0.05 N rms. We achieved a surface accuracy of 8 nm rms after simulated correction with forces of 0.02 N rms.
LBT adaptive secondary preliminary design
Daniele Gallieni, Ciro Del Vecchio, Enzo Anaclerio, et al.
We report on the design of the two Gregorian adaptive secondary mirrors of the Large Binocular Telescope. Each adaptive secondary is a Zerodur shell having an external diameter of 911 mm and a thickness of about 1.5 mm. The deformable mirror is controlled by a pattern of 918 electromagnetic actuators. Its shape is referred to a stable ULE back plate by means of capacitive sensors co-located to the actuators pattern. The preliminary design of the system is addressed with particular attention to the reference plate optimization.
Numerical simulations of the LBT adaptive secondary mirror
In this paper we describe the design of the deformable mirror of the Large Binocular Telescope adaptive secondary unit. Starting from the optical design, a numerical model of the ultra-thin, aspherical glass shell, accommodating the 918 magnets on the selected actuator geometry, has been run. As a result, we can evaluate the response of this crucial component of the telescope optics with great accuracy. The DM is analyzed from the mechanical standpoint -- gravity deformations, wavefront residue, residue of low-order Zernike aberrations, corrections of magnetic interactions -- in order to compute the optical performances in the most demanding operational circumstances.
Adaptive secondary for the 2.1-m telescope at SPM Observatory
Arturo I. Iriarte Valverde, Salvador Cuevas, J. Elon Graves, et al.
Atmospheric turbulence distorts the wavefront of the incoming light from an astronomical object and so limits the ability of a telescope to form a perfect image. The AO systems for astronomy had come the most powerful tool for infrared observation in the near thermal domain. A conventional AO system requires quite a few reflections that are needed to transfer and correct an image. A typical system would have a collimator, deformable mirror and a camera at the bare minimum. For the thermal region the gains are substantial where one can eliminate extra optical surfaces and their associated thermal background, that occurs when you put the deformable mirror at the secondary. We study the possibility of development an adaptive secondary with the techniques of a Current Bimorph mirror with the necessaries number of actuators for control the edge slope. Also we simulate the performance of a 19 channels curvature adaptive optics system in order to demonstrate the gain achievable with an adaptive secondary. The adaptive secondary for the 2.1 m Telescope at SPM Observatory is designed for a f/50 beam, 100 mm in diameter with 19 actuators necessaries to control the edge slope and curvature.
50-mm MEMS deformable mirror
Sigmund Manhart, Werner J. Hupfer, Susanne Nikolov, et al.
A 50 mm diameter micromachined membrane mirror was designed and manufactured at TU Delft/DIMES in the framework of ESA's technology research program on adaptive optics. Equipped with 39 actuator electrodes the membrane mirror can be electrostatically deformed and used as a wavefront corrector. Functional and performance tests were carried out at Dornier Satellitensysteme GmbH to show its suitability for quasi- static correction of low-order aberrations caused by thermal and gravitational stresses in the future generation of large space telescopes.
Low-cost membrane-type deformable mirror with high-density actuator spacing
A design for a Deformable Mirror (DM) with closely spaced actuators is presented. The DM surface is made of a thin membrane type glass with a thickness of 300 microns. It is supported by a series of piezoelectric actuator tubes with a square grid spacing of 4 mm. A conventional epoxy bond is used to hold the actuators to the membrane, with a small steel ball interfacing between the two (at each actuator) for desirable deformation characteristics. The actuators are also bonded to a base structure made of commercially pure Titanium to help athermalize the overall design. The base structure is designed to protect the epoxy bonds from atmospheric moisture. With this design, an actuator could be capable of up to 7 microns of displacement with respect to neighboring actuators, allowing for considerable ability to correct wavefront error in a compact design. Such capability is required to achieve the goals of adaptive coronagraph systems on large, ground based telescopes. Low cost is achieved through the use of inexpensive actuators and a relatively simple fabrication process.
Wavefront Sensing
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NAOS visible wavefront sensor
This paper describes the Visible Wave Front Sensor (visible WFS) for the VLT Nasmith Adaptive Optics system (NAOS). This Shack-Hartman-based wave front sensor instrument includes within a continuous flow liquid nitrogen cryostat: (1) a low noise fast readout CCD camera controlled by the ESO new generation CCD controller FIERA. The readout noise of this system is 3 e- at 50 kilopixel/sec/port, and is only limited by the CCD intrinsic noise. FIERA proposes remotely controlled readout modes with optional binning, windowing and flexible integration time. (2) two remotely exchangeable micro-lens arrays focusing the analyzed wave front directly on the CCD sensitive surface. The wave front sensor includes also its own atmospheric dispersion compensator. Due to the continuous rotation of the NAOS adapter, the mechanical stiffness of the visible wave front sensor must be very high not to disturb the loop operation (no more than 0.1 micrometer of lenslet array displacement compared to the CCD location over a 30 degree rotation angle of the instrument). The following simulations and tests are described: (1) simulation results providing an estimation of the NAOS maximum operating magnitude, (2) camera optimization, (3) mechanical stiffness measurements.
Laboratory test of a pyramid wavefront sensor
A laboratory characterization of a new wavefront sensor for adaptive optics applications called a pyramid sensor is presented. This characterization is aimed at establishing the sensor accuracy and sensitivity. To investigate the operation of the sensor in low and high order correction adaptive optics systems, its behavior for different amplitudes of incoming wavefront aberrations is studied. The sensor characterization is carried out using a two arm optical set-up that allows the comparison of the PS measurements with those of a commercial Fizeau interferometer. This is done when a certain aberration is introduced into the optical path of both instruments via a deformable mirror. The experimental data are analyzed and discussed using both geometrical and diffractive optics theory. The closed loop sensor accuracy is investigated experimentally and demonstrates closed loop wavefront correction down to 30 nm root mean square for starting aberrations whose root mean square ranges from 170 nm to 300 nm. Modal noise propagation coefficients are determined and are compared with Shack-Hartmann sensor coefficients.
EEV CCD39 wavefront sensor cameras for AO and interferometry
Raymond C. DuVarney, Charles A. Bleau, Garry T. Motter, et al.
SciMeasure, in collaboration with Emory University and the Jet Propulsion Laboratory (JPL), has developed an extremely versatile CCD controller for use in adaptive optics, optical interferometry, and other applications requiring high-speed readout rates and/or low read noise. The overall architecture of this controller system will be discussed and its performance using both EEV CCD39 and MIT/LL CCID-19 detectors will be presented. Initially developed for adaptive optics applications, this controller is used in the Palomar Adaptive Optics program (PALAO), the AO system developed by JPL for the 200' Hale telescope at Palomar Mountain. An overview of the PALAO system is discussed and diffraction-limited science results will be shown. Recently modified under NASA SBIR Phase II funding for use in the Space Interferometry Mission testbeds, this controller is currently in use on the Micro- Arcsecond Metrology testbed at JPL. Details of a new vacuum- compatible remote CCD enclosure and specialized readout sequence programming will also be presented.
High-speed frame store CCD for use in optical and near-infrared astronomy
The concept of fully depleted, backside illuminated pn-CCD's with an integrated frame store area for high-speed applications will be presented. The device with a format of 128 by 256 pixel exhibits a readout time faster than 1000 frames per second with an electronic noise of less than 5 electrons. Due to the large sensitive volume of the detector and the development of an ultra-thin radiation entrance window, near theoretical quantum efficiencies are achieved from the ultra violet to the near infrared region. The high speed, low noise and high quantum efficiency makes these devices especially suited for the use as wavefront sensors in adaptive optics systems.
Lasers: Technology, Techniques, and Monitoring
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Update on 589-nm sodium guide star pump laser requirements
John M. Telle, Peter W. Milonni, Robert Q. Fugate
We extend our prior work on continuous-wave and pulsed excitation of sodium guidestars. We discus the effects of atomic recoil, saturation of absorption, and optical pumping on guidestar brightness for the cases of continuous-wave (CW) single-mode and multimode excitation.
Single-mode fiber relay for the ESO laser guide star facility
We report on efficient transportation of visible high power (> 5 W) continuous-wave laser radiation in polarization maintaining, medium-long (> 10 m) optical fibers with single-mode output. Solutions are presented to achieve both maximum launching efficiency of a free-space laser beam as well as efficient reduction of stimulated Brillouin back- scattering together with optimum excitation of mesospheric sodium atoms. The fiber throughput experiments carried out so far with shorter fibers agree well with the theoretical predictions. Single-mode transmission above 70% have been achieved in 5 m and 10 m long test fiber units at input powers of 4.2 W at 532 nm. We do not experience fiber glass damage nor changes in throughput over 200 hours operation, at power densities of 25 MW/cm2. An outlook is given on the final development steps required for successful implementation of a single-mode fiber relay system within ESO's laser guide star facility.
Systems Under Development
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Concept and performance of multiple laser guide stars for 8-m-class telescopes
In this paper, we describe a concept of multiple laser guide stars system based on tomographic reconstruction and multi- conjugate correction for 8-m class telescopes. We show that this type of adaptive optics (AO) systems can be considered as the next generation of AO systems for 8-m telescopes and represents a necessary intermediate step toward AO systems for Extremely Large Telescopes. Multiple guide stars allow to correct the cone effect which affects single LGS systems and prevents from going to wavelengths shorter than approximately 1 micrometer. With 4 LGS plus 1 NGS, it is expected that correction with a high Strehl can be obtained at least in the R and I bands, with an extended corrected field of view (FOV). An analytic AO model is used to assess the expected on-axis performance. Using recent results on 3D mapping of turbulence (i.e. tomography), we estimate the sky coverage of such a system. We also discuss the implications of a large corrected field of view on the system design: large wavefront sensor field, constrains on the optics and deformable mirrors, and size of the science detector. With an MCAO system, large telescopes would be able to observe faint extragalactic objects and wide crowded fields (2 arcmin) at the diffraction limit.
Multiconjugate
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3D mapping of turbulence: theory
In this paper we study a multi-conjugate adaptive optics system which has 4 laser guide stars and 2 deformable mirrors. An interaction matrix describing the whole system is computed, and its eigenmodes are analyzed. The implication of the inability to measure tilt from the artificial guide stars are investigated. We show that a single natural guide star anywhere in the corrected FOV is sufficient to correct the singular modes, provided 5 modes are measured from that guide star. These modes are tilts, defocus and astigmatisms. The performance of this AO system are estimated, taking into account only fitting, aliasing and matrix inversion errors. It is shown that good correction performances are obtained in the visible, with an 8 m class telescope.
Lasers: Technology, Techniques, and Monitoring
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Atmospheric tomography with Rayleigh laser beacons for correction of wide fields and 30-m-class telescopes
James Roger P. Angel, Michael Lloyd-Hart
Single sodium beacons will likely be the most convenient for adaptive systems to correct 6 - 10 m class telescopes over a small field of view (the isoplanatic angle), provided reliable, powerful 589 nm lasers become available and affordable. However, when adaptive optics are applied to extended fields of view and correction of telescopes as large as 32 m diameter, it seems likely that laser beacons produced by Rayleigh scattering will be preferred. For these more demanding applications which require atmospheric tomography, Rayleigh beacons come into their own for two reasons. First, the cone effect, which causes the high turbulence to be sampled at a different scale, is no longer problematic when multiple lasers are used and height dependence is solved for explicitly. Second, the tomographic solution can make use of the beacon created by a laser pulse during all of its journey through the upper atmosphere, not just scattering from a thin layer selected by range gating. In this way a laser that costs an order of magnitude less to buy and maintain than a sodium laser of the same power can yield a brighter beacon and more information about the atmospheric turbulence. This is important because both the number and brightness of beacons or stars must increase with the number of layers included in the tomographic solution. For the same reason, tomography with natural stars is unlikely to be valuable for very large telescopes because in general the number and required brightness of each star increase with corrected field angle, while current narrow-field adaptive optics systems relying on natural stars are already very limited in sky coverage. Our method for tomography to take advantage of Rayleigh scattering over a wide range of heights uses short pulses from near diffraction-limited, ultraviolet lasers, projected from a small aperture above the telescope's secondary mirror. Each pulse subtends less than 1 arcsec at any instant as it travels up through many kilometers. An imaging detector at the main telescope focus conjugate to mid-height is used to record fast movies of the rising pulses as they come into and out of focus. Phase diversity analysis of the movies taken together then yields the three-dimensional turbulence of the atmosphere.
