Proceedings Volume 6272

Advances in Adaptive Optics II

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Proceedings Volume 6272

Advances in Adaptive Optics II

View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 31 July 2006
Contents: 44 Sessions, 174 Papers, 0 Presentations
Conference: SPIE Astronomical Telescopes + Instrumentation 2006
Volume Number: 6272

Table of Contents

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Table of Contents

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  • Laser Guide Star and Multi-Conjugate AO Field Tests I
  • Laser Guide Star and Multi-Conjugate AO Field Tests II
  • Adaptive Optics Programs I
  • Astronomical Results and Performance Characterization
  • Extreme AO Systems
  • AO Systems for Extremely Large Telescopes
  • Large Adaptive Mirrors
  • Real-Time-Control Systems
  • Natural Guide Star AO Field Tests
  • Conventional Wavefront Sensors and Concepts
  • Laser Systems, Facilities, and Concepts
  • High Precision Wavefront Control
  • Multi-Conjugate AO and Control System Lab Tests
  • Analytical Studies
  • MEMS and Deformable Mirror Modeling
  • Pyramid and Layer-Oriented Wavefront Sensing
  • High Contrast Imaging Studies
  • Adaptive Optics Programs II
  • Extreme AO Lab Tests
  • Innovative Deformable Mirror Technologies
  • Wavefront Reconstruction Theory
  • Turbulence, Anisoplanatism, and Sky Coverage
  • Simulation Studies and Tools
  • Innovative Wavefront Sensing Concepts
  • Calibration and Control Topics
  • Poster Session: Laser Guide Star and Multi-Conjugate AO Field Tests
  • Poster Session: Adaptive Optics Programs I
  • Poster Session: Astronomical Results and Performance Characterization
  • Poster Session: Real-Time Control Systems
  • Poster Session: Natural Guide Star AO Field Tests
  • Poster Session: Laser Systems, Facilities, and Concepts
  • Poster Session: Wavefront Sensors and Detectors
  • Poster Session: Multi-Conjugate AO and Control System Lab Tests
  • Poster Session: Analytical Studies
  • Poster Session: MEMS and Deformable Mirror Modeling
  • Poster Session: Pyramid and Layer-Oriented Wavefront Sensing
  • Poster Session: Adaptive Optics Programs II
  • Poster Session: AO Lab Tests
  • Poster Session: Innovative Deformable Mirror Technologies
  • Poster Session: Wavefront Reconstruction Theory
  • Poster Session: Turbulence, Anisoplanatism, and Sky Coverage
  • Poster Session: Simulation Studies and Tools
  • Poster Session: Innovative Wavefront Sensing Concepts
  • Poster Session: Calibration and Control Topics
Laser Guide Star and Multi-Conjugate AO Field Tests I
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LGS AO at W.M. Keck Observatory: routine operations and remaining challenges
The Laser Guide Star Adaptive Optics (LGS AO) at the W.M. Keck Observatory is the first system of its kind being used to conduct routine science on a ten-meter telescope. In 2005, more than fifty nights of LGSAO science and engineering were carried out using the NIRC2 and OSIRIS science instruments. In this paper, we report on the typical performance and operations of its LGS AO-specific sub-systems (laser, tip-tilt sensor, low-bandwidth wavefront sensor) as well as the overall scientific performance and observing efficiency. We conclude the paper by describing our main performance limitations and present possible developments to overcome them.
First results from the UCSC Laboratory for Adaptive Optics multi-conjugate and multi-object adaptive optics testbed
S. Mark Ammons, Renate Kupke, Edward A. Laag, et al.
We present first results from the Multi-Conjugate and Multi-Object Adaptive Optics (MCAO and MOAO) testbed, at the UCO/Lick Laboratory for Adaptive Optics (LAO) facility at U.C. Santa Cruz. This testbed is constructed to simulate a 30-m telescope executing MCAO and/or open loop MOAO atmospheric compensation and imaging over 5 arcminutes. It is capable of performing Shack-Hartmann wavefront sensing on up to 8 natural or laser guide stars and 2-3 additional tip/tilt stars. In this paper, we demonstrate improved on-axis correction relative to ground layer adaptive optics (~ 15% Strehl relative to ~ 12%) with a simulated 28-m aperture at a D/r0 corresponding to a science wavelength of 2.6 microns using three laser guide stars on a simulated 41 arcsec radius with a central science object and one deformable mirror at the ground layer.
Tomographic reconstruction of stellar wavefronts from multiple laser guide stars
Experiments have been carried out at the MMT telescope in June 2005 and again in April 2006 to validate open loop tomographic wavefront reconstruction using five dynamically refocused Rayleigh laser beacons (RLGS) and multiple tilt natural guide stars (NGS). Wavefront sensing in this manner is recognized as a critical precursor to the development of adaptive optics for Extremely Large Telescopes. At the MMT, wavefronts from the laser beacons are recorded by five 60-element Shack-Hartmann sensors implemented on a single CCD. A wide-field camera measures image motion from multiple field stars to calculate global tilt and distinguish effects of contributions to second order aberrations from low and high altitude turbulence. Together, the signals from these sensors are used to estimate the first 45 Zernike modes in the wavefront of a star within the LGS constellation. The reconstruction is compared off line to simultaneous wavefront measurements made of the star with a separate Shack-Hartmann sensor. We will present the results in this paper and quantify the wavefront improvement expected from tomographic adaptive optics correction.
Multi-conjugate solar adaptive optics with the VTT and GREGOR
After the successful demonstration of the solar multi-conjugate adaptive optics (MCAO) system at the German 70cm Vacuum Tower Telescope (VTT), Observatorio del Teide, Tenerife, in the last years, we are continuing the development of the system as a testbed for the future MCAO of the 150cm GREGOR solar telescope. We describe an improved reconstruction scheme that increases the number of corrected off-axis degrees of freedom and will be tested at the VTT in September 2006. We present a modified optical setup of the GREGOR MCAO that has the advantage of being adjustable to a wide height range of the turbulence.
Progress with solar multi-conjugate adaptive optics at NSO
Thomas Rimmele, Kit Richards, Jacqueline Roche, et al.
We have implemented a MCAO experiment at the Dunn Solar Telescope. The MCAO system uses 2 deformable mirrors, one conjugated to the telescope entrance pupil and other one conjugated to a layer in the upper atmosphere. For our initial experiments we have used a staged approach in which the 97 actuator, 76 subaperture correlating Shack-Hartmann solar adaptive optics system normally operated at the DST is followed by the second DM and the tomographic wavefront sensor, which used three "solar guide stars". We have successfully and stably locked the MCAO system on solar structure. We varied the height of the upper conjugate between 3km and 9 km. A large number of images were recorded in order to evaluate the performance of the system. The data analysis is still ongoing. We present preliminary results and discuss future plans.
Laser Guide Star and Multi-Conjugate AO Field Tests II
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First light of the ESO laser guide star facility
D. Bonaccini Calia, E. Allaert, J. L. Alvarez, et al.
Two teams of scientists and engineers at Max Planck Institut fuer Extraterrestrische Physik and at the European Southern Observatory have joined forces to design, build and install the Laser Guide Star Facility for the VLT. The Laser Guide Star Facility has now been completed and installed on the VLT Yepun telescope at Cerro Paranal. In this paper we report on the first light and first results from the Commissioning of the LGSF.
Adaptive Optics Programs I
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Adaptive optics developments at Keck Observatory
Peter L. Wizinowich, Jason Chin, Erik Johansson, et al.
The purpose of this paper is to report on new adaptive optics (AO) developments at the W. M. Keck Observatory since the 2004 SPIE meeting.1 These developments include commissioning of the Keck II laser guide star (LGS) facility, development of new wavefront controllers and sensors, design of the Keck I LGS facility and studies in support of a next generation Keck AO system.
First light AO system for LBT: toward on-sky operation
The paper is describing the present status of the LBT first light AO system. The system design started in January 2002 and is now approaching the final test in the Arcetri solar tower. Two key features of this single conjugate AO system are the use of an adaptive secondary mirror having 672 actuators and a pyramid wavefront sensor with a maximum sampling of 30x30 subapertures. The paper is reporting about the adaptive secondary mechanical electrical and optical integration, and the wavefront sensor unit integration and acceptance test. Finally some lab test of the AO system done using an adaptive secondary prototype with 45 actuators, the so called P45 are described. The aim of these test was to get an estimate of the system limiting magnitude and to demonstrate the feasibility of a new technique able to measure AO system interaction matrix in a shortest time and with higher SNR with respect to the classical interaction matrix measurement. We are planning to use such a technique to calibrate the AO system in Arcetri and later at the LBT telescope.
The ESO Adaptive Optics Facility
The Adaptive Optics Facility is a project to convert one VLT-UT into a specialized Adaptive Telescope. The present secondary mirror (M2) will be replaced by a new M2-Unit hosting a 1170 actuators deformable mirror. The 3 focal stations will be equipped with instruments adapted to the new capability of this UT. Two instruments are in development for the 2 Nasmyth foci: Hawk-I with its AO module GRAAL allowing a Ground Layer Adaptive Optics correction and MUSE with GALACSI for GLAO correction and Laser Tomography Adaptive Optics correction. A future instrument still needs to be defined for the Cassegrain focus. Several guide stars are required for the type of adaptive corrections needed and a four Laser Guide Star facility (4LGSF) is being developed in the scope of the AO Facility. Convex mirrors like the VLT M2 represent a major challenge for testing and a substantial effort is dedicated to this. ASSIST, is a test bench that will allow testing of the Deformable Secondary Mirror and both instruments with simulated turbulence. This article describes the Adaptive Optics facility systems composing associated with it.
Status of Subaru laser guide star AO system
Hideki Takami, Stephen Colley, Matt Dinkins, et al.
The laser guide star adaptive optics (AO188) system for Subaru Telescope is presented. The system will be installed at the IR Nasmyth platform of Subaru 8 m telescope, whereas the current AO system with 36 elements is operating at the Cassegrain focus. The new AO system has a 188 element wavefront curvature sensor with photon counting APD modules and 188 element bimorph mirror. The laser guide star system has a 4.5 W solid state sum-frequency laser on the Nasmyth platform. The laser launching telescope with 50 cm aperture will be installed at behind the secondary mirror. The laser beam will be transferred to the laser launching telescope using photonic crystal single mode fiber cable. The instrument with the AO system is IRCS, infrared camera and spectrograph which has been used for Cassegrain AO system and new instrument, HiCIAO, high dynamic range infrared camera for exsolar planet detection. The first light of the AO system is planned in 2006.
A conceptual design for the Thirty Meter Telescope adaptive optics systems
In this paper, we provide an overview of the adaptive optics (AO) program for the Thirty Meter Telescope (TMT) project, including an update on requirements; the philosophical approach to developing an overall AO system architecture; the recently completed conceptual designs for facility and instrument AO systems; anticipated first light capabilities and upgrade options; and the hardware, software, and controls interfaces with the remainder of the observatory. Supporting work in AO component development, lab and field tests, and simulation and analysis is also discussed. Further detail on all of these subjects may be found in additional papers in this conference.
Design of the adaptive optics systems for GMT
Michael Lloyd-Hart, Roger Angel, N. Mark Milton, et al.
The Giant Magellan Telescope (GMT) includes adaptive optics (AO) as an integral component of its design. Planned scientific applications of AO span an enormous parameter space: wavelengths from 1 to 25 μm, fields of view from 1 arcsec to 8 arcmin, and contrast ratio as high as 109. The integrated systems are designed about common core elements. The telescope's Gregorian adaptive secondary mirror, with seven segments matched to the primary mirror segments, will be used for wavefront correction in all AO modes, providing for high throughput and very low background in the thermal infrared. First light with AO will use wavefront reconstruction from a constellation of six continuous-wave sodium laser guide stars to provide ground-layer correction over 8 arcmin and diffraction-limited correction of small fields. Natural guide stars will be used for classical AO and high contrast imaging. The AO system is configured to feed both the initial instrument suite and ports for future expansion.
PALM-3000: visible light AO on the 5.1-meter Telescope
PALM-3000 is proposed to be the first visible-light sodium laser guide star astronomical adaptive optics system. Deployed as a multi-user shared facility on the 5.1 meter Hale Telescope at Palomar Mountain, this state-of-the-art upgrade to the successful Palomar Adaptive Optics System will have the unique capability to open the visible light spectrum to diffraction-limited scientific access from the ground, providing angular imaging resolution as fine as 16 milliarcsec with modest sky coverage fraction.
Astronomical Results and Performance Characterization
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Astronomical science with laser guide star adaptive optics: a brief review, a current snapshot, and a bright future
We briefly discuss the past, present, and future state of astronomical science with laser guide star adaptive optics (LGS AO). We present a tabulation of refereed science papers from LGS AO, amounting to a total of 23 publications as of May 2006. The first decade of LGS AO science (1995-2004) was marked by modest science productivity (≈1 paper/year), as LGS systems were being implemented and commissioned. The last two years have seen explosive science growth (≈1 paper/month), largely due to the new LGS system on the Keck II 10-meter telescope, and point to an exciting new era for high angular resolution science. To illustrate the achievable on-sky performance, we present an extensive collection of Keck LGS performance measurements from the first year of our brown dwarf near-IR imaging survey. We summarize the current strengths and weaknesses of LGS compared to Hubble Space Telescope, offer a list of desired improvements, and look forward to a bright future for LGS given its wide-scale implementation on large ground-based telescopes.