Laser guide star in the form of crossed lines: additional capabilities
Yuri G. Serezhkin, Alexander I. Yakovlev
The capabilities of making of a laser guide star in the form of the crossed lines produced by laser beams into high atmosphere are considered. It is shown, that there is some optimum distance (APEQ 1 km) between a telescope and lasers, at which one the requirements of excitation of a star are optimum. The requirements to energy and time responses of lasers are determined. The estimation of additional opportunities given a two dimensional spatial configuration of a laser guide star for neutralization of distortions, which are produced by atmosphere turbulence, is made. These capabilities are bound with that distortions produced by turbulence on a ray path hill up and downwards have different character and the processing can separate them. The future processing logic of the information should isolate inphase motions of separate regions of a linear LGS describing oscillations of a laser beam at distribution hill up and to reduce to minimum reaction of system correction of a wavefront on these oscillations. The analysis of capabilities of neutralization of an atmospheric turbulence at usage of two- dimensional laser guide star in adaptive optical systems consisting of two telescopes is made.
Models of atmospheric turbulence and reference beacon design
Some problems, connected with the development of ground-based adaptive telescope, particularly, with equipping it with an additional optical system for laser guide star formation, are treated in the paper. Different bistatic schemes for laser guide star formations are analyzed.
New approach to Rayleigh guide beacons
Michael Lloyd-Hart, Stuart M. Jefferies, E. Keith Hege, et al.
We present analysis and numerical simulations of a new method to sense atmospheric wavefront distortion in real time with Rayleigh beacons. Multiple range-gated images of a single pulse from the laser are used to determine each phase map, providing an advantage over other methods in that photon noise is substantially reduced for a given brightness of the beacon. A laser at about 350 nm projects collimated pulses of light adjacent to the telescope. Rayleigh-scattered light from each pulse is recorded through the full telescope aperture in a sequence of video frames, each a few microseconds long. Images are captured as the pulse approaches and passes through the height at which the camera is focused. Phase diversity is thus naturally introduced between the frames. An iterative algorithm is used to extract the pupil-plane phases from the recorded intensity distributions. We anticipate that such beacons are likely to be valuable in future advanced systems for adaptive optics on very large telescopes with multiple laser beacons and deformable mirrors that aim to provide a large corrected field of view by tomography of the atmospheric turbulence.
Monitoring of atmospheric sodium: new method to get the telluric spectrum and influence of the water lines in the performances of LGS
The monitoring of the column density of atmospheric sodium could be crucial for the efficiency of LGS systems. However, its detection is affected by the presence of water lines around the sodium doublet lines; this is really dramatic in the D2 line. We have developed a new method to detect the D2 sodium telluric line, obtaining previously the solar spectrum clean of telluric contamination. This method does not need any theoretical supposition on the solar line profile. We analyze the influence of the water lines in the detection of the D2 line and discuss the effects on the efficiency of LGSs.
Laser guide star: monitoring and light pollution
Nancy Ageorges, R. Michael Redfern, Francoise Delplancke, et al.
This paper summarizes three years of observations of the resonant optical backscatter of laser, used to produce a mesospheric sodium-layer laser guide star for an adaptive optics system. Observations were obtained from a neighboring telescope. The aim of this work was two-fold: to study the Na plume (altitude and profile variations) and to study the Rayleigh cone in order to achieve scattering measurements relevant to the light pollution created by a sodium laser guide star. We report on the short-term characteristics of the sodium layer and stress the consequences of these variations for Laser Guide Star Adaptive Optics System operations. From the measurements of the background intensity measured while observing the laser guide star and the top of the Rayleigh cone, we can derive information on the light pollution produced by the laser as well as the resulting implications for an observatory laser management policy. Information on the laser intensity, size and shape along the Rayleigh cone are also presented.
Laser beacon for monitoring the mesospheric sodium layer at La Palma
Laurent Michaille, Tony A.D. Canas, Christopher Dainty, et al.
We report the results of the first laser beacon experiment at the astronomical site of La Palma (Canary Islands). A continuous wave low power laser (a few hundreds of mW) system has been set up. The laser, tuned on the sodium D2 line at 589 nm, is launched close to zenith angle. The emission of the mesospheric sodium layer is observed from a telescope located 160 meters away from the laser. The layer is therefore resolved in altitude and the different features of its dynamics are investigated.
PASS-2: quantitative photometric measurements of the polychromatic laser guide star
We present results from measurements of the return flux from a polychromatic sodium laser guide star produced in Pierrelatte, France during the PASS-2 experiment. In the experiment, photometry of light at 330, 569, 589, and 589.6 nm emitted by mesospheric sodium under two-color laser excitation (569 and 589 nm) was performed. The variation of oscillator and laser configurations as well as simultaneous measurements of the atmospheric coherence length and the mesospheric sodium density permit a comparison of the results with atomic physics models. Using the results, we can determine the setup that produces the maximum return flux from the polychromatic laser guide star. The knowledge gained will be used to aid the ELP- OA project, which has as its goal the design, testing, and implementation of an adaptive optics system that uses a polychromatic laser guide star for wave front tilt measurements.
Systems Under Development
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Preliminary experiments of prototype laser guide star system for the Subaru telescope
Yutaka Hayano, Hideki Takami, Naruhisa Takato, et al.
Prototype laser guide star system for Subaru telescope has been developed at Communications Research Laboratory (CRL). The laser system comprises two commercially available lasers: a modified continuous wave (CW) dye laser and a 10 W all-solid CW laser of 532 nm wavelength for pumping the dye laser. The natural guide star adaptive optics system for Subaru telescope on Cassegrain focus will be upgraded to a laser guide star adaptive optics system using this laser system. The experiment of transmitting laser beam to the sodium layer is performed at CRL using a 1.5 m telescope. The laser beam is emitted from a 20 cm telescope mounted next to the 1.5 m telescope. A laser guide star is observed by a cooled CCD camera, which is equipped on the Nasmyth platform. We report the preliminary results of the experiment of observing a sodium laser guide star.
Lasers: Technology, Techniques, and Monitoring
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Monitoring of the mesospheric sodium layer using a magneto-optical filter
Patrizio Patriarchi, Alessandro Cacciani
Adaptive optics (AO) systems for atmospheric turbulence compensation require a bright reference star for measuring and correcting wavefront distortion. For these reasons a general use of AO requires a laser guide star (LGS) to provide the wavefront information. An artificial guide star can be created by focusing a laser beam tuned to the sodium D2 line at 5890 Angstrom on the mesospheric sodium layer at about 90 km altitude and observing the resonant scattering. The D1 line of the sodium layer can be observed in absorption on early type unreddened (or slightly reddened) stars using a high resolution spectroscopic system. Recent studies have reported that the column density of the layer is temporally variable with timescales from a year down to tens of minutes. Therefore a monitoring of the column density is necessary when observing with sodium LGS. It is demonstrated that, with a sufficiently high spectral resolution, a relatively small (50 cm) telescope observing a bright star can collect in minutes the number of photons necessary to measure the equivalent width of the sodium line with a good signal-to-noise ratio, provided that an efficient spectroscopic device of suitable resolution is attached to it. We propose here a system to monitor the column density of the mesospheric sodium layer which is based on the sodium magneto-optical filter developed by A. Cacciani and already widely used in solar observations since the 70s. This filter is compact, stable in wavelength and profile shape, with high efficiency (up to 40%) and narrow profile width (20 mA). It is based on the magneto-optical activity of the sodium gas embedded in a strong transverse magnetic field. The filter has a central bandpass that can be fitted to the width of the D1 mesospheric line, plus two lateral symmetric bandpasses that observe the adjacent stellar continuum. The D1 equivalent width can be derived by observing the mesospheric sodium line in absorption on the spectrum of a bright early type (O, B, A) star.
ELPOA: data processing of chromatic differences of the tilt measured with a polychromatic laser guide star
In this paper we present an experiment for measuring the chromatic differences of the tilt used for polychromatic laser guide star, a suitable solution to overcome the monochromatic laser guide star limitation: the tilt indetermination. A comparative study between two types of data processing is done: the classical estimation of angle of arrival by the image center of gravity, and a new one: an estimation of tilts by fitting a phase map in the polychromatic case. From these studies, expected precision is derived and comparison between simulations and data is done.
Rayleigh scattering profiles with altitude at Teide Observatory produced by laser
Rayleigh scattering produced in the lower atmosphere by laser launched from the ground, could be very relevant for performances of Laser Guide Stars systems and for light contamination measurements in other close telescopes. We have studied the scattering above Teide Observatory (Canary Islands) produced by a potassium laser. We show measurements during two weeks with different atmospheric transparencies, and got information about the variation of aerosol and dust related with altitude and the impact on other close telescopes.
Systems Under Development
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Adaptive optics with four laser guide stars: cone effect correction on large telescopes
In this paper, we study the performance of an adaptive optics system with 4 laser guide stars (LGS) and a natural guide star (NGS), and compare it to the system with 1 LGS, both installed on an 8-m telescope. Focus anisoplanatism is obtained with a numerical simulation. The typical errors of NGS wavefront are computed with analytical formulae. The entire system is studied to obtain its performance in terms of achievable Strehl ratio. This 4-LGS method allows to push adaptive optics system towards the visible part of the spectrum without tomographic reconstruction of 3D atmospheric perturbation. The cone effect is two times smaller with 4-LGS than with 1-LGS, allowing to reach almost a Strehl ratio of 0.5 at 500 nm. Considering the NGS errors and the focus anisoplanatism, the Strehl ratio (SR) can reach 0.45 at 1.25 micrometer under good seeing (GS) conditions with the Nasmyth Adaptive Optics System (14 X 14 sub-pupils wavefront sensor) at the Very Large Telescope.
Wavefront Compensation Optimization
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Practical approach to modal basis selection and wavefront estimation
The MPIA/MPE adaptive optics with a laser guide star system ALFA works excellent with natural guide stars up to 13th magnitude in R-band. Using fainter natural guide stars or the extended laser guide star, ALFA's performance does not entirely satisfy our expectations. We describe our efforts in optimizing the wavefront estimation process. Starting with a detailed system analysis, this paper will show how to construct a modal basis set which efficiently uses Shack- Hartmann measurements while keeping a certain number of low order modes close to analytical basis sets like Zernikes or Karhunen-Loeve functions. We will also introduce various phase estimators (least squares, weighted least squares, maximum a posteriori) and show how these can be applied to the ALFA AO. A first test done at the Calar Alto 3.5-m-telescope will be discussed.
Linear zonal atmospheric prediction for adaptive optics
Patrick C. McGuire, Troy A. Rhoadarmer, Hanna Avriel Coy, et al.
We compare linear zonal predictors of atmospheric turbulence for adaptive optics. Zonal prediction has the possible advantage of being able to interpret and utilize wind-velocity information from the wavefront sensor better than modal prediction. For simulated open-loop atmospheric data for a 2- meter 16-subaperture AO telescope with 5 millisecond prediction and a lookback of 4 slope-vectors, we find that Widrow-Hoff Delta-Rule training of linear nets and Back- Propagation training of non-linear multilayer neural networks is quite slow, getting stuck on plateaus or in local minima. Recursive Least Squares training of linear predictors is two orders of magnitude faster and it also converges to the solution with global minimum error. We have successfully implemented Amari's Adaptive Natural Gradient Learning (ANGL) technique for a linear zonal predictor, which premultiplies the Delta-Rule gradients with a matrix that orthogonalizes the parameter space and speeds up the training by two orders of magnitude, like the Recursive Least Squares predictor. This shows that the simple Widrow-Hoff Delta-Rule's slow convergence is not a fluke. In the case of bright guidestars, the ANGL, RLS, and standard matrix-inversion least-squares (MILS) algorithms all converge to the same global minimum linear total phase error (approximately 0.18 rad2), which is only approximately 5% higher than the spatial phase error (approximately 0.17 rad2), and is approximately 33% lower than the total 'naive' phase error without prediction (approximately 0.27 rad2). ANGL can, in principle, also be extended to make non-linear neural network training feasible for these large networks, with the potential to lower the predictor error below the linear predictor error. We will soon scale our linear work to the approximately 108-subaperture MMT AO system, both with simulations and real wavefront sensor data from prime focus.
Multiconjugate
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Comparison of multiconjugate adaptive optics configurations and control algorithms for the Gemini South 8-m telescope
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.
Wavefront Compensation Optimization
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Compensating for pupil rotation in the W.M. Keck Observatory adaptive optics system
The non-circular shape of the Keck telescopes primary mirror, combined with the fact that the telescope is on an alt/az mount with the adaptive optics system on the left Nasmyth platform, means that as the telescope tracks a star field the hexagonal pupil image rotates on the wavefront sensor camera and on the deformable mirror. This creates a need for a dynamic control strategy. The control matrix, the list of which subapertures are sufficiently illuminated and the list of which actuators are active and which are slaved, all must change on the fly, with the loops closed and without perturbing a science observation in progress. This control strategy must also be robust in the presence of modest pupil translation. These needs are met in the Keck AO system by the Pupil Rotation Compensation (PRC) software tool. The PRC software is responsible for maintaining system parameters that are functions of the pupil position and orientation. This paper will present the details of the PRC software design including algorithms used for selecting active subapertures, dealing with variable subaperture illumination, calculating control matrices, and slaving actuators outside the pupil illuminated area on the deformable mirror.