Analysis of K-band imaging of the wide binary system σ CrB with the Lick Observatory NGS AO system
We present astronomical results from K-band adaptive optics (AO) observations of the wide binary system σ Corona Borealis with the Lick Observatory natural guide star adaptive optics system on 2004 August 27-29. Seeing conditions were excellent and the AO compensation was very good, with Strehl ratios reaching 50% at times. The stellar images were reduced using three different analysis techniques: (1) Parametric Blind Deconvolution, (2) Multi-Frame Blind Deconvolution, and (3) the MATPHOT stellar photometry code. The relative photometric and astrometric precision achievable with these three analysis methods are compared. Future directions that this research can go towards achieving the goal of routinely obtaining precise and accurate photometry and astrometry based on near-infrared AO observations are described.
Characterization of the Lick adaptive optics point spread function
We have investigated both the temporal and spatial structure of the point spread function (PSF) produced by the Lick Observatory adaptive optics (AO) system using the FastSub readout mode of the IRCAL camera using short-exposure images with exposure times of 22ms at a frame rate of ~ 20Hz suitable for "freezing" the compensation under typical K-band observing conditions. These short exposures are a useful diagnostic tool for determining the system performance and permit measurement of the instantaneous Strehl ratio. Data taken from a number of observing runs, spanning over four months, show the underlying morphology of the PSF to be very stable with instantaneous Strehl ratios varying from ~ 20%-70% in NGS mode. Estimates of the instantaneous Strehl distribution have also been obtained from which we have determined the probability density function for the distribution of the instantaneous Strehl ratios.
Extreme AO Systems
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Design of the extreme AO system for SPHERE, the planet finder instrument of the VLT
T. Fusco, C. Petit, G. Rousset, et al.
The SPHERE system aims at the detection of extremely faint sources (giant extra-solar planet) in the vinicity of bright stars. Such a challenging goal automatically requires the use of a coronagraphic device to cancel out the flux coming from the star and smart imaging technics which have to be added to reach the required contrast for exo-planet detection (typically 10-6 - 10-7 in contrast). In this frame of the SPHERE project a global system study has demonstrated the feasibility of an AO system for the direct exoplanets detection. A detailed description of this system is proposed in this paper. The main trade-offs are discussed and justified and all the subsystems briefly presented. The realization phase has begun in 2006 and we foresee to obtain a first light at the VLT in 2010.
The Gemini Planet Imager
The next major frontier in the study of extrasolar planets is direct imaging detection of the planets themselves. With high-order adaptive optics, careful system design, and advanced coronagraphy, it is possible for an AO system on a 8-m class telescope to achieve contrast levels of 10-7 to 10-8, sufficient to detect warm self-luminous Jovian planets in the solar neighborhood. Such direct detection is sensitive to planets inaccessible to current radial-velocity surveys and allows spectral characterization of the planets, shedding light on planet formation and the structure of other solar systems. We have begun the construction of such a system for the Gemini Observatory. Dubbed the Gemini Planet Imager (GPI), this instrument should be deployed in 2010 on the Gemini South telescope. It combines a 2000-actuator MEMS-based AO system, an apodized-pupil Lyot coronagraph, a precision infrared interferometer for real-time wavefront calibration at the nanometer level, and a infrared integral field spectrograph for detection and characterization of the target planets. GPI will be able to achieve Strehl ratios > 0.9 at 1.65 microns and to observe a broad sample of science targets with I band magnitudes less than 8. In addition to planet detection, GPI will also be capable of polarimetric imaging of circumstellar dust disks, studies of evolved stars, and high-Strehl imaging spectroscopy of bright targets. We present here an overview of the GPI instrument design, an error budget highlighting key technological challenges, and models of the system performance.
The EPICS project for the European Extremely Large Telescope: outcome of the Planet Finder concept study for OWL
C. Vérinaud, N. Hubin, M. Kasper, et al.
The Exo-Planets Imaging Camera and Spectrograph (EPICS), is the Planet Finder Instrument concept for the European Extremely Large Telescope (ELT). The study made in the frame of the OWL 100-m telescope concept is being up-dated in direct relation with the re-baselining activities of the European Extremely Large Telescope.
Extreme adaptive optics for the Thirty Meter Telescope
Direct detection of extrasolar Jovian planets is a major scientific motivation for the construction of future extremely large telescopes such as the Thirty Meter Telescope (TMT). Such detection will require dedicated high-contrast AO systems. Since the properties of Jovian planets and their parent stars vary enormously between different populations, the instrument must be designed to meet specific scientific needs rather than a simple metric such as maximum Strehl ratio. We present a design for such an instrument, the Planet Formation Imager (PFI) for TMT. It has four key science missions. The first is the study of newly-formed planets on 5-10 AU scales in regions such as Taurus and Ophiucus - this requires very small inner working distances that are only possible with a 30m or larger telescope. The second is a robust census of extrasolar giant planets orbiting mature nearby stars. The third is detailed spectral characterization of the brightest extrasolar planets. The final targets are circumstellar dust disks, including Zodiacal light analogs in the inner parts of other solar systems. To achieve these, PFI combines advanced wavefront sensors, high-order MEMS deformable mirrors, a coronagraph optimized for a finely- segmented primary mirror, and an integral field spectrograph.
AO Systems for Extremely Large Telescopes
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MAD star oriented: laboratory results for ground layer and multi-conjugate adaptive optics
Enrico Marchetti, Roland Brast, Bernard Delabre, et al.
The Multi-Conjugate Adaptive Optics Demonstrator (MAD) built by ESO with the contribution of two external consortia is a powerful test bench for proving the feasibility of Ground Layer (GLAO) and Multi-Conjugate Adaptive Optics (MCAO) techniques both in the laboratory and on the sky. The MAD module will be installed at one of the VLT unit telescope in Paranal observatory to perform on-sky observations. MAD is based on a two deformable mirrors correction system and on two multi-reference wavefront sensors (Star Oriented and Layer Oriented) capable to observe simultaneously some pre-selected configurations of Natural Guide Stars. MAD is expected to correct up to 2 arcmin field of view in K band. MAD is completing the test phase in the Star Oriented mode based on Shack-Hartmann wavefront sensing. The GLAO and MCAO loops have been successfully closed on simulated atmosphere after a long phase of careful system characterization and calibration. In this paper we present the results obtained in laboratory for GLAO and MCAO corrections testing with bright guide star flux in Star Oriented mode paying also attention to the aspects involving the calibration of such a system. A short overview of the MAD system is also given.
NFIRAOS: TMT narrow field near-infrared facility adaptive optics
Glen Herriot, Paul Hickson, B. L. Ellerbroek, et al.
Although many of the instruments planned for the TMT (Thirty Meter Telescope) have their own closely-coupled adaptive optics systems, TMT will also have a facility Adaptive Optics (AO) system, NFIRAOS, feeding three instruments on the Nasmyth platform. This Narrow-Field Infrared Adaptive Optics System, employs conventional deformable mirrors with large diameters of about 300 mm. The requirements for NFIRAOS include 1.0-2.5 microns wavelength range, 30 arcsecond diameter science field of view (FOV), excellent sky coverage, and diffraction-limited atmospheric turbulence compensation (specified at 133 nm RMS including residual telescope and science instrument errors.) The reference design for NFIRAOS includes six sodium laser guide stars over a 70 arcsecond FOV, and multiple infrared tip/tilt sensors and a natural guide star focus sensor within instruments. Larger telescopes require greater deformable mirror (DM) stroke. Although initially NFIRAOS will correct a 10 arcsecond science field, it uses two deformable mirrors in series, partly to provide sufficient stroke for atmospheric correction over the 30 m telescope aperture, but mainly to improve sky coverage by sharpening near-IR natural guide stars over a 2 arcminute diameter "technical" field. The planned upgrade to full performance includes replacing the ground-conjugated DM with a higher actuator density, and using a deformable telescope secondary mirror as a "woofer." NFIRAOS feeds three live instruments: a near-Infrared integral field Imaging spectrograph, a near-infrared echelle spectrograph, and after upgrading NFIRAOS to full multi-conjugation, a wide field (30 arcsecond) infrared camera.
Adaptive optics designs for an infrared multi-object spectrograph on TMT
The Thirty Meter Telescope (TMT), the next generation giant segmented mirror telescope, will have unprecedented astronomical science capability. Since science productivity is greatly enhanced through the use of adaptive optics, the TMT science team has decided that adaptive optics should be implanted on all the IR instruments. We present the results of a feasibility study for the adaptive optics systems on the infrared multi-object spectrograph, IRMOS and report on the design concepts and architectural options. The IRMOS instrument is intended to produce integral field spectra of up to 20 objects distributed over a 5 arcminute field of regard. The IRMOS adaptive optics design is unique in that it will use multiple laser guidestars to reconstruct the atmospheric volume tomographically, then apply AO correction for each science direction independently. Such a scheme is made technically feasible and cost effective through the use of micro-electromechanical system (MEMS) deformable mirrors.
MIRAO: a mid-IR adaptive optics system design for TMT
We present a design of a thermal-infrared optimized adaptive optics system for the TMT 30-meter telescope. The approach makes use of an adaptive secondary but during an initial implementation contains a more conventional ambient-temperature optical relay and deformable mirror. The conventional optical relay is used without sacrificing the thermal background by using multiple off-axis laser guide stars to avoid a warm dichroic in the common path. Three laser guide stars, equally spaced 75" off axis, and a "conventional" 30×30 deformable mirror provide a Strehl > 0.9 at wavelengths longer than 10 microns and the LGS beams can be passed to the LGS wavefront sensors with pickoff mirrors while a one-arcminute field is passed unvignetted to the science instrument and NGS WFSs. The overall design is relatively simple with a wavefront correction similar to existing high-order systems (e.g. 30×30) but still provides competitive performance over the higher-order TMT NIR AO design at wavelengths as short as 3 microns due to its reduced thermal emissivity. We present our figures of merit and design considerations within the context of the science drivers for high-spectral resolution NIR/MIR spectroscopy at 5-28 microns on a 30-meter ground-based telescope.
ONIRICA: an infrared camera for OWL with MCAO low order partial correction
Roberto Ragazzoni, Renato Falomo, Carmelo Arcidiacono, et al.
ONIRICA, standing for OWL Near InfraRed Imaging Camera, is a pre-Phase A, conceptual design study to assess the feasibility of an imaging camera for a 100m class telescope. In this paper the main scientific driven and the adopted preliminary choices for its optomechanical implementation are reviewed.
Large Adaptive Mirrors
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Deformable secondary mirrors for the LBT adaptive optics system
We describe the manufacture of thin shells for the deformable secondary mirrors of the LBT adaptive optics system. The secondary mirrors are thin shells, 910 mm in diameter and 1.6 mm thick. Each mirror will have its shape controlled by 672 voice-coil actuators. The main requirement for manufacture of the shell is smoothness on scales too small to be adjusted by the actuators. An additional requirement is that the rear surface match the reference body within 30 μm peak-to-valley. A technique was developed for producing smooth surfaces on the very aspheric surfaces of the shells. We figure the optical surfaces on a thick disk of Zerodur, then turn the disk over and thin it to 1.6 mm from the rear surface. Figuring is done primarily with a 30 cm diameter stressed lap, which bends actively to match the local curvature of the aspheric surface. For the thinning operation, the mirror is blocked with pitch, optical surface down, onto a granite disk with a matching convex surface. Because the shell may bend during the blocking operation and as its thickness is reduced to 1.6 mm, figuring of the rear surface is guided by precise thickness measurements over the surface of the shell. This method guarantees that both surfaces of the finished shell will satisfy their requirements when corrected with small actuator forces. Following the thinning operation, we edge the shell to its final dimensions, remove it from the blocking body, and coat the rear surface with aluminum to provide a set of conductive plates for capacitive sensors.
A deformable secondary mirror for the VLT
ESO has initiated in June 2004 a feasibility study to investigate the possibility to retro-fit one of the VLT 8 m telescope with a deformable secondary mirror (DSM). The scope of this effort has been broadened to a concept of Adaptive Optics Facility (adaptive telescope with adapted instrument park). The feasibility study, conducted by MicroGate, ADS Intl and the INAF-Osservatorio Astrofisico di Arcetri, has been successful (no show stopper identified) and has provided an elegant design of an alternate M2-Unit for the VLT. It features a 1170 actuators DSM based on the voice coil force actuators coupled with capacitive sensors. An 80 kHz internal control loop allows implementing of electronic damping. The simulations performed have shown a fitting error of 62.5 nm rms (ro = 12.1 cm @ 30 deg. zenith) with a 2mm thin shell and 1.5 kW of heat dissipation. The design shall provide a full stroke of ~50 μm and a rise time of < 1 msec. The DSM will be focused and "centered" by a Hexapod and a bi-positions electro-mechanism will allow switching from Nasmyth to Cassegrain focus configuration. Several features are planned to ease maintenance and diagnostic.
Manufacturing of lightweight glass segments for adaptive optics
The next generation of large telescopes now on the drawing boards (30-100 m. diam) will need adaptive optics to deliver their full potential. Today the thin glass meniscus necessaries for example for the adaptive secondary mirrors are produced by tinning conventional thick mirrors: a technique expensive and time consuming. A cost effective technique for the manufacturing of these components is here proposed that will deliver thin (few mm) lightweight optics made in glass. The technique under investigation foresees the thermal slumping of thin glass segments using a high quality ceramic mold (master). The sheet of glass is placed onto the mold and then, by means of a suitable thermal cycle, the glass is softened and its shape is changed copying the master shape. At the end of the slumping the correction of the remaining errors will be performed using the Ion Beam Figuring technique, a non-contact deterministic technique. To reduce the time spent for the correction it will be necessary to have shape errors on the segments after the slumping as small as possible. To investigate this technique INAF-OAB (Astronomical Observatory of Brera) is building the necessaries facilities, in particular the oven and mold for the slumping and the Ion Beam Figuring system. The paper describes the process of production of the optical segments and the status of the investigation.
A new concept for large deformable mirrors for extremely large telescopes
For extremely large telescopes, there is strong need for thin deformable mirrors in the 3-4 m class. So far, feasibility of such mirrors has not been demonstrated. Extrapolation from existing techniques suggests that the mirrors could be highly expensive. We give a progress report on a study of an approach for construction of large deformable mirrors with a moderate cost. We have developed low-cost actuators and deflection sensors that can absorb mounting tolerances in the millimeter range, and we have tested prototypes in the laboratory. Studies of control laws for mirrors with thousands of sensors and actuators are in good progress and simulations have been carried out. Manufacturing of thin, glass mirror blanks is being studied and first prototypes have been produced by a slumping technique. Development of polishing procedures for thin mirrors is in progress.
ASSIST: the adaptive secondary setup and instrument stimulator
Remko Stuik, Robin Arsenault, Bernard Delabre, et al.
In telescopes with a Deformable Secondary Mirror, the testing and calibration of both the DSM itself as well as the instruments using this DSM are expensive and time consuming processes. Especially in telescopes without an intermediate focus before the DSM, a number of calibrations can only be performed on a real star during night time. A full suite of Adaptive Optics systems and AO-assisted instruments is currently under development for the VLT, also know as the VLT Adaptive Telescope. ASSIST was developed to assist in the integration and testing of three elements of the VLT Adaptive Telescope Facility; the DSM; the MUSE AO system 'GALACSI' and the HAWK-I AO system 'GRAAL.' The core of ASSIST is a support infrastructure to integrate the DSM in a compact and stable test setup. A Nasmyth rotator simulator will be provided for attaching the two AO systems, while ASSIST will be fed by a star simulator and turbulence generator for realistic performance measurements of both the DSM as well as the AO system under test. An on-axis high-speed interferometer will be used for additional testing of the functional operation of the DSM. In this paper we present the requirements and design of ASSIST and the projected performance of the test bench for both the testing and calibration of the DSM as well as for the two AO systems under test.
Real-Time-Control Systems
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SPARTA: the ESO standard platform for adaptive optics real time applications
Enrico Fedrigo, Robert Donaldson, Christian Soenke, et al.
ESO is starting a number of new projects collectively called Second Generation VLT instrumentation. Several of them will use Adaptive Optics (AO). In comparison with today's ESO AO systems, the 2nd Generation VLT AO systems will be much bigger (in terms of degrees of freedom) and faster (in terms of loop frequency). Consequently the Real-Time Computer controlling these AO systems will be significantly bigger and more challenging to build compared with today's AO systems in operation. To support the new requirements ESO started the development of a common flexible platform called SPARTA for Standard Platform for Adaptive optics Real Time Applications. The guidelines along which SPARTA is developed recognize the importance of industry standards over custom development to lower the development costs, ease the maintenance and make the system upgradeable thus delivering the performance required. SPARTA is based on a hybrid architecture that comprises all the major computing architectures available today: the high computational throughput is achieved through the combination of FPGA and DSP usage, where DSP are used as fast coprocessors and FPGA are used as front and as communication infrastructure, thus guaranteeing also the low latency. The flexibility is spread between the usage of both high-end CPUs and again the DSPs. All three technologies are organized in a parallel system interconnected by fast serial fabrics based on standard protocols. External input / output interfaces are also based on industry standard protocols, thus enabling the usage of commercially available tools for development and testing.
TMT adaptive optics systems control architecture
Achieving the science goals of TMT will require AO subsystems of unprecedented power and sophistication, including a Real Time Controller (RTC) subsystem that will implement wavefront reconstruction and control algorithms for up to four different laser guide star (LGS) AO systems. The requirements for the RTC represent a significant advance over the current generation of astronomical AO control systems, both in terms of the wavefront reconstruction algorithms to be employed and the new hardware approaches that will be required. Additionally, the number of active components included in the AO systems and the complexity of their interactions will require a highly automated AO Sequencer that will work in concert with the TMT Telescope and Instrument Sequencers. In this paper, we will describe the control and software requirements for the whole AO system, and in particular for the RTC and the AO Sequencer. We will describe the challenges involved in developing these systems and will present a conceptual design.
The wavefront correction system for the Advanced Technology Solar Telescope
T. Rimmele, K. Richards, J. M. Roche, et al.
An important part of a large solar telescope is the ability to correct, in real time, optical alignment errors caused by gravitational bending of the telescope structure and wavefront errors caused by atmospheric seeing. The National Solar Observatory is currently designing the 4 meter Advanced Technology Solar Telescope (ATST). The ATST wavefront correction system, described in this paper, will incorporate a number of interacting wavefront control systems to provide diffraction limited imaging performance. We will describe these systems and summarize the interaction between the various sub-systems and present results of performance modeling.
Tomagraphic wavefront correction for the LSST
The Large Synoptic Survey Telescope (LSST) is a three mirror modified Paul-Baker design with an 8.4m primary, a 3.4m secondary, and a 5.0m tertiary followed by a 3-element refractive corrector producing a 3.5 degree field of view. This design produces image diameters of <0.3 arcsecond 80% encircled energy over its full field of view. The image quality of this design is sufficient to ensure that the final images produced by the telescope will be limited by the atmospheric seeing at an excellent astronomical site. In order to maintain this image quality, the deformations and rigid body motions of the three large mirrors must be actively controlled to minimize optical aberrations. By measuring the optical wavefront produced by the telescope at multiple points in the field, mirror deformations and rigid body motions that produce a good optical wavefront across the entire field may be determined. We will describe the details of the techniques for obtaining these solutions. We will show that, for the expected mirror deformations and rigid body misalignments, the solutions that are found using these techniques produce an image quality over the field that is close to optimal. We will discuss how many wavefront sensors are needed and the tradeoffs between the number of wavefront sensors, their layout and noise sensitivity.
Modal Fourier wavefront reconstruction using graphics processing units
José M. Rodríguez-Ramos, José G. Marichal-Hernández, Fernando Rosa
Large degree-of-freedom real-time adaptive optics control requires reconstruction algorithms computationally efficient and readily parallelized for hardware implementation. Lysa Poyneer (2002) has shown that the wavefront reconstruction with the use of the fast Fourier transform (FFT) and spatial filtering is computationally tractable and sufficiently accurate for its use in large Shack-Hartmann-based adaptive optics systems (up to 10,000 actuators). We show here that by use of Graphical Processing Units (GPUs), a specialized hardware capable of performing FFTs on big sequences almost 7 times faster than a high-end CPU, a problem of up to 50,000 actuators can be already done within a 6 ms limit. The method to adapt the FFT in an efficient way for the underlying architecture of GPUs is given.
Natural Guide Star AO Field Tests
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On-sky results of the adaptive optics MACAO for the new IR-spectrograph CRIRES at VLT
J. Paufique, P. Biereichel, B. Delabre, et al.
The adaptive optics MACAO has been implemented in 6 focii of the VLT observatory, in three different flavors. We present in this paper the results obtained during the commissioning of the last of these units, MACAO-CRIRES. CRIRES is a high-resolution spectrograph, which efficiency will be improved by a factor two at least for point-sources observations with a NGS brighter than R=15. During the commissioning, Strehl exceeding 60% have been observed with fair seeing conditions, and a general description of the performance of this curvature adaptive optics system is done.
Results from the GMT ground-layer experiment at the Magellan Telescopes
A. Athey, S. Shectman, M. Phillips, et al.
We present results from our two year study of ground-layer turbulence as seen through the 6.5-meter Magellan Telescopes at Las Campanas Observatory. The experiment consists of multiple, moderate resolution, Shack- Hartmann wavefront sensors deployed over a large 16 arcminute field. Over the two years of the experiment, the ground-layer turbulence has been sampled on eleven nights in a variety of seeing and wind conditions. On most nights the ground-layer turbulence contributes 10% to the total visible-band seeing, although a few nights exhibit ground-layer contributions up to 30%. We present the ground-layer turbulence on the sampled nights as well as a demonstration of its strength as a function of field size. This information is combined with data from a MASS-DIMM seeing monitor adjacent to the Magellan Telescopes to infer the annual ground-layer contribution to seeing at Las Campanas.
PYRAMIR: first on-sky results from an infrared pyramid wavefront sensor
PYRAMIR is a pyramid wavefront sensor (PWFS) for the 97-actuator AO system installed on the Calar Alto 3.5 m telescope. With its linear pupil sampling of 18 pixels, its maximum loop frequency of 140 Hz, and its sensing wavelength range from 1.1 micron to 2.4 micron it should be able to deliver reasonably high Strehl ratios at the sensing wavelength. This feature is still unique in the world of pyramid sensors. The first on-sky test of the system was carried out in March 2006. In this paper we will present the first results of this test. Strehl measurements medium atmospheric conditions, using reference stars of mJ=8mag and mJ=4 mag and were performed during this first on-sky run. A detailed comparison to simulation results will also be presented in order to confirm whether the system works up to expectances. While this experiment has not yet the potential to show for the very first time the superiority of the pyramid principle over corresponding Hartmann-Shack systems in a real telescope environment, it was confirmed that PYRAMIR performs up to expectances and a detailed comparison to the Shack-Hartmann system can be carried out in the next run.
Conventional Wavefront Sensors and Concepts
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Optimization of center of gravity algorithms in a Shack-Hartmann sensor
We propose analytical studies supported by simulation of various centroiding algorithms for Shack-Hartmann based wavefront sensor. We focused on the simple center of gravity as well as one of its optimization, the weighted center of gravity. Noise effects, as well as linearity issues and high flux bias induced by sub-aperture size and PSF structures are investigated. For each method, optimal parameters are defined in function of photon flux, readout noise, and turbulence level.
Laser guide star Shack-Hartmann wavefront sensor modeling: matched filtering, wavefront sensor nonlinearity, and impact of sodium layer variability for the Thirty Meter Telescope
This paper describes modeling and simulation results for the Thirty Meter Telescope on the degradation of sodium laser guide star Shack-Hartmann wavefront sensor measurement accuracy that will occur due to the spatial structure and temporal variations of the mesospheric sodium layer. Using a contiguous set of LIDAR measurements of the sodium profile, the performance of a standard centroid and of a more refined noise-optimal matched filter spot position estimation algorithm is analyzed and compared for a nominal mean signal level equal to 1000 photo-detected electrons per subaperture per integration time, as a function of subaperture to laser launch telescope distance and CCD pixel read out noise. Both algorithms are compared in terms of their rms spot position estimation error due to noise, their associated wavefront error when implemented on the Thirty Meter Telescope facility adaptive optics system, their linear dynamic range and their bias when detuned from the current sodium profile.
High SNR measurement of interaction matrix on-sky and in lab
The fundamental task of AO system calibration is the acquisition of the Interaction Matrix (IM). This task is usually performed in a laboratory or at the telescope using a reference fiber illuminating both deformable mirror and wavefront sensor. The problem of measuring the IM on a bright reference star has been attacked by some authors. The principal problem of this measurement is to achieve a high SNR when atmospheric turbulence is present. This is very difficult if sensor signals are simply time averaged to get rid of the turbulence effects. The paper presents a new technique to perform an on sky measurement of the IM with high SNR and reducing the overall measurement time by an order of magnitude. This technique can be very useful for AO systems using large size DMs like MMT, LBT and possibly VLT and OWL. In these cases fiber-based IM measurements require challenging optical set-up that in some cases, like for OWL, are unpractical to build. The technique is still relevant for classical small DM AO systems that could be calibrated on sky avoiding misregistration errors. Finally this technique is valuable for laboratory measurements when the IM of an AO system has to be measured with great accuracy against external disturbances like bench vibrations, local turbulence effects and so on. Again IM measurement SNR is increased and the overall measurement time can be significantly reduced. The paper will introduce and detail the technique physical principle and quantify with numerical simulations the SNR improvement achieved using this technique. Finally laboratory results obtained during the test of the LBT AO system prototype are given and compared to simulations.
Lick Observatory adaptive optics wavefront sensor upgrades
The wavefront sensor camera and tip/tilt sensor APDs that were on Lick Observatory's Shane 3 Meter Adaptive Optics system were over a decade old and showing their age. They were recently upgraded. The first upgrade was to convert from quad-APDs in the laser guidestar mode natural star tip-tilt sensor to a sensitive low-noise CCD. The new CCD in this position, an 80x80 E2V CCD-39 inside a SciMeasure camera, has a low enough read noise, ~3 e-/pixel, that the tip/tilt measurement in closed-loop operation is photon noise limited and thus benefits from the improved quantum efficiency of the CCD. We have demonstrated on-sky up to two magnitudes of improvement in viable tip/tilt star brightness, which greatly extends the available sky coverage in the laser guidestar AO mode. Also, the increased field of view of the new tip/tilt sensor provides a much more reliable means of acquiring and locking on dim tip/tilt stars, making the whole system operationally more efficient. In the second phase of the upgrade project, the high order wavefront sensor has been replaced, also with a CCD-39 chip in a SciMeasure camera. In this paper we will describe these upgrades and present preliminary performance results.