Centroid gain compensation in a Shack-Hartmann adaptive optics system: implementation issues
In an adaptive optics system with an undersampled Shack- Hartmann wavefront sensor (WFS), variations in seeing, laser guide star quality, and sodium layer thickness and range distance all combine to vary WFS centroid gain across the pupil during an exposure. While using the minimum of four pixels per WFS sub-aperture improves frame rate and read noise, the WFS centroid gain uncertainty may introduce static aberrations and degrade servo-loop phase margin. In a recent paper, we have presented a novel method to estimate and compensate WFS gains of each sub-aperture individually in real time for both natural and laser guide stars. In this paper, we address additional issues related to the implementation of this method in a real system such as Altair for Gemini North.
Wavefront Sensing
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Gemini prime focus wavefront sensor
A Prime Focus Wavefront Sensor (PFWFS) has been designed and built at the Gemini Observatory. The system contains a Shack- Hartmann (SH) wavefront sensor and has been designed to use commercial components. The primary mirror of the 8 m Gemini Telescope has a complex active optics system that needs to be calculated during commissioning. The wavefront sensor was built to measure the image quality at prime focus, this eliminates the secondary mirror introducing supplementary aberrations. It has been successfully used during commissioning, to test the active optics.
Testing the pyramid wavefront sensor on the sky
The pyramid wavefront sensor is a novel concept device whose features are attractive for adaptive optics for several reasons. We show here the first loop closure of an AO system using this kind of sensor at the focal plane of a 4m-class telescope. One of the critical optical elements of our wavefront sensor is the pyramid that splits the light from the star used for the wavefront correction. This component is essentially a four faces prism having actually a full vertex angle of 7 degrees with specifications on its edges and roof of 4 - 5 microns or better. The best turned edges obtained on the prototypes already built have shown values of the order of 6 microns, with roofs of the same order, not far from the required tolerances. In this article we describe the techniques and the system used for the construction of this optical component and the improvements to the polishing procedure that we plan to adopt in order to increase the quality of its edges and optical surfaces. Pixel processing is suitable to fit with existing Shack-Hartmann systems, making this device an attractive add-on option for existing SH-based AO systems. The plans for future developments in order to firmly establish the performances of the pyramid wavefront sensor are briefed out.
Extremely high resolution tip-tilt-piston mirror mechanism for the VLT-NAOS field selector
Peter Spanoudakis, Lorenzo Zago, O. Chetelat, et al.
NAOS (Nasmyth Adaptive Optics System) is the adaptive optics system presently developed for the ESO VLT. The field selectors are to feed the NAOS wavefront sensor with the light coming from an appropriate reference source which can be up to 1 arcmin (on the sky) distant from the center of the field of view. A large input tip-tilt-piston mirror selects the required part of the field of view. A second active mirror redirects the selected field to the wavefront sensor. The displacement of both mirrors are synchronized. The NAOS Field Selector consists of two extremely accurate tip-tilt-piston mirror mechanisms controlled in closed loop. Each mechanism provides a mechanical angle amplitude of +/- 6 degrees with a resolution and mechanical stability of 0.42 arcsec rms over 20 minutes. This implies a dynamic range of 100,000 which requires an extremely accurate, very high resolution closed loop control. Both mirrors are made in SiC for low mass and inertia. The design configuration of the mechanism in based on three electromagnetic actuators 120 degrees apart with the mobile magnets mounted on flexure guides. The mirror is supported by a combination of flex pivots and a membrane for flexibility in tilt and high radial stiffness. All kinematic joints consist of flexure elements so that the mechanism is essentially frictionless. The control system is implemented on a VME bus operated with the VXWorks OS with high electrical resolution (>= 18-bit) AD and DA interface boards. The controller has been carefully designed to achieve the best overall performances, i.e., a very good noise rejection, and a relatively low settling time.
Computer simulation comparison of CCDs and APDs for curvature wavefront sensing
In this paper, the results from a computer simulation comparison between curvature AO-systems using APDs and a special-purpose curvature-CCD, the MIT/LL CCID-35, are presented. The simulated AO-system is the MACAO 60-element curvature system, which is being developed by ESO for the Very Large Telescope (VLT) and VLT Interferometer (VLTI). The results indicate a difference in performance of 5% Strehl in K-band for a 15th magnitude guide star, 2% Strehl for a 16th magnitude star, and less than 0.6% Strehl for all other magnitude guide stars.
Apparent wavefront aberration of a Gaussian source in a Hartmann wavefront sensor with focus error
Matthew R. Whiteley
We examined the effect of a Gaussian pupil-plane irradiance profile on the wave-front aberration reported by a Hartmann wave-front sensor with focus error when observing a collimated point source. It was found that even when a planar wave-front is incident upon the Hartmann sensor pupil, centroid deflections are registered in the detector plane due only to the non-uniformity of the Gaussian irradiance profile. As a result, the reconstructed wave-front may contain large focus and spherical aberration terms. The magnitude of the apparent aberrations depends upon the amount of focus error and the width of the Gaussian profile. Results are presented for a model system, indicating aberration terms in excess of (lambda) /3 or equivalent Strehl ratio less than 0.5 for certain configurations.
Wavefront sensing and guiding units for the Large Binocular Telescope
Jesper Storm, Walter Seifert, Svend-Marian Bauer, et al.
The Large Binocular Telescope (LBT) will see first light with a single primary mirror in January 2003. It will be equipped with fully adaptive secondary mirrors from the beginning as well as a complete on-axis wavefront sensing and tip-tilt guiding system. Here we present the preliminary design of the Acquisition, Guiding, and Wavefront sensing system for the LBT. The system is divided in an off-axis system for target acquisition, guiding, and slow wavefront sensing, and an on- axis system for rapid wavefront sensing and tip-tilt guiding. The on-axis system operates on the optical light reflected off a tilted entrance window for the instrument science camera. In this way both the correction to the wavefront (done with the secondary mirrors) and the wavefront sensing (done on the light reflected off the dewar entrance windows) is performed without introducing a single additional optical surface in the science beam. In this way the design follows the lead of the upgraded MMT system. However, the present design differs from the MMT design, in that it does tip-tilt sensing in addition to the rapid wavefront sensing. To enable the system to use a tip-tilt guiding star up to 1 arcmin away from the science target, a significantly larger field of view is required for the on-axis system. The off-axis part of the system can do classical guiding and slow wavefront sensing in parallel which will enable the control system to maintain the optimum setting of the optical system during observations. It will also include a high resolution wavefront sensing mode which will allow quick and detailed checks of the secondary mirrors.
Wavefront Compensation Optimization
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MANO: the modal analysis and noise optimization program for the W.M. Keck Observatory adaptive optics system
In the photon starved regime (which is usually the case in the real world), adaptive optics can benefit from advanced or innovative schemes such as modal control. The concept is quite natural, as one looks for the natural modes (eigen-modes, Karhunen-Loeve) that occur in the atmosphere and that the deformable mirror can produce; once these modes are identified, the loop gain that minimizes the residual phase variance is determined. Because the modes form an orthogonal and independent basis, optimizing each mode individually ensures that the global minimum is found, producing the highest Strehl ratio possible. In the case of Keck Adaptive Optics System, there were two fundamental difficulties that made the modal control scheme difficult. The first one is that the pupil is not circular but hexagonal. The second one is that being a the Nasmyth focus, the (non-circular) pupil rotates on the wavefront sensor. Some sub-apertures are gradually illuminated as others get extinguished, and the natural modes of the system evolve as a function of pupil rotation. In this poster, we describe the first step of building a modal control, which is to determine what the appropriate modes are, and whether they need to be computed in real time, to compensate for pupil rotation. We also show two methods that were tried to optimize the loop gain (random walk and modified Newton-Raphson). These methods were used in zonal control, as a proof of concept. On-sky trials with a modal scheme showed us that the real time mode determination was not practical or necessary, and a set of mode that worked well under any geometry was found: the covariance matrices of these modes had a diagonal that was factors of ten higher than non- diagonal elements indicating that the modes that we computed were in fact independent in the atmosphere. However, the optimizing schemes required too many samples to produce adequate statistics to find the minimum residual phase variance. Changes to the real time code to provide statistics in real time will help to produce a reliable and automatic modal control.
Atmospheric Characterization and Modeling
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Simulating phase screens
Donald R. McGaughey, George J. M. Aitken
The simulation and performance evaluation of high-order, adaptive-optics (AO) corrective systems for the new, large telescopes require the generation of distorted wavefronts (WF) that accurately model the behavior of the real atmosphere. Roggemann et al. have introduced a method of accurately generating distorted wavefronts based on the covariance matrix of wavefront distortions and Cholesky factorization. Several authors have proposed fractional-Brownian-motion algorithms that are more computationally efficient as alternatives for generating distorted wavefronts but do not give accurate correlations. In this paper, the fractional Gaussian process (FGP) algorithm is introduced as a new method of generating distorted wavefronts. The algorithm is presented on both one and two dimensions. The correlation function and power spectra of simulated wavefronts are shown to compare well to the expected correlation and power spectra respectively. It is concluded that the FGP algorithm is an accurate and efficient method of generating simulated wavefronts.
Wavefront Compensation Optimization
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NAOS real-time computer for optimized closed-loop and online performance estimation
Didier Rabaud, Frederic Chazallet, Gerard Rousset, et al.
The Real Time Computer RTC is a key component of the Nasmyth Adaptive Optics System, controlling the 185 actuators of the deformable mirror from a 144 Shack-Hartmann subapertures wavefront sensor at a maximum frequency of 500 Hz. It also provides additional capabilities such as real time optimization of the control loop which is the warranty for NAOS to achieve a very good Strehl Ratio in a broad magnitude range (Mv equals 8 up to 18), on-line turbulence and performance estimations and finally capability to store and process the data necessary to the off-line PSF reconstruction algorithm. This RTC is also designed to be easily upgraded as for Laser Guide Star. Moreover all softwares can be easily adapted to control a curvature sensor as well as the hardware which can be used with the two types of wave front sensors.
Adaptive optics tip-tilt system with fuzzy control
Ruben Flores-Meza, Pablo Diaz Sotelo, Fernando Garfias, et al.
The performance of adaptive optics systems not only depends on its number of actuators and optics quality but also on the performance of the controller used to compensate the wave- front distortions. Due to the temporal bandwidth required to realize a suitable tracking of the atmospheric turbulence dynamics it is necessary that the controller have a short time delay and high stability and robustness indices. A fuzzy logic controller, a technique related with Artificial Intelligence, accomplish all the characteristics aforementioned. In this paper, we present some laboratory tests with the LOLA adaptive optics tip-tilt system in closed loop with a fuzzy controller. In addition, we present some results obtained with LOLA and fuzzy control at the 1 meter Telescope of the Observatorio Astronomico Nacional in Tonantzintla, Peubla, Mexico. We analyze these results with a signal analysis approach such as the power spectrum of the image centroid motion and its correspondent residual variance.
Systems Under Development
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Adaptive optics simulations for imaging with the Large Binocular Telescope interferometer: a first application
Marcel Carbillet, Serge Correia, Bruno Femenia, et al.
In this contribution we present a first application of the ongoing numerical simulations that are carried out in order to study the adaptive optics (AO) correction and the subsequent imaging post-processing when observing with the Large Binocular Telescope (LBT) interferometer. The simulation tool used as a starting point for this study is the software package CAOS 2.0 (Code for Adaptive Optics Systems, version 2.0), for its AO-simulation capabilities and its modular structure. It is used here in order to generate the turbulence-corrupted and subsequently adaptive-optics- corrected interferometric point-spread functions corresponding to the simultaneous observation of both a scientific object and a reference star, for three parallactic angles corresponding to three observation runs during the night. The obtained data are therefore used as the inputs of a multiple deconvolution method planned for imaging with the LBT interferometer. As an example, we have simulated the observation, in the R-band, of a Betelgeuse-like stellar object of 15th magnitude, 30 mas diameter, and with a 3 mas bright spot, under two different conditions of turbulence and AO-correction (leading to Strehl ratios of approximately 0.15 and approximately 0.45, respectively). Final results are found to be very encouraging.