The design and optimization of detectors for adaptive optics wavefront sensing
The most common detector configuration for Shack Hartmann (SH) wavefront sensors used for adaptive optics (AO) wavefront sensing is the quad cell. Advances in detectors, such as the CCDs being developed in a project on which we are collaborators (funded by the Adaptive Optics Development Program), make it possible to use larger pixel arrays. The CCD designs incorporate improved read amplifiers and novel pixel geometries optimized for laser guide star (LGS) AO wavefront sensing. While it is likely that finer sampling of the SH spot will improve the ability of the wavefront sensor to accurately determine the spot displacement, particularly for elongated or aberrated spots such as those seen in LGS AO systems, the optimal sampling is not dependent simply on the number of pixels but must also take into account the effects of photon and detector noise. The performance of a SH wavefront sensor also depends on the performance of the algorithm used to find the spot displacement. In the literature alternatives have been proposed to the common center of mass algorithm, but these have not been simulated in detail. In this paper we will describe the results of our study of the performance of a SH wavefront sensor with a well sampled spot. We will present results for simulations of the wavefront sensor that enable us to optimize the design of the detector for varying conditions of signal to noise and spot elongation. We will also discuss the application of correlation algorithms to SH wavefront sensors and present results regarding the performance and statistics of this algorithm.
Laser Systems, Facilities, and Concepts
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20-W and 50-W solid-state sodium beacon guidestar laser systems for the Keck I and Gemini South telescopes
Allen K. Hankla, Jarett Bartholomew, Ken Groff, et al.
Lockheed Martin Coherent Technologies (LMCT) is developing 20 W and 50 W commercial solid-state sodium beacon Guidestar Laser Systems (GLS) for the Keck I and Gemini South telescopes, respectively. This work represents a critical step toward addressing the need of the astronomical adaptive optics (AO) community for a standardized, robust, turn-key, commercial GLS that can be configured for different observatory facilities and for different AO formats - including multi-conjugate AO (MCAO) and future extremely large telescopes. These modular systems build on the proven laser technologies, user-friendly interface, and low maintenance design that were developed for the successful 12 W GLS delivered by LMCT to the Gemini North telescope in February 2005. This paper describes the GLS requirements for the Keck I and Gemini South telescopes, the design of the laser oscillators, amplifiers, sum-frequency generator, and diagnostics; the functionality of the automated remote laser control system; size, weight, power, and performance data; and the current status of the programs.
The Laser Guide Star Facility for the Thirty Meter Telescope
Richard Joyce, Corinne Boyer, Larry Daggert, et al.
The Thirty Meter Telescope (TMT) will utilize adaptive optics to achieve near diffraction-limited images in the near-infrared using both natural and laser guide stars. The Laser Guide Star Facility (LGSF) will project up to eight Na laser beacons to generate guide stars in the Earth's Na layer at 90 - 110 km altitude. The LGSF will generate at least four distinct laser guide star patterns (asterisms) of different geometry and angular diameter to meet the requirements of the specific adaptive optics modules for the TMT instruments. We describe the baseline concept for this facility, which draws on the heritage from the systems being installed at the Gemini telescopes. Major subsystems include the laser itself and its enclosure, the optics for transferring the laser beams up the telescope structure and the asterism generator and launch telescope, both mounted behind the TMT secondary mirror. We also discuss operational issues, particularly the required safety interlocks, and potential future upgrades to higher laser powers and precompensation of the projected laser beacons using an uplink adaptive optics system.
Focus errors from tracking sodium layer altitude variations with laser guide star adaptive optics for the Thirty Meter Telescope
Glen Herriot, Paul Hickson, Brent Ellerbroek, et al.
Laser guide star (LGS) adaptive optics systems for extremely large telescopes must handle an important effect that is negligible for current generation telescopes. Wavefront errors, due to improperly focusing laser wavefront sensors (WFS) on the mesospheric sodium layer, are proportional to the square of the telescope diameter. The sodium layer, whose mean altitude is approximately 90 km, can move vertically at rates of up to a few metres per second; a few seconds lag in refocusing can substantially degrade delivered image quality (15 m of defocus can cause 120 nm residual wavefront error on a 30-m telescope.) As well, the range of temporal frequencies of sodium altitude focus, overlaps the temporal frequencies of focus caused by atmospheric turbulence. Only natural star wavefront sensors can disentangle this degeneracy. However, applying corrections with representative focus mechanisms having modest control bandwidths causes appreciable tracking errors. In principle, electronic offsets measured by natural guide star detectors could be rapidly applied to laser WFS measurements, but to provide useable sky coverage, integrating sufficient photons causes an unavoidable time delay, again resulting in potentially serious focus tracking errors. However, our analysis depends on extrapolating to temporal frequencies greater than 1 Hz from power spectra of sodium profile time series taken at 1-2 minute intervals. In principle, with a pulsed laser, (e.g. 3-μs pulses) and dynamic refocusing on a polar-coordinate CCD, this focus tracking error may be eliminated. This result is an additional benefit of dynamic refocusing beyond the commonly recognized amelioration of LGS WFS spot elongation.
A compact modular scalable versatile laser guidestar system architecture for 8-100 m telescopes
Allen J. Tracy, John Hobbs, Iain McKinnie, et al.
Lockheed Martin Coherent Technologies (LMCT) reports on the development of a compact, scalable versatile optical waveform sodium guidestar laser system (GLS) suitable for Adaptive Optics (AO) systems on Extremely Large Telescopes (ELT's) and smaller telescopes. We have successfully completed phase 1 of the three-phase, 4½ year, NSF funded, National Optical Astronomy Observatory (NOAO) sponsored program. The GLS can be optimized for efficient sodium layer interaction for each telescope / AO system with mitigation of parasitic effects such as Rayleigh or cirrus cloud scatter of adjacent beacon light in multi-conjugate adaptive optics (MCAO) and spot elongation in the sodium layer from off-axis light launch in an ELT. The proposed solid-state laser architecture incorporates patent-pending self-imaging waveguide technology and is based on a set of requirements that was determined after extensive discussions with the astronomy adaptive optics community. This paper presents data on single beacon, Rayleigh compensating, and elongation compensating waveforms that were demonstrated through all stages of the architecture, as well as demonstrated and anticipated 589 nm power levels for each waveform. The design of the master oscillator power amplifier (MOPA) architecture, modulation methodology, power amplification, and sum-frequency generation stages is also described. System attributes, including size, weight, and power will also be discussed.
Characteristics of sodium guidestars created by the 50-watt FASOR and first closed-loop AO results at the Starfire Optical Range
Craig A. Denman, Jack D. Drummond, Mark L. Eickhoff, et al.
A computer-automated cw sodium guidestar FASOR (Frequency Addition Source of Optical Radiation) producing a single frequency 589-nm beam with up to 50 W for mesospheric beacon generation has been integrated with the 3.5-m telescope at the Starfire Optical Range, Kirtland AFB, New Mexico. Radiance tests have produced a peak guidestar V1 magnitude = 5.1 (~7000 photons/s/cm2 at zenith) for 30 W of circularly polarized pump power in November 2005. Estimated theoretical maximum guidestar radiance is about 3 times greater than measured values indicating saturation due to atoms possibly becoming trapped in F'=1 and/or atomic recoil. From sky tests over 3.5 years, we have tracked the annual variation of the sodium column density by measuring the return flux as a function of fasor power and determining the slope at zero power. The maximum occurs on October 30 and the minimum on May 30, with corresponding predicted returns of 8000 (V1 = 4.8) and 3000 (V1 = 5.8) ph/s/cm2 with 50 W of fasor power and circular polarization. The effect of the Earth's magnetic field on the radiance of the sodium laser guidestar (LGS) from various azimuths and elevations has been measured. The peak return flux over our observatory occurs at [az=198o; el=+71o], compared with the direction of the magnetic field lines at [190o; +62o], and it can vary by a factor of 3 over the sky above el = 30o. First results for non-optimized sodium LGS adaptive optics (AO) closed-loop operation have been obtained using binary stars. Strehl ratios of 0.03 have been measured at 850 nm and a 0.14 arc second binary star has been resolved during first closed loop observations. Guidestar characteristics, including radiance, size, and Rayleigh backscatter, the sodium LGS wavefront sensor (WFS) AO system, and recent closed-loop results on binary stars are presented.
AFIRE: fiber Raman laser for laser guide star adaptive optics
D. Bonaccini Calia, W. Hackenberg, S. Chernikov, et al.
Future adaptive optics systems will benefit from multiple sodium laser guide stars in achieving satisfactory sky coverage in combination with uniform and high-Strehl correction over a large field of view. For this purpose ESO is developing with industry AFIRE, a turn-key, rack-mounted 589-nm laser source based on a fiber Raman laser. The fiber laser will deliver the beam directly at the projector telescope. The required output power is in the order of 10 W in air per sodium laser guide star, in a diffraction-limited beam and with a bandwidth of < 2 GHz. This paper presents the design and first demonstration results obtained with the AFIRE breadboard. 4.2W CW at 589nm have so far been achieved with a ~20% SHG conversion efficiency.
High Precision Wavefront Control
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Wavefront control for the Gemini Planet Imager
Lisa A. Poyneer, Jean-Pierre Véran, Daren Dillon, et al.
The wavefront control strategy for the proposed Gemini Planet Imager, an extreme adaptive optics coronagraph for planet detection, is presented. Two key parts of this strategy are experimentally verified in a testbed at the Laboratory for Adaptive Optics, which features a 32 × 32 MEMS device. Detailed analytic models and algorithms for Shack-Hartmann wavefront sensor alignment and calibration are presented. It is demonstrated that with these procedures, the spatially filtered WFS and the Fourier Transform reconstructor can be used to flatten to the MEMS to 1 nm RMS in the controllable band. Performance is further improved using the technique of modifying the reference slopes using a measurement of the static wavefront error in the science leg.
Amplitude and phase controlled adaptive optics system
Ich Pham, Sam Ma
An adaptive optics (AO) system is used to control the deformable mirror (DM) actuators for compensating the optical effects introduced by the turbulence in the Earth's atmosphere and distortions produced by the optical elements between the distant object and its local sensor. The typical AO system commands the DM actuators while minimizing the measured wave front (WF) phase error. This is known as the phase conjugator system, which does not work well in the strong scintillation condition because both amplitude and phase are corrupted along the propagation path. In order to compensate for the wave front amplitude, a dual DM field conjugator system may be used. The first and second DM compensate for the amplitude and the phase respectively. The amplitude controller requires the mapping from DM1 actuator command to DM2 intensity. This can be obtained from either a calibration routine or an intensity transport equation, which relates the phase to the intensity. Instead of a dual-DM, a single Spatial Light Modulator (SLM) may control the amplitude and phase independently. The technique uses the spatial carrier frequency and the resulting intensity is related to the carrier modulation, while the phase is the average carrier phase. The dynamical AO performance using the carrier modulation is limited by the actuator frequency response and not by the computational load of the controller algorithm. Simulation of the proposed field conjugator systems show significant improvement for the on-axis performance compared to the phase conjugator system.
Woofer-tweeter tip-tilt control for NFIRAOS on TMT
NFIRAOS is the facility AO system for TMT. NFIRAOS does not have a separate tip-tilt mirror. Instead one of its two deformable mirrors will be mounted on a tip-tilt platform, which, due to the weight of the deformable mirror, will only have a ~ 20 Hz bandwidth. This is too slow to correct the tip-tilt disturbance well enough, especially the windshake, which, in the case of TMT, is much more challenging to correct than the atmospheric tip-tilt. Tip-tilt can also be corrected directly on the deformable mirrors, but only a fraction of the incoming tip-tilt can be corrected that way, because of the limited stroke of the actuators. In this paper, we propose a woofer-tweeter approach, by which the high amplitude low temporal frequencies of tip-tilt are corrected by the tip-tilt platform, whereas the low amplitude high temporal frequencies are corrected by the deformable mirrors. This approach is based on a double integration control scheme and provides a much better attenuation of the windshake: 0.4 mas rms instead of 3.6, corresponding to an equivalent high order error of 20 nm rms instead of 180 nm rms. We find that only ~ 10% of the initial 25 mas rms windshake needs to be corrected by the DMs. With a 5-σ margin, this corresponds to only an extra ~1 micron of stroke for the actuators at the edge of the pupil, which are the most affected.
Nonlinear control for pyramid sensors in adaptive optics
Ole Wulff, Douglas Looze
This paper presents a nonlinear modification of the standard linear adaptive optics controller that greatly reduces the startup lag due to saturation in pyramid wavefront sensors (or any wavefront sensor with 4-cell subapertures). Whenever the pyramid sensor is saturated (as determined by an internal model of the saturation in the controller), the deformable mirror signal is adjusted to quickly remove the saturation of the sensor. When the pyramid sensor is not saturated, a standard linear adaptive optics controller is used to generate the deformable mirror signal. The nonlinear controller is compared with a standard linear controller using a CAOS simulation. It is shown that the startup lag is significantly reduced, and that the steady-state performance is identical to the linear controller.
Multi-Conjugate AO and Control System Lab Tests
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First laboratory demonstration of closed-loop Kalman based optimal control for vibration filtering and simplified MCAO
Classic Adaptive Optics (AO) is now successfully implemented on a growing number of ground-based imaging systems. Nevertheless some limitations are still to cope with. First, the AO standard control laws are unable to easily handle vibrations. In the particular case of eXtreme AO (XAO), which requires a highly efficient AO, these vibrations can thus be much penalizing. We have previously shown that a Kalman based control law can provide both an efficient correction of the turbulence and a strong vibration filtering. Second, anisoplanatism effects lead to a small corrected field of view. Multi-Conjugate AO (MCAO) is a promising concept that should increase significantly this field of view. We have shown numerically that MCAO correction can be highly improved by optimal control based on a Kalman filter. This article presents the first laboratory demonstration of these two concepts. We use a classic AO bench available at Onera with a deformable mirror (DM) in the pupil and a Shack-Hartmann Wave Front Sensor (WFS) pointing at an on-axis guide-star. The turbulence is produced by a rotating phase screen in altitude. First, this AO configuration is used to validate the ability of our control approach to filter out system vibrations and improve the overall performance of the AO closed-loop, compared to classic controllers. The consequences on the RTC design of an XAO system is discussed. Then, we optimize the correction for an off-axis star although the WFS still points at the on-axis star. This Off-Axis AO (OAAO) can be seen as a first step towards MCAO or Multi-Object AO in a simplified configuration. It proves the ability of our control law to estimate the turbulence in altitude and correct in the direction of interest. We describe the off-axis correction tests performed in a dynamic mode (closed-loop) using our Kalman based control. We present the evolution of the off-axis correction according to the angular separation between the stars. A highly significant improvement in performance is demonstrated.