Wavefront Compensation Optimization
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Design and current status of the reconstructor for Altair: the Gemini North adaptive optics system
Altair is the facility Adaptive Optics (AO) system for the Gemini North Telescope on Mauna Kea, Hawaii. Designed to take advantage of the excellent natural seeing conditions that Gemini North will experience, Altair is also unique in that the Deformable Mirror (DM) is conjugate to a fixed altitude of 6.5 kilometers. Running at a control loop speed of at least one kHz, the reconstructor for this high order AO system is subject to a number of conditions that drove its design and implementation. Initial studies indicated that a single RISC CPU would be capable of performing the reconstruction for Altair, as opposed to the more common solution of multiple DSP processors. We present some of these conditions, the results of a throughput benchmark test that verified the choice of hardware, some components of the processing steps of the reconstructor and a summary of the current status of the project.
Wavefront sensor-driven variable-geometry pupil for ground-based aperture synthesis imaging
I describe a variable-geometry pupil (VGP) to increase image resolution for ground-based near-IR and optical imaging. In this scheme, a curvature-type wavefront sensor provides an estimate of the wavefront curvature to the controller of a high-resolution spatial light modulator (SLM) or micro- electromechanical (MEM) mirror, positioned at an image of the telescope pupil. This optical element, the VGP, passes or reflects the incident beam only where the wavefront phase is sufficiently smooth, viz., where the curvature is sufficiently low. Using a computer simulation, I show the VGP can sharpen and smooth the long-exposure PSF and increase the OTF SNR for tilt-only and low-order AO systems, allowing higher resolution and more stable deconvolution with dimmer AO guidestars.
Atmospheric Characterization and Modeling
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Measuring and modeling the low-frequency behavior of atmospheric distortion
Donald R. McGaughey, George J. M. Aitken
The low-frequency behavior of atmospherically distorted wavefronts is not accurately known at this time. It is expected that the phase delays will be a zero mean random process. A new zero-low-frequency model of spatial PSD of the phase distortions is presented in this paper. This model has a low frequency, called the roll-over frequency, for which the PSD starts to approach zero as the frequency decreases and approaches zero. This roll-over frequency is proportional to the outer-scale of turbulence. To verify the zero-low- frequency model, wavefront slope data was analyzed from the GSM experiment with a nonlinear modelling technique called Robust Orthogonal Search (ROS). One property of ROS is that it can detect frequency components at frequencies lower than the resolution of the discrete Fourier transform. This investigation consistently indicates a lowest frequency in the data for which any lower frequency components have lower power. This results supports the low-frequency model which rolls over and approaches zero for low spatial frequencies.
Measurement of atmospheric turbulence with a Shack-Hartmann wavefront sensor at the new MMT's prime focus
Patrick C. McGuire, Maud P. Langlois, Michael Lloyd-Hart, et al.
In order to prepare for the adaptive secondary program at the new 6.5 meter MMT, we have begun a campaign to measure atmospheric turbulence with a Shack-Hartmann wavefront sensor (WFS) camera. Our first efforts, prior to second aluminization of the primary, consisted of taking turbulence data with our WFS camera with special coma-correcting optics at the MMT prime focus, without a Cassegrain secondary mirror. Our first measurements consisted of 1000 frames of approximately 10 millisecond duration, taken approximately 3 seconds apart. We measure a 5/3 power law structure function, suggesting Kolmogorov turbulence, with an r0 equals 15 cm, but with possible hints of an outer scale and tracking jitter in the structure function. At the end of our data acquisition, we deliberately put 2 micrometer of astigmatism into the primary mirror with its actuators, and in our analysis, we recover 1.7 +/- 0.3 micrometer of astigmatism. A brief analysis of the low-order modal amplitudes with the 3 second frame delay shows that there are significant self-correlations of the low-order modes even on this long time-scale.
3D mapping of turbulence: a laboratory experiment
Miska Le Louarn, Christopher Dainty, Carl Paterson, et al.
In this paper, we present the first experimental results of the 3D mapping method. 3D mapping of turbulence is a method to remove the cone effect with multiple laser guide stars and multiple deformable mirrors. A laboratory experiment was realized to verify the theoretical predictions. The setup consisted of two turbulent phase screens (made with liquid crystal devices) and a Shack-Hartmann wavefront sensor. We describe the interaction matrix involved in reconstructing Zernike commands for multiple deformable mirror from the slope measurements made from laser guide stars. It is shown that mirror commands can indeed be reconstructed with the 3D mapping method. Limiting factors of the method, brought to light by this experiment are discussed.
Systems on the Sky
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Performance of the W.M. Keck Observatory Natural Guide Star Adaptive Optic Facility: the first year at the telescope
First light for the Keck II Natural Guide Star (NGS) Adaptive Optics (AO) facility was on the night of February 4, 1999. On the firs attempt at closing the AO loops the image full-width- at-half-maximum (FWHM) went from 0.6 to 0.04 arcsec at H-band (1.65 micrometer wavelength), with a Strehl ratio of 25%. The AO system became an officially scheduled Keck science facility in August 1999; 30 science nights are scheduled in the first half of 2000. The primary purpose of this paper is to present results from this first year at the telescope.
First light for Hokupa'a 36 on Gemini North
J. Elon Graves, Malcolm J. Northcott, Francois J. Roddier, et al.
The University of Hawaii adaptive optics program has recently moved its 36 actuators system, named 'Hokupa'a 36', to the Gemini North Telescope. First light for Hokupa'a 36 was in time for the dedication of this telescope during June 1999 and most of the images presented were taken with this adaptive optics system. This paper will cover the modifications to the CFHT, Hokupa'a 36 system that were necessary to accommodate the larger 8 meter aperture of the Gemini Telescope. Performance at the telescope has now been measured and compares favorably with that predicted.
Prospects
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Prospects for benefits to astronomical adaptive optics from U.S. military programs
This paper outlines potential benefits to the development of astronomical adaptive optics by the transfer of technology from US military laboratories. Areas described include wavefront correctors, focal planes for wavefront sensing and tracking, rugate coatings, lasers for generating sodium guide stars in the mesosphere, the potential of using femtosecond lasers to generate bright, white-light beacons at high altitude.
Astronomical Science
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Review of published galactic and solar system science: a bright future for adaptive optics science
In this review, the current (Feb. 2000) refereed astronomy journals have been surveyed for all AO papers dealing with galactic or solar system astronomy. In the last 6 years, 61 refereed pure science AO papers in solar system and galactic astronomy were selected. Of these 61 papers, 41% have been published in the last 14 months. The vast majority of these papers (63%) utilize broad-band near infrared imaging (J, H & K'). More complex techniques (L band imaging, AO spectroscopy (2D & 3D), polarimetry, coronography) are becoming robust AO techniques in the IR. Of less current published success is visible AO, due in part from the complex technical issues, and also in part from the stiff competition from HST. As well there appears to have been little AO science in the M band. As the latest generation of 6 - 10 m AO systems (and their facility AO cameras/spectrographs) come on-line we should see further robust growth in AO galactic and solar system IR science well into the next decade.
eXtragalactic astronomy: the X-games of adaptive optics
Observing active nuclei, Ultra-Luminous Infrared Galaxies, starburst and merging galaxies, is both a challenge and a requirement for adaptive optics. It is a requirement, because models needed to explain the high infrared flux and the physics of these monsters need constraints that come, in part, from the fine details gleaned on high angular resolution images, and it is a challenge because, being distant, these objects are usually faint in apparent visual magnitude, meaning that the wavefront sensors have to operate in a photon starved regime. Many observations have been controversial in the past, and it is always difficult to tell an artifact such as astigmatism from an inner bar. The importance of observing the point spread function is therefore even more crucial than on bright objects, as PSF reconstruction methods 'a la Veran' break down when the photon noise dominates the statistics of the wave front, or when locking the loop on extended objects. Yet, while some cases have been controversial, some very clear and profound results have been obtained in the extragalactic domain, such as the detection of host galaxy to quasars and star formation studies. It turns out that the fundamental prerequisite to such success stories is a stable, well understood and well calibrated PSF.
Studying the star formation process with adaptive optics
Francois Menard, Catherine Dougados, Gaspard Duchene, et al.
Young Stellar Objects (YSOs) are the builders of worlds. During its infancy, a star transforms ordinary interstellar dust particles into astronomical gold: planets to say the process is complex, and largely unknown to data. Yet, violent and spectacular events of mass ejection are witnessed, disks in keplerian rotation are detected, multiple stars dancing around each other are found. These are as many traces of the stellar and planet formation process. The high angular resolution provided by adaptive optics, and the related gain in sensitivity, have allowed major breakthrough discoveries to be made in each of these specific fields and our understanding of the various physical processes involved in the formation of a star has leaped forward tremendously over the last few years. In the following, meant as a report of the progress made recently in star formation due to adaptive optics, we will describe new results obtained at optical and near- infrared wavelengths, in imaging and spectroscopic modes. Our images of accretion disks and ionized stellar jets permit direct measurements of many physical parameters and shed light into the physics of the accretion and ejection processes. Although the accretion/ejection process so fundamental to star formation is usually studied around single objects, most of young stars form as part of multiple systems. We also present our findings on how the fraction of stars in binary systems evolves with age. The implications of these results on the conditions under which these stars must have formed are discussed.
Image Analysis and Deconvolution
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Deconvolution of adaptive optics images: from theory to practice
Practical guidelines are proposed for deconvolution of partially adaptive optics corrected images: from raw data to high photometric precision object restoration. Preliminary processing is discussed (detector calibration, background subtraction...). The deconvolution scheme itself is then presented. It takes into account the noise statistics in the image, the imprecise knowledge of the point spread function (PSF), and the available a priori information on the object (spatial structure, positivity...). This deconvolution scheme is first validated on simulated images of NAOS, the AO system of the VLT and then applied to astronomical images. In particular, an edge preserving regularization is tested on several solar system objects: Io, Uranus and Neptune.
ESO photometric and astrometric analysis program for AO: a programmatic and numerical analysis
The European Southern Observatory is currently developing an array of software analysis packages to perform Photometry and Astrometry (P&A) on both stellar and diffuse objects observed with Adaptive Optics (AO) Systems. As they are complete, the component programs of ESO-PAPAO will be made available to AO observers using ADONIS on the 3.6 meter telescope at La Silla and, later, those using the various AO systems being developed for the 8.2 meter VLT telescopes at Paranal, such as NAOS- CONICA and MACAO-SINFONI. The performances of the ESO-PAPAO package are being extensively quantified; both to support their use in astrophysical analysis and as a guide for the definition of AO observing programs. The algorithms are being developed in IDL. A user interface allows immediate access to the ESO-PAPAO by observers not familiar with IDL. We will describe the objectives of the ESO-PAPAO; the calibrated ADONIS data sets that have been collected for distribution to contributors to the ESO-PAPAO program, and the methods and results of numerical tests of photometric precision in comparisons of the various different analysis packages. In particular, the STARFINDER program, developed at the University of Bologna in a collaborative effort with ESO, has been applied to data from ADONIS at La Silla, UHAO at Mauna Kea, and HST. Results from the analysis of this astronomical AO data will be presented -- photometric precision of 0.02 to 0.05 magnitudes, and astrometric precision of approximately 0.1 pixel in crowded fields with strong isoplanatic effects.
Atmospheric Characterization and Modeling
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Anisoplanicity studies within NGC6871
Jason A. Marshall, Mitchell Troy, Richard G. Dekany, et al.
Images corrected with adaptive optics benefit from an increase in the amount of flux contained within the diffraction-limited core. The degree of this correction is measured by the Strehl ratio, equal to the ratio of the maximum observed intensity to the maximum theoretical intensity. Natural guide star adaptive optics systems are limited by the need for a guide star of adequate magnitude within suitable proximity to the science target. Thus, the above-described benefit can only be obtained for objects over a fraction of the total sky. Two nights of imaging the central region of the open star cluster NGC6871 with the Palomar Adaptive Optics System has supplied measurements of the Strehl ratio for numerous stars within the field. These measurements were used to calculate K band isoplanatic angles of 39 arcseconds (UT 1999 May 31) and 50 arcseconds (UT 1999 August 1). These isoplanatic angles are compared to those derived from Kolmogorov atmospheric theory, and their implications for adaptive optics systems are discussed.
Image Analysis and Deconvolution
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StarFinder: an IDL GUI-based code to analyze crowded fields with isoplanatic correcting PSF fitting
Emiliano Diolaiti, Orazio Bendinelli, Domenico Bonaccini, et al.
StarFinder is a new code for the deep analysis of stellar fields, designed for well-sampled images with high and low Strehl ratio. It is organized in the form of a self-contained IDL widget-based application, with a 'user-friendly' graphic interface. We give here a general description of the code along with some applications to real data with space-invariant Point Spread Function (PSF). We present also some methods to handle anisoplanatic effects in wide-field Adaptive Optics (AO) observations.