Laboratory for adaptive optics at UC Santa Cruz: project status and plans
The UCO/Lick Observatory Laboratory for Adaptive Optics charter goal is to advancing the state of the art in adaptive optics technology for instruments on the current and next generation of extremely large telescopes. We are investigating the architecture and techniques for implementing wide field adaptive optics systems for general purpose imaging and spectroscopy and high contrast adaptive optics systems for imaging extrasolar planets. The laboratory has two testbeds, a high contrast extreme adaptive optics (ExAO) testbed and a multi-guidestar tomography adaptive optics testbed. The later is reconfigurable between multi-conjugate AO (MCAO) and multi-object AO (MOAO) architectures. The testbeds are scaled to emulate 10 to 30 meter aperture telescope AO systems and allow systematic study of the performance and practicalities of such systems. Additionally, we are developing and testing new AO component technologies including novel wavefront sensors and MEMS deformable mirrors. In this paper we highlight the status and direction of the laboratory experiments and summarize the latest results.
The woofer-tweeter experiment
The Woofer-Tweeter experiment started in the Adaptive Optics Laboratory of the University of Victoria in February 2005 has recently achieved completion. The goal of this experiment is to validate the woofer-tweeter AO concept i.e. instead to have a single deformable mirror conjugated at the ground, two DMs conjugated at the ground are used to achieve both the necessary stroke and actuators density required for a single DM for an ELT. Recently, the loop has been closed on the turbulence with a loop rate of 100Hz. Two closed-loop controllers have been tested so far: a global integrator and a tweeter off-loading integrator. This paper describes the UVic Woofer-Tweeter bench layout and components and the Woofer-Tweeter simulation tool used to both model and control the experiment. A glimpse on the very first results from the closed-loop operations is also given.
Analytical Studies
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Task performance in astronomical adaptive optics
In objective or task-based assessment of image quality, figures of merit are defined by the performance of some specific observer on some task of scientific interest. This methodology is well established in medical imaging but is just beginning to be applied in astronomy. In this paper we survey the theory needed to understand the performance of ideal or ideal-linear (Hotelling) observers on detection tasks with adaptive-optical data. The theory is illustrated by discussing its application to detection of exoplanets from a sequence of short-exposure images.
ELTs adaptive optics for multi-objects 3D spectroscopy: key parameters and design rules
In the last few years, new Adaptive Optics [AO] techniques have emerged to answer new astronomical challenges: Ground-Layer AO [GLAO] and Multi-Conjugate AO [MCAO] to access a wider Field of View [FoV], Multi-Object AO [MOAO] for the simultaneous observation of several faint galaxies, eXtreme AO [XAO] for the detection of faint companions. In this paper, we focus our study to one of these applications : high red-shift galaxy observations using MOAO techniques in the framework of Extremely Large Telescopes [ELTs]. We present the high-level specifications of a dedicated instrument. We choose to describe the scientific requirements with the following criteria : 40% of Ensquared Energy [EE] in H band (1.65μm) and in an aperture size from 25 to 150 mas. Considering these specifications we investigate different AO solutions thanks to Fourier based simulations. Sky Coverage [SC] is computed for Natural and Laser Guide Stars [NGS, LGS] systems. We show that specifications are met for NGS-based systems at the cost of an extremely low SC. For the LGS approach, the option of low order correction with a faint NGS is discussed. We demonstrate that, this last solution allows the scientific requirements to be met together with a quasi full SC.
Strehl ratio and image sharpness for adaptive optics
Julian C. Christou, Kenneth J. Mighell, Russell B. Makidon
The performance of an adaptive optics system is typically given in terms of the Strehl ratio of a point spread function (PSF) measured in the focal plane of the system. The Strehl ratio measures the normalized peak intensity of the PSF compared to that of an ideal PSF, i.e. aberration-free, through the system. One advantage of this metric is that it has been shown to be proportional to the rms wavefront error via the Marechel approximation. Thus, Strehl ratio measurements are used to determine the performance of the system. Measurement of the Strehl ratio is frequently problematic in the presence of noise as can be the peak determination for critically sampled data. We have looked at alternative metrics, in particular the S1 sharpness metric. This metric measures the compactness of the PSF by the normalized sum of the squared image intensity and therefore relates to the intensity variance of the image. Using simulated AO PSFs, we show that there is a unique relationship between S1 and the Strehl ratio and we can therefore relate it back to the rms wavefront error.
Correctability limitations imposed by spherical-wave scintillation in multi-conjugate adaptive optics
Spherical-wave scintillation is shown to impose multi-conjugate adaptive optics (MCAO) correctability limitations that are independent of wavefront sensing and reconstruction. Residual phase and log-amplitude variances induced by scintillation in weak turbulence are derived using (diffraction-based) diffractive MCAO spatial filters or (diffraction-ignorant) geometric MCAO proportional gains as linear open-loop control parameters. In the case of Kolmogorov turbulence, expressions involving the Rytov variance and/or weighted Cn2 integrals apply. Differences in performance between diffractive MCAO and geometric MCAO resemble chromatic errors. Optimal corrections based on least squares imply irreducible performance limits that are validated by wave-optic simulations.
Large DM AO systems: synthetic IM or calibration on sky?
Several designs of future Adaptive Optics (AO) systems propose to use a large Deformable Mirror (DM), regarding the size as well as the number of actuators. Most of the time, there is no focal plane upstream the DM. Therefore, the classical way of calibrating the interaction matrix on an artificial source cannot be applied. Furthermore, the requirements in terms of calibration error budget are tight and the high order modes of such DMs are stiff and hence they achieve only a small stroke. This is why novel ways to determine the system Interaction Matrix (IM) have to be investigated. Several paths have been studied. One solution would be to simulate a synthetic IM. However, calibration on sky is also an option. Different techniques were simulated, tested and optimized on real AO systems. The results are presented in this paper.
MEMS and Deformable Mirror Modeling
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Characterizing the potential of MEMS deformable mirrors for astronomical adaptive optics
Current high-contrast "extreme" adaptive optics (ExAO) systems are partially limited by deformable mirror technology. Mirror requirements specify thousands of actuators, all of which must be functional within the clear aperture, and which give nanometer flatness yet micron stroke when operated in closed loop.1 Micro-electrical mechanical-systems (MEMS) deformable mirrors have been shown to meet ExAO actuator yield, wavefront error, and cost considerations. This study presents the performance of Boston Micromachines' 1024-actuator continuous-facesheet MEMS deformable mirrors under tests for actuator stability, position repeatability, and practical operating stroke. To explore whether MEMS actuators are susceptible to temporal variation, a series of long-term stability experiments were conducted. Each actuator was held fixed and the motion over 40 minutes was measured. The median displacement of all the actuators tested was 0.08 nm surface, inclusive of system error. MEMS devices are also appealing for adaptive optics architectures based on open-loop correction. In experiments of actuator position repeatability, 100% of the tested actuators returned repeatedly to their starting point with a precision of < 1 nm surface. Finally, MEMS devices were tested for maximum stroke achieved under application of spatially varying one-dimensional sinusoids. Given a specified amplitude in voltage, the measured stroke was 1 μm surface at the low spatial frequencies, decreasing to 0.2 μm surface for the highest spatial frequency. Stroke varied somewhat linearly as inverse spatial frequency, with a flattening in the relation at the high spatial frequency end.
Electrostatic micro-deformable mirror for adaptive optics
Frederic Zamkotsian, Arnaud Liotard, Patrick Lanzoni, et al.
Next generation adaptive optical (AO) systems require deformable mirrors with very challenging parameters, up to 250 000 actuators and inter-actuator spacing around 500μm. MOEMS-based devices are promising for the development of a complete generation of new deformable mirrors. We are currently developing a micro-deformable mirror (MDM) based on an array of electrostatic actuators with attachments to a continuous mirror on top. The originality of our approach lies in the elaboration of layers made of polymer materials. Mirrors with very efficient planarization and active actuators have been demonstrated, with a piston motion of 2μm for 30V. Using our dedicated characterization bench, we have measured a 6.5kHz resonance frequency, well suited for AO applications. Based on the design of this actuator and our polymer process, realization of a complete polymer-MDM is under way. The electrostatic force provides a non-linear actuation, while AO systems are based on linear matrices operations. Then, we have developed a dedicated 14-bit electronics in order to "linearize" the actuation. After calibrating the behavior of each actuator and fitting the curve by a sixth order polynomial, the electronics delivers a linearized output. The response is nearly perfect over our 3×3 MDM prototype with a standard deviation of 3.5 nm, and we have then obtained the influence function of the central actuator. First evaluation on the cross non-linarities has also been evaluated on the OKO mirror and a simple look-up table is sufficient for determining the location of each actuator whatever the locations of the neighbor actuators. Electrostatic MDM are particularly well suited for AO applications.
Modeling and open-loop control of point-actuated continuous facesheet deformable mirrors
In this paper we present a mathematical model for a point-actuated, continuous facesheet deformable mirror. The model consists of a single partial differential equation for the facesheet coupled with a number of nonlinear algebraic constraints (one constraint per actuator). We also present a nonlinearly constrained quadratic minimization problem whose solution gives the quasi-steady state control for the mirror, given a target wavefront aberration.
A fast high-fidelity model for the deformation of continuous facesheet deformable mirrors
A new analytic approach for the description of deformable mirror facesheet deformation is presented. This new approach contains a high degree of physics fidelity, yet is relatively simple to implement and quick to use when compared with the equally-accurate finite element approach. Modeled physics include thin plate treatment for the deformable mirror facesheet and full mechanical coupling between the facesheet and the underlying actuators. Example influence functions for a circular DM are presented.
Actuator tests for a large deformable membrane mirror
Roger Hamelinck, Nick Rosielle, Maarten Steinbuch, et al.
In the design of a large adaptive deformable membrane mirror, variable reluctance actuators are used. These consist of a closed magnetic circuit in which a strong permanent magnet provides a static magnetic force on a ferromagnetic core which is suspended in a membrane. By applying a current through the coil which is situated around the magnet, this force is influenced, providing movement of the ferromagnetic core. This movement is transferred via a rod imposing the out-of-plane displacements in the reflective deformable membrane. In the actuator design a match is made between the negative stiffness of the magnet and the positive stiffness of the membrane suspension. If the locality of the influence functions, mirror modes as well as force and power dissipation are taken into account, a resonance frequency of 1500 Hz and an overall stiffness of 1000 N/m for the actuators is needed. The actuators are fabricated and the dynamic response tested in a dedicated setup. The Bode diagram shows a first eigenfrequency of 950 Hz. This is due to a lower magnetic force than expected. A Helmholtz coil setup was designed to measure the differences in a large set of permanent magnets. With the same setup the 2nd quadrant of the B-H curve is reconstructed by stacking of the magnets and using the demagnetization factor. It is shown that the values for Hc and Br of the magnets are indeed lower than the values used for the initial design. New actuators, with increased magnet thickness, are designed and currently fabricated.
Pyramid and Layer-Oriented Wavefront Sensing
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PYRAMIR: construction and implementation of the world's first infrared pyramid sensor
In this paper we present an overview of the construction and implementation of the unmodulated infrared pyramid wavefront sensor PYRAMIR at the Calar Alto 3.5 m telescope. PYRAMIR is an extension of the existing visible Shack-Hartmann adaptive optics system ALFA, which allows wavefront sensing in the near-infrared wavefront regime. We describe the optical setup and the calibration procedure of the pyramid wavefront sensor. We discuss possible drawbacks of the calibration and show the results gained on Calar Alto.
Laboratory testing the layer oriented wavefront sensor for the multiconjugate adaptive optics demonstrator
The Multiconjugate Adaptive optics Demonstrator (MAD) for ESO-Very Large Telescopes (VLT) will demonstrate on sky the MultiConjugate Adaptive Optics (MCAO) technique. In this paper the laboratory tests relative to the first preliminary acceptance in Europe of the Layer Oriented (LO) Wavefront Sensor (WFS) for MAD will be described: the capabilities of the LO approach have been checked and the ability of the WFS to measure phase screens positioned at different altitudes has been experimented. The LO WFS was opto-mechanically integrated and aligned in INAF - Astrophysical Observatory of Arcetri before the delivering to ESO (Garching) to be installed on the final optical bench. The LO WFS looks for up to 8 reference stars on a 2arcmin Field of View and up to 8 pyramids can be positioned where the focal spot images of the reference stars form, splitting the light in four beams. Then two objectives conjugated at different altitudes simultaneously produce a quadruple pupil image of each reference star. An optical bench setup and transparent plastic screens have been used to simulate telescope and static atmospheric layers at different altitudes and a set of optical fibers as (white) light source. The plastic screens set has been characterized using an inteferometer and the wave-front measurements compared to the LO WFS ones have shown correlation up to ~95%.
Two-sided pyramid wavefront sensor in the direct phase mode
The two-sided pyramid wavefront sensor has been extensively simulated in the direct phase mode using a wave optics code. The two-sided pyramid divides the focal plane so that each half of the core only interferes with the speckles in its half of the focal plane. A relayed image of the pupil plane is formed at the CCD camera for each half. Antipodal speckle pairs are separated so that a pure phase variation causes amplitude variations in the two images. The phase is reconstructed from the difference of the two amplitudes by transforming cosine waves into sine waves using the Hilbert transform. There are also other corrections which have to be applied in Fourier space. The two-sided pyramid wavefront sensor performs extremely well: After two or three iterations, the phase error varies purely in y. The twosided pyramid pair enables the phase to be completely reconstructed. Its performance has been modeled closed loop with atmospheric turbulence and wind. Both photon noise and read noise were included. The three-sided and four-sided pyramid wavefront sensors have also been studied in direct phase mode. Neither performs nearly as well as does the two-sided pyramid wavefront sensor.
The MCAO wavefront sensing system of LINC-NIRVANA: status report
LINC-NIRVANA is an infrared camera that will work in Fizeau interferometric way at the Large Binocular Telescope (LBT). The two beams that will be combined in the camera are corrected by an MCAO system, aiming to cancel the turbulence in a scientific field of view of 2 arcminutes. The MCAO wavefront sensors will be two for each arm, with the task to sense the atmosphere at two different altitudes (the ground one and a second height variable between a few kilometers and a maximum of 15 kilometers). The first wavefront sensor, namely the Ground layer Wavefront sensor (GWS), will drive the secondary adaptive mirror of LBT, while the second wavefront sensor, namely the Mid High layer Wavefront Sensor (MHWS) will drive a commercial deformable mirror which will also have the possibility to be conjugated to the same altitude of the correspondent wavefront sensor. The entire system is of course duplicated for the two telescopes, and is based on the Multiple Field of View (MFoV) Layer Oriented (LO) technique, having thus different FoV to select the suitable references for the two wavefront sensor: the GWS will use the light of an annular field of view from 2 to 6 arcminutes, while the MHWS will use the central 2 arcminutes part of the FoV. After LINC-NIRVANA has accomplished the final design review, we describe the MFoV wavefront sensing system together with its current status.
SUPY: an infrared pyramid wavefront sensor for Subaru
M. Feldt, Y. Hayano, H. Takami, et al.
The 8 m SUBARU telescope atop Mauna Kea on Hawaii will shortly be equipped with a 188 actuator adaptive optics system (AO 188). Additionally it will be equipped with a Laser guide star (LGS) system to increase the sky coverage of that system. One of the additional tip-tilt sensor which is required to operate AO 188 in LGS mode will be working in the infrared to further enhance the coverage in highly obscured regions of the sky. Currently, various options for this sensor are under study, however the baseline design is a pyramid wavefront sensor. It is currently planned to have this sensor be able to provide also information on higher modes in order to feed AO 188 alone, i.e. without the LGS when NIR-bright guide stars are available. In this paper, we will present the results of the basic design tradeoffs, the performance analysis, and the project plan. Choices to be made concern the number of subapertures available across the primary mirror, the number of corrected modes, control of the AO system in combination with and without LGS, the detector of the wavefront sensor, the operation wavelength range and so forth. We will also present initial simulation results on the expected performance of the device, and the overall timeline and project structure.
High Contrast Imaging Studies
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Post processing of differential images for direct extrasolar planet detection from the ground
The direct imaging from the ground of extrasolar planets has become today a major astronomical and biological focus. This kind of imaging requires simultaneously the use of a dedicated high performance Adaptive Optics [AO] system and a differential imaging camera in order to cancel out the flux coming from the star. In addition, the use of sophisticated post-processing techniques is mandatory to achieve the ultimate detection performance required. In the framework of the SPHERE project, we present here the development of a new technique, based on Maximum A Posteriori [MAP] approach, able to estimate parameters of a faint companion in the vicinity of a bright star, using the multi-wavelength images, the AO closed-loop data as well as some knowledge on non-common path and differential aberrations. Simulation results show a 10-5 detectivity at 5σ for angular separation around 15λ/D with only two images.
Effects of diffraction and static wavefront errors on high-contrast imaging from the Thirty Meter Telescope
Mitchell Troy, Ian Crossfield, Gary Chanan, et al.
High-contrast imaging, particularly direct detection of extrasolar planets, is a major science driver for the next generation of extremely large telescopes such as the segmented Thirty Meter Telescope. This goal requires more than merely diffraction-limited imaging, but also attention to residual scattered light from wavefront errors and diffraction effects at the contrast level of 10-8-10-9. Using a wave-optics simulation of adaptive optics and a diffraction suppression system we investigate diffraction from the segmentation geometry, intersegment gaps, obscuration by the secondary mirror and its supports. We find that the large obscurations pose a greater challenge than the much smaller segment gaps. In addition the impact of wavefront errors from the primary mirror, including segment alignment and figure errors, are analyzed. Segment-to-segment reflectivity variations and residual segment figure error will be the dominant error contributors from the primary mirror. Strategies to mitigate these errors are discussed.
Contrast limits with the Simultaneous Differential Extrasolar Planet Imager (SDI) at the VLT and MMT
We discuss contrast limits obtained during a survey of young (<300 Myr), close (<50 pc) stars with the Simultaneous Differential Extrasolar Planet Imager (SDI) implemented at the VLT and the MMT. SDI uses a double Wollaston prism and a quad filter to take images simultaneously at 3 wavelengths surrounding the 1.62 μm methane bandhead found in the spectrum of cool brown dwarfs and gas giants. By performing a difference of images in these filters, speckle noise from the primary can be significantly attenuated, resulting in photon noise limited data. In our survey data, we achieved H band contrasts >25000 (5σ ΔF1(1.575μm)>10 mag, ΔH>10.6 mag for a T6 spectral type) at a separation of 0.5" from the primary star. With this degree of attenuation, we can image (5σ detection) a 2-4 Jupiter mass planet at 5 AU around a 30 Myr star at 10 pc. We are currently completing our survey of young, nearby stars. We have obtained complete datasets for 40 stars in the southern sky (VLT) and 11 stars in the northern sky (MMT). We believe that our SDI images are the highest contrast astronomical images ever made from ground or space for methane rich companions.
Computationally efficient analytic transformation from phase psd to intensity psf
A simple analytic form for the intensity point spread function is obtained in terms of the power spectral density function for the phase. Two fourier transforms are required to compute the psf from the psd (A third fourier transform is needed to give the fraction of the light in the core). The analytic form is an infinite sum of convolution integrals of increasing order in the psd function multiplied by the simple renormalization factor exp(-sigma^2), where sigma^2 is the two-dimensional integral of the psd in radians squared. Computationally, the psf is evaluated on a discrete grid in kx-ky space. This infinite sum can be evaluated at all pixels other than the zero frequency pixel by taking the two-dimensional complex fourier transform X of the psd, computing exp(X)-1, and then taking the inverse fourier transform. There is also a simple expression for the value at the zero spatial frequency pixel. Like the psd, the psf is smooth because the psf is an ensemble average over all realizations for the phase: Each realization of the phase gives an intensity speckle pattern in the focal plane. The psf is the ensemble average over all realizations. This transformation has been extensively tested for azimuthally symmetric phase psd functions by comparing the computed psf using the analytic transformation with the azimuthally averaged psf computed using a specific realization for the phase. The psd functions that were compared this way were all azimuthally symmetric, but the analytic transformation from psd to psf doesn't require this. The final result for the halo is equivalent to the result in Hardy1 when the pupil is infinite. The derivation in this paper is simple and direct.
Effects of atmospheric dispersion on the PSF background level
Atmospheric dispersion represents a relatively overlooked problem in connection with the ultimate quality of ELT images corrected by adaptive optics (AO). The aim of this paper is to evaluate the contribution from atmospheric dispersion to the background level of the point-spread function (PSF). Since proper suppression of this level is important for the prospects for direct exo-planet observation, it is necessary to quantify the contributions from all possible sources to it. Atmospheric dispersion will in principle result in three different kinds of contributions. The first one is related to the fact that two rays of different color following the same path through the atmosphere to the telescope do not have the same optical path-length difference (OPD). The second one is related to the fact that two coinciding rays of different color entering the atmosphere at a non zero zenith angle will be separated due to refraction before they reach the telescope. The third one is related to the fact that rays are diffracted by inhomogeneities in the atmosphere and that the diffraction angle is dependent on color. This last effect is small and will not be treated here. As a consequence of dispersion phase fluctuations can, in principle, only be compensated at a single wavelength by AO systems with deformable mirrors (DMs). Hence looking for an exo-planet in a certain spectral bandwidth there will be a contribution from the parent star uncorrected background level. Hence it will be crucial to perform observations in a narrow spectral bandwidth and to ensure that the wavefront measurements used for AO correction are performed within the same narrow bandwidth. The last point affects the needed magnitude of the parent star, which is used for wavefront measurements.
Adaptive Optics Programs II
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Status progress of AdOpt@TNG and offer to the international astronomical community
M. Cecconi, A. Ghedina, P. Bagnara, et al.
The Adaptive Optics Module of the Telescopio Nazionale Galileo (AdOpt@TNG) has enjoyed a huge refurbishment. A new WaveFront Sensing CCD (EEV39 80x80pixels by SciMeasure) has been mounted, allowing for up to 1KHz frame rate. Thanks to the versatility of the pyramid wavefront sensor, the fast changing of the 4x4 and 8x8 pupil sampling has been easily and successfully implemented. A dual pentium processor PC with Real-Time Linux has substituted the old VME as Real Time Computer. The implementation of the new Deformable Mirror by Xinetics will be also discussed. A new Graphical User Interface has been built to allow for user-friendly utilization of the module by astronomers. On-sky observations will be presented in terms of FWHM and Strehl Ratio for different values of guiding star magnitudes and seeing conditions. The encouraging on-sky results and overall system stability pushed to offer AdOpt@TNG to the international astronomical community.
GLAS: engineering a common-user Rayleigh laser guide star for adaptive optics on the William Herschel Telescope
Gordon Talbot, Don Carlos Abrams, Nikolaos Apostolakos, et al.
The GLAS (Ground-layer Laser Adaptive-optics System) project is to construct a common-user Rayleigh laser beacon that will work in conjunction with the existing NAOMI adaptive optics system, instruments (near IR imager INGRID, optical integral field spectrograph OASIS, coronagraph OSCA) and infrastructure at the 4.2-m William Herschel Telescope (WHT) on La Palma. The laser guide star system will increase sky coverage available to high-order adaptive optics from ~1% to approaching 100% and will be optimized for scientific exploitation of the OASIS integral-field spectrograph at optical wavelengths. Additionally GLAS will be used in on-sky experiments for the application of laser beacons to ELTs. This paper describes the full range of engineering of the project ranging through the laser launch system, wavefront sensors, computer control, mechanisms, diagnostics, CCD detectors and the safety system. GLAS is a fully funded project, with final design completed and all equipment ordered, including the laser. Integration has started on the WHT and first light is expected summer 2006.
VASAO: visible all sky adaptive optics
Christian Veillet, Olivier Lai, Derrick Salmon, et al.
Building on an extensive and successful experience in Adaptive Optics (AO) and on recent developments made in its funding nations, the Canada-France-Hawaii-Telescope Corporation (CFHT) is studying the VASAO concept: an integrated AO system that would allow diffraction limited imaging of the whole sky in the visible as well as in the infrared. At the core of VASAO, Pueo-Hou (the new Pueo) is built on Pueo, the current CFHT AO bonnette. Pueo will be refurbished and improved to be able to image the isoplanetic field at 700 nm with Strehl ratios of 30% or better, making possible imaging with a resolution of 50 milliarcseconds between 500 and 700nm, and at the telescope limit of diffraction above. The polychromatic tip-tilt laser guide star currently envisioned will be generated by a single 330nm mode-less laser, and the relative position of the 330nm and 589nm artificial stars created on the mesosphere by the 330nm excitation of the sodium layer will be monitored to provide the atmospheric tip-tilt along the line of sight, following the philosophy developed for the ELP-OA project. The feasibility study of VASAO will take most of 2006 in parallel with the development of a science case making the best possible use of the unique capabilities of the system, If the feasibility study is encouraging, VASAO development could start in 2007 for a full deployment on the sky by 2011-2012.
Extreme AO Lab Tests
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The Extreme Adaptive Optics Testbed at UCSC: current results and coronagraphic upgrade
Scott A. Severson, Brian Bauman, Daren Dillon, et al.
We present a summary of our current results from the Extreme Adaptive Optics (ExAO) Testbed and the design and status of its coronagraphic upgrade. The ExAO Testbed at the Laboratory for Adaptive Optics at UCO/Lick Observatory is optimized for ultra-high contrast applications requiring high-order wavefront control. It is being used to investigate and develop technologies for the Gemini Planet Imager (GPI). The testbed is equipped with a phase shifting diffraction interferometer (PSDI), which measures the wavefront with sub-nm precision and accuracy. The testbed also includes a 1024-actuator Micro Electro Mechanical Systems (MEMS) deformable mirror manufactured by Boston Micromachines. We present a summary of the current results with the testbed encompassing MEMS flattening via PSDI, MEMS flattening via a Shack-Hartmann wavefront sensor (with and without spatial filtering), the introduction of Kolmogorov phase screens, and contrast in the far-field. Upgrades in progress include adding additional focal and pupil planes to better control scattered light and allow alternative coronagraph architectures, the introduction and testing of high-quality reflecting optics, and a variety of input phase aberrations. Ultimately, the system will serve as a full prototype for GPI.