Astronomical Science
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Hypervelocity jets and homuncular motion in eta Carinae: an application of Fabry-Perot, ADONIS, and AO sofware
Douglas G. Currie, David Le Mignant, Birgitta Svensson, et al.
Eta Carinae is an extremely massive and highly evolved member of the Carinae starburst region. It has undergone numerous eruptions over the past millennium. In 1841, a Giant Eruption ejected several solar masses or more of material. Most of this material is currently in the dusty nebula denoted as the 'Homunculus.' ADONIS observations with the Fabry-Perot Interferometer at the Brackett gamma line around 2.16 microns have allowed probing the interior of the major elements of the Homunculus. These elements were not visible in our WFPC observations at the various visible wavelengths. The preliminary results of the analysis of this data have confirmed of the 'Double-Flask' model of the Homunculus; a model that we proposed based upon the Hubble data and spectroscopic data. This approach can also determine the opacity of the Homunculus walls, as well as the total mass and the grain structure of the dust. In a separate recent analysis, small clumps of material have been discovered that were ejected at velocities almost 1% of the speed of light. These 'Malin Bullets' and the material following the bullets, the 'contrails' called the 'Malin Spikes' have been measured using the STARFINDER program. This analysis has discovered about one hundred additional Malin Bullets. This data, as well as other data taken in direct support of the PAPAO program provide new information on the structure of the Homunculus. Inter-comparison of the WFPC data and the ADONIS data is an integral part of the PAPAO program for the validation diffuse astrometry and deconvolution software. Portions of this data will form the database for general community use for the calibration of different programs.
Adaptive optics imaging of Pluto-Charon and the discovery of a moon around the Asteroid 45 Eugenia: the potential of adaptive optics in planetary astronomy
Laird Miller Close, William J. Merline, David J. Tholen, et al.
We outline two separate projects which highlight the power of adaptive optics (AO) to aid planetary research. The first project utilized AO to discover a moon around asteroid 45 Eugenia by use of the PUEO AO facility at CFHT. We also utilized AO to resolve the Pluto-Charon system by producing 0.15' FWHM images. We used the University of Hawaii AO system (Roddier et al. 1991) at CFHT to obtain deep (20 min) narrow band images in and out of the molecular bands of water and methane ices.
Studies of Herbig-Haro objects with the Palomar adaptive optics system
Herbig-Haro objects are bright optical emission-line sources associated with tightly collimated jets ejected from pre-main- sequence stars. Only a few hundred are known. In optical images, they appear to be dense knots of material at the outer ends of the jets, and often exhibit streaming wake morphologies suggestive of bow shocks. Their optical spectra show characteristics of high-velocity shocks, with line-widths typically 100 km/s. HH objects often occur in pairs consistent with the bipolar morphology of outflows from YSOs; when radio maps of NH3 are made, high-density central regions consistent with collimating disks are seen. HH objects also often appear in a series along a jet, presumably where the jet undergoes a particularly energetic interaction with the ambient medium. Adaptively-corrected near-infrared studies of HH objects can reveal much about their workings at fine spatial scales. Narrow-band NIR filters sensitive to transitions of molecular hydrogen and other selected species are excellent tracers of shock excitation, and many HH objects have been observed to show complex structure in these lines down to the arc second level. By pushing to higher spatial resolution with adaptive optics, much more detailed information about the nature of the shock fronts may be obtained. In this paper we describe our first observations of HH objects with the AO system on the Palomar 200-inch telescope.
Image Analysis and Deconvolution
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Digital adaptation algorithms of adaptive optics corrected images
Sergey D. Polskikh, Konstantin N. Sviridov
The technology is considered of space object image obtainment with high angular resolution, based on the adaptive tuning of image spatial spectra (digital adaptation), corrected by adaptive optics. As the basis of the technology, the algorithm is taken of the integral equation of the I-st kind of convolution type with unknown core and imprecisely given right part. It's shown the procedure of the inverse operator construction for this equation solution is connected with minimization of nonlinear regularizing multiextremel functionals and could be realized on the base of global optimization methods. The structure of multiextremel functionals is analyzed, and the main global extremum search methods are researched. It is shown, that as the basis of the optimal construction of the channel for the obtainment of images with high resolution, the principle must be taken of the sequential reduction of the global extremum search space dimensionality, and what's more, the predetector processing of the wavefront by the adaptive optics is the first stage of this reduction. The results are given of numerical modelling including the examples of the distorted and restorated images of model objects under different signal-to-noise ratios.
Astronomical Science
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Impact of adaptive optics on star formation research
In this paper, we discuss the benefits of ground-based, adaptive optics (AO) aided observations for star formation research. After outlining the general advantages, we present results obtained during the ALFA science demonstration program in 1999. These results underline the absolute necessity of AO assistance for almost any kind of observations regarding star formation regions.
Image Analysis and Deconvolution
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Stability of the adaptive-optic point spread function: metrics, deconvolution, and initial Palomar results
Eric E. Bloemhof, Kenneth Alan Marsh, Richard G. Dekany, et al.
The essential benefit of adaptive optics is delivering a telescope point-spread function (PSF) limited by diffraction rather than by atmospheric turbulence. In practice, achieving diffraction-limited PSF diameters is relatively routine for modern high-order systems, at least within a restricted isoplanatic patch containing the guide star. The lower- intensity wings of the PSF, though, are often highly complex in their structure and subject to variability over short time scales. Spurious bright knots can occur among the secondary Airy maxima, and the 'waffle-mode' artifact may be problematic with a broad class of adaptive optics approaches. Good temporal stability of the adaptive-optic PSF is generally highly desirable if the full advantage of that spectacular PSF is to be realized; it is absolutely critical for many specific high resolution programs that can only be attempted with adaptive optics. In the course of commissioning the high-order adaptive optics system built at JPL for the Palomar 200-inch telescope, we have investigated PSF stability in the field under a variety of conditions. We discuss here our experimental findings at Palomar, and their implications for some key scientific programs.
Companion detection limits with adaptive optics coronagraphy
Ben R. Oppenheimer, Richard G. Dekany, Thomas L. Hayward, et al.
We presented a detailed observational study of the capabilities of the Palomar Adaptive Optics System and the PHARO near infrared camera in coronagraphic mode. The camera provides two different focal plane occulting masks consisting of completely opaque circular disks of diameter 0.433 arcsec and 0.965 arcsec, both within the cryogenic dewar. In addition, three different pupil plane apodizing masks (a.k.a. Lyot masks) are provided which downsize the beam. The six different combinations of Lyot mask and focal plane mask provide for different levels of suppression of the point spread function of a bright star centered on the focal plane mask. We obtained images of the bright nearby star Gliese 614 with all six different configurations in the K-band filter. Herein, we provide an analysis of the dynamic range achievable with these configurations. The dynamic range (the ratio of the primary star intensity to the intensity of the faintest point source detectable in the images) is a complicated function of not only the angular separation of the primary star and companion, but also of the azimuthal angle because of the complex point spread function of the primary star, which is also wavelength dependent. However, beyond 2.5 arcseconds from the star, regardless of the wavelength of the observation, the detection limit of a companion is simply the limiting magnitude of the image, as determined by the sensitivity of the PHARO camera. Within that radius, the dynamic range is at least 8 magnitudes at the 5(sigma) level and as high as 12 in a one second exposure. This represents a substantial gain over similar techniques without adaptive optics, which are generally limited to radii beyond two arcsec. We provide a quantitative discussion and recommendation for the optimal configuration along with a detailed comparison with recent theoretical predictions of AO coronagraphic performance.
Astronomical Science
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ALFA crowded star field observations: experiences, data reduction, and future strategies
David James Butler, Richard I. Davies, Markus Feldt
In objects like the Galactic center and star clusters, including Globular Clusters, the aim is to achieve the highest possible Strehl in order to separate objects with large luminosity differences. Although crowded star fields can be considered as prime targets for adaptive optics assisted NIR imaging, a number of challenges exist at both the observing and data reduction stages. While preparing for an observation, it may be useful to consider the effect of close guide star neighbors on centroiding and a sensible total integration time to achieve the desired S/N in the point spread function (PSF) halo. The centroiding issue is important because jittering of guide star neighbors in and out of the ALFA Shack-Hartmann wavefront sensor, as a result of a high stellar number density, can reduce the achievable Strehl. Total exposure time considerations may allow an empirical PSF determination with the desired PSF halo signal. In this contribution, we present our observational experiences, with ALFA, of a complicated region of stars in a Globular Cluster in both natural and a laser guide star observing mode as well as some relevant future plans. In particular, we concentrate on natural guide stars and consider the following: the use of a constrained centroiding algorithm, the change in the mean PSF halo pixel flux versus Strehl; and the use of 'a priori' astrometry and photometry in a deconvolution assisted profile fitting approach to crowded field photometry. This method reduced photometry errors and the systematic magnitude offset produced by a competing technique.
Atmospheric Characterization and Modeling
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Efficiency of off-axis astronomical adaptive systems: comparison of experimental data for different astronomical sites
Leonardo J. Sanchez, Valeri V. Voitsekhovich, Valeri G. Orlov, et al.
The efficiency of off-axis adaptive astronomical systems is estimated for four astronomical sites: Paranal (Chili), Roque de los Muchachos (Canary Islands), San Pedro Martir (Mexico) and Observatoire de Haute Provence (France). The efficiency of interest is considered through the Strehl ratio of the corrected image calculated for V, J and K bands and for 8-m class telescopes. The experimental optical turbulence strength Cn2(z) profiles necessary for calculations have been measured with balloon flights and with the Generalized Scidar. It is found that this efficiency depends mainly on the Cn2(z) profile layered structure and on the turbulence strength.
Image Analysis and Deconvolution
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Analysis of the off-axis point spread function of astronomical adaptive systems at San Pedro Martir
Valeri G. Orlov, Valeri V. Voitsekhovich, Leonardo J. Sanchez, et al.
A detailed analysis of anisotropy in the point spread function (PSF) associated with off-axis adaptive astronomical correction is presented. The results obtained are based on the experimental Cn2 profile which has been recently measured at San Pedro Martir observatory (Mexico). It is found that the PSF has a complicated shape that depends strongly on the wavelength and on the separation between the guide and observed stars.
Instrumentation for Astronomy with AO
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GriF: an infrared 3D spectroscopic mode for KIR/PUEO
When combined with Adaptive Optics, integral field spectroscopy, i.e. observation of a sky field simultaneously in a number of spectral passbands, is the most efficient way to perform spectro-imaging at high angular resolution. GriF will provide the CFHT community with such a capability in the near infrared K-band. This extension will be completed by means of two simple optical devices to be installed in the KIR cryostat (the infrared camera of PUEO): a cooled grism in the filter-wheel and a cold aperture on an entrance focal plane wheel. They will be completed by a room-temperature Fabry- Perot (FP) interferometer in front of KIR. The FP selects narrow bandpass images while the grism spatially separates them, giving a 3-D spectroscopic capacity within a compact and light design. At each exposure, several (up to 9) monochromatic images of a rectangular field of about 36 arcseconds X 4 arc-seconds will be simultaneously acquired, allowing a precise subtraction of continuum and background. The cooled grism will guarantee a low background environment, thus a good sensitivity at K. The medium spectral resolution (about 2600) will fit to a number of programs and will represent a considerable improvement on imaging with narrow- band filters. Thus, combining high angular resolution with the spectroscopic diagnosis, GriF will allow the study of a large class of compact objects or structures, especially in the extragalactic domain where its sensitivity should be unique.
SPID: a high spectral resolution diffraction-limited camera
Michel Tallon, Andre Baranne, Alain Blazit, et al.
SPID aims at offering a high spectral resolution in both short-exposure (speckle imaging) and long-exposure (adaptive optics with partial compensation) modes. It offers an adjustable spectral resolution (from 60 up to 3000) in the range 400 - 750 nm. For differential observation of astronomical objects, SPID gives images in two spectral bandwidths at the same time. The width and the central wavelength of each bandwidth can be chosen independently. A high image quality is achieved thanks to a new design derived from a Courtes' monochromator. SPID also includes a wavefront sensor for post-compensation processing. A short-exposure mode allows us to achieve diffraction limited images but with a low signal-to-noise ratio. Depending on the object brightness and on the seeing quality, adaptive optics will allow us to improve significantly the signal-to-noise ratio and sometimes to observe a diffraction limited core in long-exposure mode. Depending on the scientific goal, the availability of the two modes will drive the best choice. The current status of SPID is presented together with first results obtained at CFHT in the short-exposure mode.