Extreme adaptive optics system optimization with the high order test bench
Elise Vernet, Markus Kasper, Christophe Vérinaud, et al.
Extreme adaptive optics systems dedicated to the search for extrasolar planets are currently being developed for most 8-10 meter telescopes. Extensive computer simulations have shown the ability of both Shack-Hartmann and pyramid wave front sensors to deliver high Strehl ratio correction expected from extreme adaptive optics but few experiments have been realized so far. The high order test bench implements extreme adaptive optics on the MACAO test bench with realistic telescope conditions reproduced by star and turbulence generators. A 32×32 actuator micro deformable mirror, one pyramid wave front sensor, one Shack-Hartmann wave front sensor, the ESO SPARTA real time computer and an essentially read-noise free electron multiplying CCD60 (E2V CCD60) provide an ideal cocoon to study the different behavior of the two types of wave front sensors in terms of linearity, sensitivity to calibration errors, noise propagation, specific issues to pyramid or Shack-Hartmann wave front sensors, etc. We will describe the overall design of this test bench and will focus on the characterization of two essential sub-systems: the micro deformable mirror and the phase screens.
A laboratory experiment for demonstrating post-coronagraph wavefront sensing and control for extreme adaptive optics
J. Kent Wallace, Randall Bartos, Shanti Rao, et al.
Direct detection of exo-planets from the ground will become a reality with the advent of a new class of extreme-adaptive optics instruments that will come on-line within the next few years. In particular, the Gemini Observatory will be developing the Gemini Planet Imager (GPI) that will be used to make direct observations of young exo-planets. One major technical challenge in reaching the requisite high contrast at small angles is the sensing and control of residual wave front errors after the starlight suppression system. This paper will discuss the nature of this problem, and our approach to the sensing and control task. We will describe a laboratory experiment whose purpose is to provide a means of validating our sensing techniques and control algorithms. The experimental demonstration of sensing and control will be described. Finally, we will comment on the applicability of this technique to other similar high-contrast instruments.
Laboratory demonstration of a focal plane coronagraphic interferometer designed for anti-halo apodization of starlight
Nicole Putnam, Johanan L. Codona, J. Roger P. Angel
The coronagraphic focal plane interferometer reflects away the core starlight with a mirror in the focal plane and uses it to form a coherent interferometric reference beam. This is used in a Mach-Zehnder configuration with phase shifting to measure the complex amplitude of the star halo speckles in the focal plane where the interference takes place. We present results from a laboratory prototype in which the speckles are suppressed over half the field by modifying the wavefront in a pupil plane with a MEMS deformable mirror, based on a Fourier transform of the complex halo derived from the focal plane interferometric data. Even deeper suppression of the residual stellar halo over the full 360 degree field will be possible by explicitly constructing an "anti-halo" from the reference beam; a new technique for exoplanet imaging (Codona and Angel, 2004). We present the design and current status of a laboratory prototype to study antihalo apodization. The spatially-filtered core starlight will be modulated by deformable mirrors in a Michelson configuration to form a temporally-coherent copy of the measured residual complex halo, with the same amplitude but opposite phase (i.e. an "anti-halo"). Using components with only modest control accuracy, the method has the potential to reduce an already low residual halo by an additional two decades.
Innovative Deformable Mirror Technologies
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Deformable nanolaminate optics
We are developing a new class of deformable optic based on electrostatic actuation of nanolaminate foils. These foils are engineered at the atomic level to provide optimal opto-mechanical properties, including surface quality, strength and stiffness, for a wide range of deformable optics. We are combining these foils, developed at Lawrence Livermore National Laboratory (LLNL), with commercial metal processing techniques to produce prototype deformable optics with aperture sizes up to 10 cm and actuator spacing from 1 mm to 1 cm and with a range of surface deformation designed to be as much as 10 microns. The existing capability for producing nanolaminate foils at LLNL, coupled with the commercial metal processing techniques being used, enable the potential production of these deformable optics with aperture sizes of over 1 m, and much larger deformable optics could potentially be produced by tiling multiple deformable segments. In addition, based on the fabrication processes being used, deformable nanolaminate optics could potentially be produced with areal densities of less than 1 kg per square m for applications in which lightweight deformable optics are desirable, and deformable nanolaminate optics could potentially be fabricated with intrinsically curved surfaces, including aspheric shapes. We will describe the basic principles of these devices, and we will present details of the design, fabrication and characterization of the prototype deformable nanolaminate optics that have been developed to date. We will also discuss the possibilities for future work on scaling these devices to larger sizes and developing both devices with lower areal densities and devices with curved surfaces.
Development of a liquid based deformable mirror
Edgar M. Vuelban, Nandini Bhattacharya, Joseph M. Braat
Recently, a new type of liquid based deformable mirror has been proposed and demonstrated. The device consists of an array of vertically oriented open capillary channels immersed in a pool of two immiscible liquids and a free-floating reflective membrane, which serves as the reflecting surface. Liquid surface and membrane deformations are facilitated by means of electrocapillary actuation that induces upward or downward flow of liquid inside the capillary. This electrocapillary movement of liquid can be individually controlled. The advantages of this proposed device include high stroke dynamic range, low power dissipation, high number of actuators, fast response time, and reduced fabrication cost. The device is mainly suitable for dynamic wavefront correction. We present some aspects of the modeling of the device.
Carbon fibre composite deformable mirrors: developments at UCL
Sarah Kendrew, Peter Doel, David Brooks, et al.
Adaptive optics performance is essential for achieving the demanding science goals set for the ground-based optical telescopes of the future - the so-called extremely large telescopes (ELTs). Research into novel technologies for lightweight and robust active and adaptive mirrors is crucial for ensuring this capability. Surface quality, form, and a high level of stability during operation are very important criteria for such mirrors. In 2004 we reported initial results from a project into the design and manufacture of a prototype carbon fibre reinforced polymer (CFRP) deformable mirror. This system has now been extensively characterised and tested, and results of dynamical testing and influence function measurements are discussed here. Manual grinding and polishing resulted in a residual form error of the order of 10 μm P-V and a surface roughness of approximately 5 nm rms. A good agreement was observed between the modeling data and experimental results.
Wavefront Reconstruction Theory
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Multi-rate control of high-order adaptive optics systems
Next generation adaptive-optics systems (AO) have unprecedented complexity. The proposed number of degrees-of-freedom has considerably increased, putting the focus on the real-time processing capabilities. Second generation instrumentation for the Very-Large Telescope (VLT) is one such case. We present a method capable of lowering the average computational effort (i.e. lowering the average frame-rate) and deliver the same performance figures. It consists of applying a distributed set of update rates to outperform the conventional vector-matrix multiplies (VMM) used for modal reconstruction and control in AO systems when Zernike polynomials or Karhunen-Loeve modes are used as basis. We analyse the low and high-noise regimes for which we outline the theoretical key points and present both semi-analytical and Monte-Carlo simulation results having the Planet-Finder (PF) as baseline system.
Adaptive optics Η2-optimal control design applied on an experimental setup
Karel Hinnen, Michel Verhaegen, Niek Doelman
Most adaptive optics systems (AO) are based on a simple control law that is unable to account for the temporal evolution of the wavefront. In this paper, a recently proposed data-driven Η2-optimal control approach is demonstrated on an AO laboratory setup. The proposed control approach does not assume any form of decoupling and can therefore exploit the spatio-temporal correlation in the wavefront. The performance of the optimal control approach is compared with a conventional method. An analysis of the dominant error sources shows that the optimal control approach leads to a significant reduction in the temporal error. Since the temporal error grows with the Greenwood to sampling frequency ratio, the performance gain is especially large at large ratios.
Fourier domain algorithm for the fitting step in multi-conjugate adaptive optics
In this paper we present a Fourier-domain preconditioned conjugate gradient algorithm for the fitting step in Multi-Conjugate Adaptive Optics (MCAO) for extremely large telescopes. This algorithm is fast and robust, and it is convenient to implement with parallel processing in a real-time system. Simulation results are presented for an MCAO system for a 30-meter telescope with 2 deformable mirrors.
FRIM: minimum-variance reconstructor with a fractal iterative method
Adaptive optics (AO) systems under study for the future generation of telescopes have to cope with a huge number of degrees of freedom. This number N is typically 2 orders of magnitude larger than for the currently existing AO systems. An iterative method using a fractal preconditioning, has recently been suggested for a minimum-variance reconstruction in O(N) operations. We analyze the efficiency of this algorithm for both the open-loop and the closed-loop configurations. We present the formalism and illustrate the assets of this method with simulations. While the number of iterations for convergence is around 10 in open-loop, the closed-loop configuration induces a reduction of the required number of iterations by a factor of 3 typically. This analysis also enhances the importance of introducing priors to ensure an optimal command. Closed-loop simulations demonstrate the loss of performance when no temporal priors are used. Besides, we discuss the importance of an accurate model for both the system and its uncertainties, so as to ensure a stable behavior in closed-loop.
Extreme adaptive optics using an off-axis subaperture on a ground-based telescope
E. Serabyn, J. K. Wallace, M. Troy, et al.
The next generation of adaptive optics (AO) systems, often referred to as extreme adaptive optics (ExAO), will use higher numbers of actuators to achieve wavefront correction levels below 100 nm, and so enable a host of new observations such as high-contrast coronagraphy. However, the number of potential coronagraph types is increasing rapidly, and selection of the most advantageous coronagraph is subject to many factors. Here it is pointed out that experiments in the ExAO regime can already be carried out with existing hardware, by using a well-corrected subaperture on an existing telescope. For example, by magnifying a 1.5 m diameter off-axis subaperture onto the AO system's deformable mirror (DM) on the Palomar Hale telescope, we have recently achieved stellar Strehl ratios as high as 92% to 94%, corresponding to wavefront errors of 85 - 100 nm. Using this approach, a wide variety of ExaO experiments can thus be carried out well before "next generation" ExAO systems are deployed on large telescopes. The potential experiments include infrared ExAO imaging and performance optimization, a comparison of coronagraphic approaches in the ExAO regime, visible wavelength AO, and predictive AO.
Adaptive optics control of wind blown turbulence via translation and prediction
In the case where wind blown turbulence is mostly adhering to frozen flow conditions the use of the Kalman Filter in an adaptive optics controller is of interest because it incorporates prior the time history of wavefront measurements as additional information to be combined with the immediate measurement of the wavefront. In prior work we have shown that indeed there is a signal to noise advantage, however the extra real-time overhead of the Kalman Filter computations can become prohibitive for larger aperture systems. In this paper we investigate a Fourier domain implementation that might approximate, and gain the advantages of, the Kalman Filter while being feasible to implement in real time control computers. Most of the advantage of using the Kalman Filter comes from its ability to predict the wind blown turbulence for the next measurement step. For the photonic and instrumentation noise levels commonly found in astronomical AO systems, we find that most of the Strehl gain is achieved by simply translating the wavefront estimate the incremental distance.
Turbulence, Anisoplanatism, and Sky Coverage
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The effect of temporal fluctuations in r0 on high-resolution observations
Previous experimental measurements and theoretical modelling indicate that atmospheric turbulence is expected to show intermittency (or "clumpiness"). The impact that this intermittency has on simulated adaptive optics (AO) and Lucky Imaging (LI) performance is assessed in this SPIE contribution using simulated phase screens with Von Karman power spectra which incorporate turbulent intermittency. The statistics of the intermittency model used are based as closely as possible on astronomical seeing measurements at real observatories. The performance of realistic AO correction of large Taylor screens with intermittent turbulence is compared directly with the performance given with traditional Gaussian random Taylor screens having the same Von Karman power spectrum. Also discussed is the improvement in performance which can be obtained by modifying the AO control parameters in response to the changing seeing conditions. Lucky Imaging simulations indicate that the performance of this method can be significantly improved under intermittent seeing.
Time-varying stochastic turbulence model
In this paper we present a new stochastic model for time-varying turbulence. The model can be viewed as a linearization of the Navier-Stokes equation, with deterministic drift and diffusion terms, plus an additional stochastic driving term. Fixed-time realizations of the model have Kolmogorov statistics, but the diffusion and stochastic driving terms yield "boiling" behavior that is different from the Taylor frozen flow model.
Angular anisoplanatism in laser guide star adaptive optics
Marcos A. van Dam, Richard J. Sasiela, Antonin H. Bouchez, et al.
The image quality obtained using laser guide star adaptive optics (LGS AO) is degraded by the fact that the wavefront aberrations experienced by light from the LGS and from the science object differ. In this paper we derive an analytic expression for the variance of the difference between the two wavefronts as a function of angular distance between the LGS and the science object. This error is a combination of focal anisoplanatism and angular anisoplanatism. We show that the wavefront error introduced by observing a science object displaced from the guide star is smaller for LGS AO systems than for natural guide star AO systems.
Sky coverage and tip/tilt error analysis for TMT
A Monte Carlo sky coverage model for laser guide star adaptive optics systems is presented. This model provides fast Monte Carlo simulations of the tip/tilt (TT) wavefront error calculated with minimum variance estimators over natural guide star constellations generated from star models. With this simulation code we are able to generate a TT error budget for the Thirty Metre Telescope (TMT) facility Narrow Field Infra-Red Adaptive Optics System (NFIRAOS), and perform several design trade studies. With the current NFIRAOS design, the median TT error at the galactic pole with median seeing is calculated to be 65 nm or 1.8 mas.