Application of adaptive optics to the GI2T/REGAIN interferometer
Christophe Verinaud, Alain Blazit, Denis Mourard
The GI2T is a multi-speckle Michelson stellar interferometer of two 1.5 m telescopes operating in the visible and near infrared wavelengths. The REGAIN project (REcombinateur du GrAnd INterferometre) which consists of the renewal of the whole instrument is now nearly completed and the next step of the developments on GI2T will be the implantation of adaptive optics systems in each interferometric arm. In this paper we show that the data reduction methods of speckle interferometry that have been applied to the GI2T under pure turbulence, are also valid when the images are partially corrected by adaptive optics. Some simulation results show the expected performances in terms of signal-to-noise ratio for typical observation conditions. We also tackle the problem of piston control by a bimorph deformable mirror. The last section presents instrumental aspects of the foreseen AO system and the current status of the integration of the first wavefront sensor.
ALFA and 3D: integral field spectroscopy with adaptive optics
One of the most important techniques for astrophysics with adaptive optics is the ability to do spectroscopy at diffraction limited scales. The extreme difficulty of positioning a faint target accurately on a very narrow slit can be avoided by using an integral field unit, which provides the added benefit of full spatial coverage. During 1998, working with ALFA and the 3D integral field spectrometer, we demonstrated the validity of this technique by extracting and distinguishing spectra from binary stars separated by only 0.26'. The combination of ALFA & 3D is also ideally suited to imaging distant galaxies or the nuclei of nearby ones, as its field of view can be changed between 1.2' X 1.2' and 4' X 4', depending on the pixel scale chosen. In this contribution we present new results both on galactic targets, namely young stellar objects, as well as extra-galactic objects including a Seyfert and a starburst nucleus.
Study and test of a hybrid coronagraph
Pierre Baudoz, Yves Rabbia, Jean Gay
We report on variations around the technique of Achromatic Interfero Coronagraph (AIC). The AIC is an imaging device providing nulling of a star without nulling the environment of this star. This instrument already shows its capabilities with observations at Observatoire de Haute Provence and Canada France Hawaii Telescope. The main drawback of the AIC is the two symmetrical images it gives on the detector for a given feature. This characteristic is not annoying for compact features such as faint companions, blobs, ... but can be confusing for diluted features. We propose a method to suppress the symmetry of the output image of the AIC. As the AIC, the hybrid coronagraph we proposed is based on a Michelson interferometer with an image plane in one arm. A mask is placed in this image plane to cover the whole plane except the central part of the Airy disk. The hybrid coronagraph is then very close to the phase mask in the concept but since the interferences of the 2 arms are still destructive, it is less wavelength dependent. The only remaining dependency due to the mask diameter versus the wavelength can be largely decreased by an appropriate width for the mask. We eventually outlined first laboratory results with a hybrid coronagraph prototype.
First observation results with the compact achromatic interfero coronagraph
Pierre Baudoz, Yves Rabbia, Jean Gay, et al.
The Achromatic Interfero Coronagraph (AIC) aims at observing faint structures in the environment of a star. Light from the star is rejected by means of destructive interference (nulling) thus allowing very close sensing at a level overshooting the telescope's diffraction limit. Initially devised for space based facilities, AIC can be efficient on ground based large telescope provided adaptive optics is available. A compact version of AIC has been built so as to install it at CFHT/Hawaii between the PUEO bonette and the KIR camera used in the K band. In this paper we report on the first observation run performed on June 99 with the compact AIC. The principle of AIC is briefly recalled and technical features of the compact AIC is described. Some images obtained are shown and close-sensing performance regarding detection of companions is evaluated. Expectable results on CFHT are outlined.
Ground-based coronagraphy with high-order adaptive optics
Russell B. Makidon, Anand Sivaramakrishnan, Christopher D. Koresko, et al.
We simulate the actions of a coronagraph matched to diffraction-limited adaptive optics (AO) systems on the Calypso 1.2 m, Palomar Hale 5 m and Gemini 8.1 m telescopes, and identify useful parameter ranges for AO coronagraphy on these systems. We model the action of adaptive wavefront correction with a tapered, high-pass filter in spatial frequency rather than a hard low frequency cutoff, and estimate the minimum number of AO channels required to produce sufficient image quality for coronagraphic suppression within a few diffraction widths of a central bright object (as is relevant to e.g., brown dwarf searches near late-type dwarf stars). We explore the effect of varying the occulting image- plane stop size and shape, and examine the trade-off between throughput and suppression of the image halo and Airy rings. We discuss our simulations in the context of results from the 241-channel Palomar Hale AO coronagraph system, and suggest approaches for future AO coronagraphic instruments on large telescopes.
Systems on the Sky
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ALFA: three years of experience in adaptive optics with a laser guide star
The Max-Planck institutes for astronomy and for extraterrestrial physics run a high order adaptive optics system with a laser guide star facility at the Calar Alto 3.5- m telescope in southern Spain. This system, called ALFA, saw first light in September 1996. Today, ALFA can compensate for atmospheric turbulences with natural guide stars as faint as 13.5th magnitude in R-band. ALFA recently succeeded in overcoming this limiting magnitude with the deployment of its laser guide star. This paper briefly reviews the ALFA project and its progress over the last 3 years. We further discuss the impact of sodium-layer laser guide stars on wavefront sensing and present results obtained with both kinds of guide stars.
ALFA laser guide star: present status and future developments
ALFA (Adaptive Optics with Laser for Astronomy) is a joint project between the Max Planck Institut fur extraterrestrische Physik in Garching and the Max Planck Institut fur Astronomie in Heidelberg. To increase the sky coverage of the adaptive optics system, we are using a laser guide star that is created in the sodium layer of the earth atmosphere. The laser consists of a 5W cw dye laser, operated at single frequency and tuned to the sodium D2 resonance transition. Here we report on the present status of the laser system, the technological means to ensure an easy operation of the laser guide star, as well as on developments we are planning for the future.
Systems Under Development
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Curvature-based laser guide star adaptive optics system for Gemini South
The Gemini Observatory and University of Hawaii are planning to install an 85-element curvature adaptive optics system with a laser guide star system on its Cerro Pachon telescope in 2001. This paper discusses the motivation, issues on implementing a laser guide star with a curvature-based system, the implementation of a laser guide star based on a commercially available 2W ring-dye laser, and the expected performance of the system. Detailed simulations show very promising results for system performance down to natural guide star magnitudes of 19 - 20th magnitude. The performance cross- over point between NGS and LGS is between 13 - 16th magnitude depending on the performance parameter of interest (e.g. Strehl, energy through a slit, etc.).
LGS AO photon return simulations and laser requirements for the Gemini LGS AO program
Laser Guide Star Adaptive Optics system operation at an 8-m class telescope like Gemini North and Gemini South require minimum laser output power in the 10 W range. Since each additional Watt of laser power increases the laser purchase and operational costs, it is highly desirable to understand how laser parameters can be optimized in order to decrease the total laser power requirement. This paper provides some tools to calculate the output power requirement of a candidate laser system and its optimized parameters in the case of continuous wave lasers and high repetition rate pulsed lasers. Laser power requirements for Gemini North and Gemini South are given as examples of this calculation.
Lasers: Technology, Techniques, and Monitoring
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ELPOA: toward the tilt measurement from a polychromatic laser guide star
Adaptive optics at astronomical telescopes aims at correcting in real time the phase corrugations of incoming wavefronts caused by the turbulent atmosphere, as early proposed by Babcock. Measuring the phase errors requires a bright source, which is located within the isoplanatic patch of the program source. The probability that such a reference source exists is a function of the wavelength of the observation, of the required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. Several papers have addressed the problem of the sky coverage as a function of these parameters (see e.g.: Le Louarn et al). It turns out that the sky coverage is disastrously low in particular in the short (visible) wavelength range where, unfortunately, the gain in spatial resolution brought by adaptive optics is the largest. Foy and Labeyrie have proposed to overcome this difficulty by creating an artificial point source in the sky in the direction of the observation relying on the backscattered light due to a laser beam. This laser guide star (hereafter referred to as LGS) can be bright enough to allow us to accurately measure the wavefront phase errors, except for two modes which are the piston (which is not relevant in this case) and the tilt. Pilkington has emphasized that the round trip time of the laser beam to the mesosphere, where the LGS is most often formed, is significantly shorter than the typical tilt coherence time; then the inverse-return- of-light principle causes deflections of the outgoing and the ingoing beams to cancel. The apparent direction of the LGS is independent of the tilt. Therefore the tilt cannot be measured only from the LGS. Until now, the way to overcome this difficulty has been to use a natural guide star to sense the tilt. Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because approximately equals 90% of the variance of the phase error is in the tilt. Therefore, correcting the tilt requires a higher accuracy of the measurements than for higher orders of the wavefront. Hence current adaptive optics devices coupled with a LGS face low sky coverage. Several methods have been proposed to get a partial or total sky coverage for the tilt, such as the dual adaptive optics concept, the elongation perspective method, or the polychromatic LGS (hereafter referred to as PLGS). We present here a progress report of the R&D program Etoile Laser Polychromatique et Optique Adaptative (ELP-OA) carried out in France to develop the PLGS concept. After a short recall of the principles of the PLGS, we will review the goal of ELP-OA and the steps to get over to bring it into play.
Systems Under Development
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Design and performance of an 85-actuator curvature system
Malcolm J. Northcott, J. Elon Graves, Francois J. Roddier, et al.
The UH 36 element curvature AO system, Hokupa'a-36, was recently moved to the Gemini 8m telescope, where it was used with great success obtaining images for the telescope dedication. Since the 36 actuator system was optimized for performance on a 4 m (CFHT) telescope it does not provide full near IR wavelength converge on the Gemini 8m telescope. In order to address this issue we are planning to upgrade the system to 85 actuators. Given the slightly better seeing expected at the Gemini telescope, the move to 85 actuators will give Strehl ratios commensurate to those obtained with 36 actuators on the CFHT. The limiting magnitude will scale with the telescope aperture giving considerably better sky coverage than at the CFHT. Curvature AO systems can scale considerably beyond 85 actuators, at this point technology presents the most important limitations to scaling.
Progress on Altair: the Gemini North adaptive optics system
The Gemini Adaptive Optics System, (Altair), under construction at the National Research Council of Canada's Herzberg Institute of Astrophysics is unique among AO systems. Altair is designed with its deformable mirror (DM) conjugate to high altitude. We summarize construction progress. We then describe Altair in more detail. Both the Wavefront sensor foreoptics and control system are unconventional, because the guide star footprint on an altitude-conjugated DM moves as the guide star position varies. During a typical nodding sequence, where the telescope moves 10 arcseconds between exposures, this footprint moves by half an actuator and/or WFS lenslet. The advantages of altitude conjugation include increased isoplanatic patch size, which improves sky coverage, and improved uniformity of the corrected field. Altitude conjugation also reduces focal anisoplanatism with laser beacons. Although the initial installation of Altair will use natural guide stars, it will be fully ready to use a laser guide star (LGS). The infrastructure of Gemini observatory provides a variety of wavefront sensors and nested control loops that together permit some unique design concepts for Altair.
Systems on the Sky
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Progress with the Lick adaptive optics system
Progress and results of observations with the Lick Observatory Laser Guide Star Adaptive Optics System are presented. This system is optimized for diffraction-limited imaging in the near infrared, 1 - 2 micron wavelength bands. We describe our development efforts in a number of component areas including, a redesign of the optical bench layout, the commissioning of a new infrared science camera, and improvements to the software and user interface. There is also an ongoing effort to characterize the system performance with both natural and laser guide stars and to fold this data into a refined system model. Such a model can be used to help plan future observations, for example, predicting the point-spread function as a function of seeing and guide star magnitude.
Systems Under Development
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Low-cost adaptive optics systems
Carl Paterson, Ian H. Munro, Christopher Dainty
Flexible low-cost adaptive optics systems are presented. These are based around low-cost actuator components, specifically electrostatic membrane and bimorph mirrors. A modular breadboard experimental framework has been constructed to investigate different system components (wavefront sensors, actuators and control-loop algorithms). The framework includes a fully controllable dynamic wavefront generator for investigating both the spatial and temporal performance characteristics of the systems experimentally. The repeatability of the wavefront generator allows the effects of different system parameters on the performance to be investigated. The performance of a typical closed-loop system is presented. This system has a 37-electrode membrane mirror, a Shack-Hartmann sensor using a Dalsa CCD camera and a control system which runs on a single processor achieving frame rates of up to 800 Hz. All the components of this system are commercially available (with a total component cost of about 25 K). Using the dynamic wavefront generator it has been possible to measure the performance of this system experimentally for a wide range of well defined atmospheric turbulent conditions.
Solar
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Workstation-based solar/stellar adaptive optics system
The microprocessors used in off-the-shelf workstations double in performance every eighteen months. The Swedish Vacuum Solar Tower (SVST) uses off-the-shelf workstations for all aspects of its on-line telescope control and data acquisition. Since 1995 workstation performance has been adequate for a correlation tracker of solar granulation controlling a tip- tilt corrector. In 2000 workstation performance permits the construction of a 20 - 50 subimage Shack-Hartmann based low- latency adaptive optics system. It is argued that workstations provide a cost-effective, upgradable, low-risk and flexible means of construction of stellar and solar adaptive optics systems. We give an overview of the adaptive optics system installed at the SVST in May 1999. The system uses a bimorph modal mirror with 19 electrodes from Laplacian Optics. For use with extended targets, such as solar fine structure, cross- correlations with 16 X 16-pixel sub-images are used. For use with point sources, a centroiding algorithm is implemented. The work station used is capable of completing all processing required by the adaptive optics system in 0.5 ms (cross-correlations) or 0.3 ms (centroiding), with potential for significant performance improvements.
Multiconjugate
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Isoplanatic angle and optimal guide star separation for multiconjugate adaptive optics
We propose a first orae performance estimation of multiconjugate adaptive optics (MCAO) systems. An important and restrictive parameter is the angular guide star (GS) separation for a field of view (FOV) of interest to be compensated. An analytical approach is proposed to estimate the residual variance of a MCAO systems for a given position in the FOV as a function of GS separation. This approach allows us to define expected isoplanatic angles for MCAO systems as a function of the atmospheric and observing conditions (turbulence profile, telescope diameter and GS separation). The analytical results are also compared with the numerical simulation of a MCAO system, in which a great care has been taken in the 3D wavefront reconstruction from 2D wavefront sensor (WFS) measurements. For a 8 m telescope, we show that 3 GS's and 3 deformable mirrors provides very good performance in a 200 arcseconds FOV at 2.2 micrometer.
Systems Under Development
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1600 actuator tweeter mirror upgrade for the Palomar Adaptive Optics system (PALAO)
Richard G. Dekany, Mitchell Troy, Gary L. Brack, et al.
We discuss conceptual design issues for a 1600 actuator tweeter mirror/multiconjugate AO upgrade to the 349 actuator Palomar Adaptive Optics System (PALAO). Based upon a 42 X 42 actuator Photonex deformable mirror technology, developed by Xinetics, Inc., this upgrade would enable unique science at visible wavelengths and deliver unprecedented near-infrared Strehl ratios for modestly bright (mV equals 9) guide stars. When used in conjunction with the existing 349 actuator Xinetics, Inc. deformable mirror, a series of pressing issues regarding the practical utility of multiconjugate adaptive correction for extremely large telescopes could be addressed. By utilizing a low noise (EEV39) wavefront sensor camera developed by SciMeasure Analytical Systems, Inc., this system would provide on-axis K-band Strehl ratio of > 95%, improving scientific throughput and enabling the detection and spectroscopy of unresolved companions in an unprecedented contrast space around nearby stars.
Curvature-based adaptive optics for the NASA IRTF
Daniel J. O'Connor, J. Elon Graves, Malcolm J. Northcott, et al.
The IRTF is a 3.0 meter, f/38, infrared optimized, cassegrain telescope operated under contract from NASA with the primary mission of providing ground-based support for NASA's planetary missions. We are currently in the design and construction phase of a 36 element, curvature-based, natural guide star, adaptive optics facility installation for the IRTF. System architecture will be modeled on the highly successful AO systems developed at the University of Hawaii. The system should achieve an AO efficiency, q >= 0.4. The Strehl ratio is expected to exceed 0.8 in the K band. We estimate a limiting guide star magnitude for full correction of mR equals 14.4.
Systems on the Sky
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Lessons learned in the design, construction, and implementation of the W.M. Keck Observatory AO system
The construction and integration of the Keck AO system presented many interesting challenges, stemming from the program's unique scientific objectives and the size and shape of the Keck telescope primary mirror. In this paper, we will discuss several technical problems encountered during the design, construction, integration, and commissioning of the Keck AO system, and describe the solutions that have been implemented in the AO system.
Systems Under Development
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Adaptive optics performance criteria for imaging and laser communications systems
Bruce Martin Levine, Allan Wirth, Clive Standley
Atmospheric turbulence over vertical paths or long horizontal paths perturbs phase in the pupil of an optical communications receiver, and also can cause severe intensity scintillation. We describe a mathematical method for predicting a bound for these fluctuations by using the Strehl ratio as a criterion for determining the variations in the intensity fluctuations. Derived is the probability function of the instantaneous Strehl ratio, in addition to methods for computing the lower confidence limit. We show how these functions can vary by the degree of partial wavefront correction via adaptive optics.
Solar
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First-order adaptive system for correction of images in solar ground-based telescopes
Leonid V. Antoshkin, A. B. Borovik, Nina N. Botygina, et al.
The possibility of applying adaptive-optics devices to ground- based solar astronomy and high-resolution spectroscopy is considered. Due to atmospheric turbulence the resolution of ground-based solar telescopes are limited to 1 arcsec on the average with short period of subarcsecond resolution at good sites. Application of adaptive correction is a ground-based telescope to improve image quality is possible to improve whole performance of telescope to obtain images with resolution about 0.3' and to improve resolution for spectral measurements of the Sun. Adaptive Optics represent a visible tool for increasing the resolution of ground-based telescopes to the level that will be needed to understand the physical processes occurring on the Sun. An experimental adaptive- optics system for image stabilization is described, as well as the results of its tests. Different ways of the further development of the adaptive-optics system for use in the Big Solar Vacuum Telescope (BSVT) of the Baikal Astrophysical Observatory are discussed.
Systems Under Development
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Correcting of thermal and gravitational distortions of primary mirror on space camera by using adaptive optics
Zhiwei Zhang, Xin Yu
The paper describes the necessity of employing AO on space camera to overcome the thermal and gravitational influences in order to improve its image quality. Simulative computation of six cases are made to the thermal distortion of primary mirror of a typical Ritchey-Chretien space optical system by using finite element method. The P-V and RMS values, the Zernike coefficients of the mirror's surface are given. An AO correcting experiment system was designed and completed, both open-loop and close-loop experiments were made. A (Phi) 100 mm K9 glass reflective plane mirror was used as a proportional model of the primary mirror to simulate its thermal distortion. Three typical cases -- axial thermal gradient, radial gradient and the local thermal gradient were selected to simulate the on-orbit environment influences. This experiment results prove that AO can correct the low frequency thermal distortion of primary mirror on space camera and decrease the difficulty of temperature control of space optical system.
Pendular seismometer for correcting telescope pointing errors
Vibrations of telescopes can be successfully corrected in real time using seismometer as an inertial reference. This technique will be needed in adaptive optics systems with polychromatic laser guide stars to remove the tilt component of mechanical origin, it can also be used to compensate wind- induced shake in any telescope. A prototype pendular seismometer is described which is sensitive to pointing errors in the frequency band from few tenths to several tens of Hz. The average pendulum position is maintained by a slow servo system which also damps its resonance. The prototype instrument has a r.m.s. noise of 3 milliarcseconds in the 0 - 25 Hz band, sufficiently low for diffraction-limited imaging with 10 m class telescopes. It was tested on a 1 m telescope, and a good agreement of the seismometer signal with the direct optical measurements of telescope's optical axis fluctuations was found. A frequency response of the seismometer is studied, and an expression for the r.m.s. residual (uncompensated) vibrations is given.
Solar
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Solar adaptive optics
High resolution observations of the Sun are of key importance in understanding fundamental astrophysical processes. Adaptive optics (AO) is an important tool that allows solar astronomers to achieve diffraction limited observations from existing ground based telescopes. AO is also a key technology required for a future 4m-class Advanced Solar Telescope (AST) that the international community of solar astronomers is planning to build. The history of the development of solar AO is reviewed and results from recent successful demonstrations of solar AO systems are presented. The main difference between solar AO and night time AO is the different, and more elaborate wavefront sensing technique that has to be applied in order to measure wavefront aberrations using solar granulation as a target. Different approaches to this problem are discussed. Multi-conjugate AO has been proposed as a technique to achieve diffraction limited resolution over a field-of-view (FOV) significantly larger than the isoplanatic patch. The Sun is an ideal object for the development and application of MCAO.
Systems on the Sky
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Palomar adaptive optics project: status and performance
Mitchell Troy, Richard G. Dekany, Gary L. Brack, et al.
We describe the current performance of the Palomar 200 inch (5 m) adaptive optics system, which in December of 1998 achieved its first high order (241 actuators) lock on a natural guide star. In the K band (2.2 micrometer), the system has achieved Strehl ratios as high as 50% in the presence of 1.0 arcsecond seeing (0.5 micrometer). Predictions of the system's performance based on the analysis of real-time wavefront sensor telemetry data and an analysis based on a fitted Kolmogorov atmospheric model are shown to both agree with the observed science image performance. Performance predictions for various seeing conditions are presented and an analysis of the error budget is used to show which subsystems limit the performance of the AO system under various atmospheric conditions.
Final commissioning phase of the AdOpt@TNG module
The AdOptTNG module is an adaptive optics facility permanently mounted at the Nasmyth focus of the 4m-class Telescopio Nazionale Galileo (TNG). Its integration on the telescope started in late November 1998 and first-light of the speckle and tip-tilt modes took place shortly after. Both modes have been offered to the astronomical community and turned out to provide performances close to the expectations. Double stars with separation below 0.1 arcsec have been resolved by the speckle facility. Improvement of the Strehl ratio of a factor two and enhancement in the FWHM from 0.65 arcsec to 0.35 arcsec have been obtained on relatively faint reference stars. The high-speed low noise CCD, namely an 80 X 80 pixel read from the four corners, has been mounted and aligned with the Shack-Hartmann wavefront sensor. A Xinetics mirror with 96 actuators has been calibrated against the wavefront sensor with on-board alignment fibers. This has been done using a modal approach and using Singular Value Decomposition in order to get a reliable interaction matrix. Filtering can be modal too, using a default integrative filter coupled with a limited FIR-fashioned technique. Open loop measurements on the sky provide data to establish open loop transfer functions and realistic estimates of limiting magnitude. High-order wavefront correction loop has been successfully tested on the sky. In this paper we give a description of the overall functionality of the module and of the procedure required to acquire targets to be used as reference in the correction. A brief overview of the very first astronomical results obtained so far on angular size and shape measurements of a few asteroids and sub-arcsec imaging of Planetary Nebulae and Herbig Haro objects is also given.
Systems Under Development
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Status of the VLT Nasmyth adaptive optics system (NAOS)
Gerard Rousset, Francois Lacombe, Pascal Puget, et al.
NAOS is the adaptive optics system to be installed at one of the Nasmyth focus of the VLT. It was designed and manufactured by a French Consortium to provide compensated images to the high angular resolution IR spectro-imaging camera (CONICA) in the 1 to 5 micrometer spectral range. For bright sources, NAOS will achieve a Strehl ratio of 70% under average seeing conditions. It is equipped with a 185 actuator deformable mirror, a tip/tilt mirror and two wavefront sensors, one in the visible and one in the near IR. All the components of NAOS have been delivered and the integration phase is in progress since the beginning of 2000. After extensive tests and performance verifications in France, the system will be shipped to Chile by the end of 2000. The first light at the VLT is foreseen in the beginning of 2001.
MACAO and its application for the VLT interferometer
Robert Donaldson, Domenico Bonaccini, Joar Brynnel, et al.
The European Southern Observatory is developing a medium order curvature adaptive optics system designed to be operable with minimal modification at any focus of the Very Large telescope (VLT). The first application of this AO system (MACAO) is to equip all four VLT Unit Telescope (UT) Coude foci with 60 element AO systems capable of delivering to the VLT Interferometer (VLTI) > 50% K band Strehl. The AO system being used by an interferometer is constrained to introduce minimal piston and operating as a sub-system of a large and complex instrument to have a robust architecture and simple operation. Installation of the first AO system is scheduled to begin first Quarter 2002 with completion of all four UTs by early 2004. Other applications of the MACAO system will be for use by the CRIRES and SPIFFI spectrographs.
Adaptive optics for the 6.5-m MMT
The adaptive optics system for the 6.5 m MMT conversion telescope will be the first to compensate the aberrated wavefront at the telescope's secondary mirror. This approach has unique advantages in terms of optical simplicity, high throughput and low emissivity. We report here the present state of construction, and the results of static and dynamic performance tests of the Cassegrain optical package.
Astronomical Science
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Neptune and Titan observed with Keck Telescope adaptive optics
Claire E. Max, Bruce A. Macintosh, Seran Gibbard, et al.
We report on observations taken during engineering science validation time using the new adaptive optics system at the 10-m Keck II Telescope. We observed Neptune and Titan at near- infrared wavelengths. These objects are ideal for adaptive optics imaging because they are bright and small, yet have many diffraction-limited resolution elements across their disks. In addition Neptune and Titan have prominent physical features, some of which change markedly with time. We have observed infrared-bright 'storms' on Neptune, and very low- albedo surface regions on Titan, Saturn's largest moon. Spatial resolution on Neptune and Titan was 0.05 - 0.06 and 0.04 - 0.05 arc sec, respectively.
Solar system science with subarcsecond slit spectroscopy
Richard G. Dekany, Donald J. Banfield, Ben R. Oppenheimer, et al.
During its first year of shared-risk observations, the PALAO/PHARO adaptive optics system has been employed to obtain near-infrared R approximately 1000 spectra of solar system targets at spectroscopic slit widths of 0.5 and 0.1 arcsec, and corresponding spatial resolution along the slit as fine as 0.08 arcsec. Phenomena undergoing initial investigation include condensate formation in the atmospheres of Neptune, and the Saturnian moon, Titan. We present the results of this AO spectroscopy campaign and discuss AO specific considerations in the reduction and interpretation of this data.
Search for asteroidal satellites using adaptive optics
Laird Miller Close, William J. Merline, C. Dumas, et al.
We utilized AO to discover a moon around asteroid 45 Eugenia by use of the PUEO AO facility at CFHT. With PUEO we performed a search for asteroidal satellites among two dozen asteroids, achieving moderate Strehl ratios (35%) and FWHM of about 0.12' at H band. During this survey, we detected a faint close companion to 45 Eugenia. The satellite was 6.14 magnitudes (at 1.65 micrometer) fainter and located at most 0.75' from Eugenia. Without the ability of AO to sharpen the contrast and increase the resolution to 0.1', the detection of this companion would have been impossible with ground-based telescopes. The companion was found to be in a 1200 km circular orbit with a period of 4.7 days. We discovered that the bulk density of the large (215 km) asteroid 45 Eugenia is a surprisingly low 1.2 g/cm3. This has lead to the exciting possibilities that either this main belt asteroid is a burned out comet or has a hollow 'rubble-pile' structure.
Multiconjugate
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Principles, limitations, and performance of multiconjugate adaptive optics
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.
Multiconjugate adaptive optics: experiments in atmospheric tomography
In 1987 I described a technique call Multi-Conjugate Adaptive Optics (MCAO) as a way of increasing the size of the area on the sky over which Adaptive Optics corrects for atmospheric wavefront distortions. An essential component of MCAO is the estimation of this wavefront distortion at different heights in the atmosphere. The technique proposed to do so was called 'Atmospheric Tomography,' or AT, since it uses tomographic techniques using the wavefront distortions at the telescope entrance pupil of objects observed in a number of different directions in the sky to infer the 3-D wavefront behavior. This paper describes a program to do so using the small scale structure on the solar surface (sunspots, pores and granulation). The Sun has the advantage of being an extended object on which the wavefront can be observed in a large number of directions using correlation Hartmann-Shack wavefront sensing. The AT experiment described in this paper uses the 76 cm Dunn Solar Telescope at NSO, 69 sub-apertures, a 2 X 2 arcmin2 field-of-view and a wavelength of 411 nm. The MCAO-AT system is being developed for the future 4 meter aperture Advanced Solar Telescope.
Extremely Large Telescopes
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Adaptive optics for 100-m-class telescopes: new challenges require new solutions
For giant, 100 m-class, telescopes, a completely new class of problems arises. Trying to solve them adopting the techniques used for the classical 4 ... 10 m-class telescope systems is indeed a reductive approach. Because of the cone effect and of the difficulties in retrieving the tilt due to the use of LGSs, tomography and multiconjugation will play a central role in the development of these systems. We review the tomographic concept and we report about recent experiments to validate the concept using sky observations. The Field of View requirement is another crucial step in the design of the optical system of these telescopes and hence it needs to be attacked in a detailed and as wide as possible approach. The adoption of classical wavefront sensors looking to different stars is here overcome with wavefront sensors directly conjugated to different layers, with big advantages in term of intrinsic complication of the system, both from the optomechanical point of view (less complexity and saving of a huge amount of light) and from the computational point of view. The adoption of these new sensing techniques (that, gaining from the close loop situation, can be efficiently coupled to a number of very faint stars, whose complexity does not scale with the number of references but, rather, to the number of sensed layers) is here reviewed. This concept of multiple object wavefront sensing leads to an example of a system with a minimum number of independent adaptive optics loop, in which each layer represented in a wavefront sensor can be conjugated to a specifically conjugated deformable mirror.
Scaling multiconjugate adaptive optics performance estimates to extremely large telescopes
Multi-conjugate adaptive optics (MCAO) is a key technology for extremely large, ground-based telescopes (ELT's) because it enables near-uniform atmospheric turbulence compensation over fields-of-view considerably larger than can be corrected with more conventional AO systems. Quantitative performance evaluation using detailed analytical or simulation models is difficult, however, due to the very large number of deformable mirror (DM) actuators, wave front sensors (WFS) subapertures, and guide stars which might comprise an MCAO system for an ELT. This paper employs more restricted minimal variance estimation methods to evaluate the fundamental performance limits imposed by anisoplanatism alone upon MCAO performance for a range of sample cases. Each case is defined by a atmospheric turbulence profile, telescope aperture diameter, field-of-view, guide star constellation, and set of DM conjugate ranges. For a Kolmogorov turbulence spectrum with an infinite outer scale, MCAO performance for a whole range of aperture diameters and proportional fields-of-view can be computed at once using a scaling law analogous to the (D/dO)5/3 formula for the cone effect. For 30 meter telescopes, useful levels of performance are possible across a 1.0 - 2.0 arc minute square field-of-view using 5 laser guide stars (LGS's) and 3 DM's, and somewhat larger fields can be corrected using 9 guide stars and 4 mirrors. 3 or more tip/tilt natural guide stars (NGS's) are necessary to detect modes of tilt anisoplanatism which cannot be detected using LGS's, however. LGS MCAO performance is a quite weak function of aperture diameter for a fixed field-of-view, and it is tempting to scale these results to larger apertures. NGS MCAO performance is moderately superior to LGS MCAO if the NGS constellation is within the compensated field-of-view, but degrades rapidly as the guide stars move away from the field. The penalty relaxes slowly with increasing aperture diameter, but how to extrapolate this trend to telescopes with diameters much larger than 30 meters is unclear.
New challenges for adaptive optics: the OWL 100-m telescope
Adaptive optics will be a key element of future Extremely Large Telescopes (ELTs) which need sharp images both to avoid object confusion and to boost sensitivity to point sources against sky background. A challenging task of correcting turbulence in the visible part of the spectrum is addressed. With an R approximately 10 natural guide star a Strehl ratio (SR) of 0.6 can be achieved at 500 nm, at the cost of an extremely low sky coverage. Thus, use of Multi-Conjugate Adaptive Optics techniques will be essential to widen the field of view (FOV). We summarize our recent theoretical results and show that with 3 deformable mirrors it is realistic to obtain a FOV diameter of 30' - 60' in the visible, or 3' - 6' in the K band. Contrary to the common belief, the layered structure of the turbulence vertical profile is not needed to achieve this limit, neither do we need to know the profile exactly. The wide corrected FOV ensures a reasonably good sky coverage in the near infrared using 4 natural guide stars (60% in the J band at medium Galactic latitudes). Artificial Laser Guide Stars will be needed to correct in the visible, using in addition a faint natural star to measure low-order wavefront aberrations. On ELTs this star can be faint, up to R equals 23, so a good sky coverage can also be obtained. A significant fraction of guide sources at these magnitudes are extragalactic.
Adaptive system of the AST-25
Victor V. Sychev, Aleksander S. Pechenov
The concept is proposed for the construction of the adaptive means complex for the astronomic supertelescope of 25-meter class with segmented primary mirror (AST-25). The correction of phase distortions appeared inside the telescope AST-25 and in free atmosphere, is provided by different contours with application of reference radiation and proper radiation from the object of observation. The segmented primary mirror with 612 controlled optical elements is used as a phase corrector. Each optical element is controlled by precision magnitorheological drives over three coordinates. The expected accuracy of the adaptive system correction in the real time is at level of 0.01'.
Atmospheric Characterization and Modeling
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Turbulence and wind profiling with generalized scidar at Cerro Pachon
We present the first simultaneous monitoring of the vertical distributions of the optical turbulence strength CN2(h) and the wind velocity V(h), along the whole optical path. For that purpose, a new algorithm has been developed for the analysis of the spatio-temporal correlation of scintillation images, obtained with the Generalized Scidar (GS). The algorithm allows for dome seeing identification. The new method has being applied to the GS data collected during four one-week runs uniformly distributed during 1998, for the characterization of Cerro Pachon, the Gemini South Telescope site. Comparison of V(h) obtained with the GS and instrumented balloons, respectively, gives a great deal of confidence in the new method. The analysis of the 6900 CN2(h) and V(h) profiles obtained, leads to a statistical study of the most important parameters relevant for the development of adaptive optical systems.
Lasers: Technology, Techniques, and Monitoring
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Sodium layer monitoring at Calar Alto by LIDAR
David James Butler, Richard I. Davies, Hayden Fews, et al.
Observations have shown the presence of sodium layer centroid height variations of a few hundred meters on timescales of tens of seconds. As quality laser guide star (LGS) plus adaptive optics (AO) assisted astronomy, especially on large (8m+) telescopes, will require optimal scheduling of observations and regular laser and wavefront sensor focusing at sites where sporadic sodium layers are frequent, an 'easy to use' sodium layer monitor is required. LIDAR offers a convenient means to achieve this. By pulsing the outgoing sodium laser and performing time-of-flight measurements on the returned photons we can acquire the altitude profile of the sodium layer. Unfortunately, conventional LIDAR requires the laser duty cycle to be very low, therefore large integration times are required. However, by using a cross-correlation technique the duty cycle can be increased to 50%, which gives far better performance. We present the details of this technique which involved amplitude modulation of the MPIA/MPE ALFA cw laser, as well as the following results of such LIDAR measurements performed in October 1999 at the 3.5 m telescope at Calar Alto Observatory in Spain. The altitude of the sodium layer at Calar Alto on 17th and 18th October 1999 was found to be at 90 +/- 3 km and there is evidence for sporadics on one of two nights with sporadic layer FWHM* varying from approximately 240 to 350 m. In addition, a noticeable layer FWHM change (excluding the sporadic layer) from approximately 13 to approximately 5 - 7 km was observed over the two nights. After flux and altitude calibration and correction of the projected altitude range, a very good agreement is found between sodium layer profiles derived from an auxiliary telescope and 3.5 m telescope LIDAR observations. Using an intensity weighted centroid algorithm the centroid height of the sodium layer was observed to have a variation of < 500 m in approximately 10 minutes. Although, shorter timescale variations may be have been present, poor observing conditions and resulting reduced S/N prevents this analysis.
Tracking the global tilt using tails of radio guide stars
The problem of global tilt arises when the downgoing light from an artificial guide star traces in reverse the upgoing laser beam. The problem also exists if the upgoing beam is in the radio, since we still cannot determine its absolute position to the required accuracy. We propose a way to solve this problem by tracing the tails of the artificial guide star. Radio pulses breaking down the air into visible plasma create the beacon, and each pulse uses the residue plasma of its predecessor to restart the break down. As the radio beams wander on their way up, the new spot will not overlap perfectly with the previous one. Thus, the spot will have a small trail, which can help trace the history of the tilt. Comparison of the measurement of the previous pulse to the tail of the new one will provide the differential movement of the downgoing beam. Integrating this movement will yield the absolute pulse location. Cumulative errors are reduced by comparison to weak nearby natural guide stars. We ran simulations of the process, where we include random atmospheric tilts for the radio beam. We also checked the effect of intentional nutations of the upgoing beam and the effects of atmospheric winds on the plasma spot, as well as detection noise. We fitted the measured signal to the head and tail of each spot, and found their difference, which amounts to the change in tilts since the last pulse. We integrated this difference to find the absolute tilt. We show that indeed the tail trail can be traced to yield information about the tilt.
Wavefront Compensation Optimization
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Initial performance of the Keck AO wavefront controller system
Erik M. Johansson, D. Scott Acton, Jong R. An, et al.
The wavefront controller for the Keck Observatory AO system consists of two separate real-time control loops: a tip-tilt control loop to remove tilt from the incoming wavefront, and a deformable mirror control loop to remove higher-order aberrations. In this paper, we describe these control loops and analyze their performance using diagnostic data acquired during the integration and testing of the AO system on the telescope. Disturbance rejection curves for the controllers are calculated from the experimental data and compared to theory. The residual wavefront errors due to control loop bandwidth are also calculated from the data, and possible improvements to the controller performance are discussed.