Analysis of crowded field adaptive optics image data
A point source deconvolution technique is described that models the effects of anisoplanatism on the adaptive optics point spread function. This technique is used in the analysis of a quadruple system observed using the Palomar Adaptive Optics system on the Hale 5 meter telescope. Two members of this system reside in a .1 asec double. Deconvolution of this close double was performed using the PSF of a third member of the system, which was offset from the double by 12 asec. Incorporation of anisoplanatism into the deconvolution procedure requires knowledge of the turbulence profile, which was measured at the time of these observations using a DIMM/MASS unit at Palomar Observatory.
Simulation Studies and Tools
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Adaptive optics simulations for the European Extremely Large Telescope
Miska Le Louarn, Christophe Vérinaud, Visa Korkiakoski, et al.
In this article we summarize the parameter space exploration for several adaptive optics systems for a 42-m European Extremely Large Telescope. These systems are modular, and based on a various number of identical high order wavefront sensor. We explore a single natural guide star single conjugate AO and multi-laser guide star systems: ground-layer AO, laser-tomography AO, and multi-conjugate AO. The performance estimates are given in terms of Strehl ratio or, for the GLAO system in terms of ensquared energy.
Laser guide star multi-conjugate adaptive optics performance of the Thirty Meter Telescope with elongated beacons and matched filtering
This paper describes wave-optics Monte Carlo simulation results to asses the impact of laser guide star wavefront sensor nonlinearity with elongated sodium beacons on the residual wavefront error for the Thirty Meter Telescope Narrow Field InfraRed Adaptive Optics System, which is a laser guide star multi-conjugate adaptive optics system intended to provide near-diffraction limited performance in the near infrared over a 30 arcsec diameter field of view.
Performance predictions of the GLAS Rayleigh laser guide star adaptive optics system for the 4.2-m William Herschel Telescope
Results of numerical simulations of the performance of GLAS (Ground-layer Laser Adaptive optics System) are presented. GLAS uses a Rayleigh laser guide star (LGS) created at a nominal distance of 20km from the 4.2m William Herschel Telescope primary aperture and a semi-analytical model has been used to determine the observed LGS properties. GLAS is primarily intended for use with the OASIS spectrograph working at visible wavelengths although a wider-field IR imaging camera can also use the AO corrected output. Image quality metrics relating to scientific performance for each instrument are used showing that the energy inside every OASIS lenslet across the 10" instrument FOV is approximately doubled, irrespective of atmospheric conditions or wavelength of observation.
Digital vs. analog comparison of adaptive optics systems
This paper presents an equivalent discrete-time model that combines the D-A converter, DM, and WFS. The performance of the adaptive optics loop can then be evaluated in terms of the discrete-time (Z) closed-loop transfer functions of the feedback loop. The closed-loop transfer functions using this discrete-time model are compared to those obtained from an analog model. A simulation shows that the digital model produces accurate stability evaluations.
Acceleration of AO simulation using programmable logic
Numerical Simulation is an essential part of the design and optimisation of astronomical adaptive optics systems. Simulations of adaptive optics are computationally expensive and the problem scales rapidly with telescope aperture size, as the required spatial order of the correcting system increases. Practical realistic simulations of AO systems for extremely large telescopes are beyond the capabilities of all but the largest of modern parallel supercomputers. Here we describe a more cost effective approach through the use of hardware acceleration using field programmable gate arrays. By transferring key parts of the simulation into programmable logic, large increases in computational bandwidth can be expected. We show that the calculation of wavefront sensor images (involving a 2D FFT, photon shot noise addition, background and readout noise), and centroid calculation can be accelerated by factor of 400 times when the algorithms are transferred into hardware. We also provide details about the simulation platform and framework that we have developed at Durham.
Sparse-aperture adaptive optics
Aperture masking interferometry and Adaptive Optics (AO) are two of the competing technologies attempting to recover diffraction-limited performance from ground-based telescopes. However, there are good arguments that these techniques should be viewed as complementary, not competitive. Masking has been shown to deliver superior PSF calibration, rejection of atmospheric noise and robust recovery of phase information through the use of closure phases. However, this comes at the penalty of loss of flux at the mask, restricting the technique to bright targets. Adaptive optics, on the other hand, can reach a fainter class of objects but suffers from the difficulty of calibration of the PSF which can vary with observational parameters such as seeing, airmass and source brightness. Here we present results from a fusion of these two techniques: placing an aperture mask downstream of an AO system. The precision characterization of the PSF enabled by sparse-aperture interferometry can now be applied to deconvolution of AO images, recovering structure from the traditionally-difficult regime within the core of the AO-corrected transfer function. Results of this program from the Palomar and Keck adaptive optical systems are presented.
Innovative Wavefront Sensing Concepts
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Whack-a-speckle: focal plane wavefront sensing in theory and practice with a deformable secondary mirror and 5-micron camera
Matthew A. Kenworthy, Philip M. Hinz, J. Roger P. Angel, et al.
Long exposures from adaptive optic systems show a diffraction limited core superimposed on a halo of uncorrected light from a science target, and the addition of various long-lived speckles that arise from uncorrected aberrations in the telescope system. The presence of these speckles limit the detection of extra-solar planets at a few diffraction widths from the primary source. Focal plane wavefront sensing uses the deformable secondary mirror of the MMT adaptive optics system to systematically remove the presence of long-lived speckles in a high-contrast image, and also test for the incoherent source that represents a separate astronomical target nearby. We use the Clio 5 micron camera (with its coronagraphic capabilities) to modulate long lived speckles and present initial on-sky results of this technique.
High contrast imaging with focal plane wavefront sensing for ground based telescopes
Olivier Guyon, Basile Gallet, Eugene A. Pluzhnik, et al.
The techniques adopted in "classical" AO (sensing in the visible, imaging in the near-IR) limit the achievable raw PSF contrast to about 105 in the central arcsecond. On nearby stars, this level is far from the theoretical PSF contrast limit imposed by photon noise in the wavefront sensor. A comparative study between wavefront sensing strategies shows that a focal-plane based wavefront sensor (WFS), combining wavefront sensing and scientific imaging on the same detector, seems optimal for high contrast imaging. This approach combines high WFS sensitivity, immunity to aliasing, chromaticity, and non common path errors and optical design simplicity. We show that such a system can be efficiently used as a second stage "Extreme-AO" system after a low-order AO system. The images acquired by the science camera are then used to drive the high-order DM (which also introduces the phase diversity needed for focal plane wavefront sensing). This scheme offers much flexibility: with the proper DM updates, the focal plane images can be simultaneously used to solve for the entrance wavefront and the presence of companions (which are incoherent with the speckles) below the speckle noise level. Control and data reduction algorithms are presented, as well as possible optical designs incorporating a coronagraph. A laboratory demonstration of this technique is currently being done at Subaru Telescope with a 1024 actuators MEMs DM. This experiment serves as a prototype to plan and design a similar system for Subaru's upcoming HiCIAO instrument (near-IR coronagraphic imager for adaptive optics).
Polychromatic laser guide star using a single laser at 330 nm
Jean-Paul Pique, Ioana C. Moldovan, Vincent Fesquet, et al.
The differential atmospheric tip-tilt can be measured using a Polychromatic Laser Guide Star. A two photon excitation has been proposed. It consists in exciting the 4D5/2 level of mesospheric sodium atoms with two identical lasers operating at 589 nm and 569 nm. With two modeless lasers of 2×15W at the mesosphere level, this scheme will produce a returned flux at 330 nm of about 4×104 photons/s/m2. Thanks to our modeless laser, we propose a new method which consists in exciting directly the 4P3/2 sodium level with one photon excitation, using a single laser operating at 330 nm. This solution was previously rejected probably because of strong saturation problems using single longitudinal mode lasers. We show that 1 W modeless laser at 330 nm can produce the same returned flux at 330 nm. This solution will save at least 400 k€ of equipment. Moreover, our new method is very promising in terms of simplicity but also in terms of flux because the returned flux above will probably be not sufficient for getting a good Strehl ratio. We propose very efficient solid state laser systems for the production of tens of watts at 330 nm.
Fast holographic wavefront sensor: sensing without computing
In this talk we will present work on a fast zonal wavefront sensor. The device consists of a multiplexed hologram which can reconstruct multiple diffracted beams to an image plane with the input of a single object beam. The hologram is prerecorded with many different gratings designed to detect any combination of Zernike term and amplitude. In operation, a wavefront incident on the hologram reconstructs a number of output beams that focus onto a distant detector. The location of these spots is specifically related to a particular Zernike polynomial of particular strength, so a complete description of the wavefront can be made by simple readout of a CCD. While this wavefront sensor is limited in spectral coverage due to dispersion in the hologram there are many benefits. Since no calculations are required, this sensor can operate at MHz rates irrespective of the number of Zernike terms used. The removal of complex computations also reduces instrument complexity and size. In this talk we will present results of the both the theory and operation of our holographic wavefront sensor.
Polychromatic phase retrieval with Kolmogorov self-adapting prior constraints
Phase retrieval is a very promising approach for wavefront sensing in the focal plane of ground-based large telescopes. It is a non-linear problem that must be solved by means of global optimization. Currently only multi-focal phase diversity algorithms are used in adaptive optics. They enable the correction of static aberrations. For speckle imaging the problem is increasingly multi-modal with the ratio D/r0. Yet thanks to an iterative Newton algorithm with self-adapting Kolmogorov prior information, we show from consistent modeling and simulations, that we could efficiently sense short exposure wavefronts at high D/r0 from a single focal plane. We show that using data at different wavelengths with a proper polychromatic model would even enforce the convergence, thus making it an envisageable method to sense the returned flux of a polychromatic laser guide star (PLGS). For instance, we show that if we suppose the PLGS is not resolved, phase retrieval would enable an improvement in the centroid estimation in agreement with the Cramer-Rao lower bound. As a post-processing technique, our algorithm already has numerous potential applications for astronomy and for other domains. Thanks to the improvement of computing workstations and the optimization of the algorithm, applications involving realtime wavefront corrections should be soon possible.
Scalable analog wavefront sensor with subpixel resolution
Standard Shack-Hartman wavefront sensors use a CCD element to sample position and distortion of a target or guide star. Digital sampling of the element and transfer to a memory space for subsequent computation adds significant temporal delay, thus, limiting the spatial frequency and scalability of the system as a wavefront sensor. A new approach to sampling uses information processing principles in an insect compound eye. Analog circuitry eliminates digital sampling and extends the useful range of the system to control a deformable mirror and make a faster, more capable wavefront sensor.
Laser interferometry guided adaptive optics for large telescopes
Novel laser guide star techniques are required if one wishes to avoid the technological and computational hurdles of implementing today's LGS designs on extremely large telescopes. Similar difficulties arise for correction at shorter wavelengths, or for multi-conjugate systems on 8-m class telescopes. To overcome many of the limitations, we propose to overlap coherently pulsed laser beams that are expanded over the full aperture of the telescope and hence travel upwards along the same path that light from the astronomical object travels downwards. Imaging the resultant interference pattern, and making use of the two polarization states, we are able to use phase shifting interferometry to retrieve the local wavefront gradients along both axes simultaneously. The technique can be generalized for shorter wavelength or wide field correction, and is applicable on any size of telescope. In this contribution we describe our experimental laboratory test-bed which we will use to verify our theoretical expectations, and to resolve a number of practical issues associated with implementation on a telescope.
Calibration and Control Topics
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Nanoengineered ferrofluid deformable mirrors: a progress report
S. Thibault, D. Brousseau, M. Rioux, et al.
We give a progress report on a new class of versatile optical elements pioneered by our laboratory. By coating ferromagnetic liquids we create reflective surfaces that can be shaped with magnetic fields, allowing us to make complex wavefronts that can vary rapidly in time. This new technology is capable of achieving complex surfaces that cannot be obtained with existing technology. The short-term objective is to perfect the technology for adaptive optics for both astronomical and ophthalmology applications. We have made a functional 112 actuator deformable mirror and characterized the ferrohydrodynamic response of the actuators. We have used high speed sensors to analyze the mirror surface subject to transient and periodic driving forces. We have developed algorithms to shape the surfaces. We have made new types of ferrofluids that are easier to coat with our nanoengineered layers. Theoretical model shown how the mirror parameters can be tuned as function of the applications. Challenges in design are outlined, as are advantages over traditional deformable mirrors.
Eliminating the static aberrations in an MCAO system
Within the last years, the Adaptive Optics (AO) systems built or designed are growing in complexity: their optics are numerous and they aim at better performances on-axis and/or in the Field of View. A limiting factor to those performances can be the Non-Common Path Aberrations (NCPA), i.e. static aberrations present in the optical path of the instrument after the dichroic, which therefore are not seen by the AO Wave Front Sensor. Since long the Deformable Mirror of the system has been used to partially compensate for them, in a more or less efficient and complex way. This paper will present a rapid to implement, rather simple and all-measured method of compensation for the NCPA tanks to the use of Phase Diversity and of a calibration matrix of the modes constituting the aberrations. Moreover for systems composed of several DMs, we will present how a clever use of those can reduce significantly the aberrations in the whole FoV. All those techniques will be illustrated by their application to ESO'S MCAO Demonstrator (MAD).
Distributed control in adaptive optics: deformable mirror and turbulence modeling
Rogier Ellenbroek, Michel Verhaegen, Niek Doelman, et al.
Future large optical telescopes require adaptive optics (AO) systems whose deformable mirrors (DM) have ever more degrees of freedom. This paper describes advances that are made in a project aimed to design a new AO system that is extendible to meet tomorrow's specifications. Advances on the mechanical design are reported in a companion paper [6272-75], whereas this paper discusses the controller design aspects. The numerical complexity of controller designs often used for AO scales with the fourth power in the diameter of the telescope's primary mirror. For future large telescopes this will undoubtedly become a critical aspect. This paper demonstrates the feasibility of solving this issue with a distributed controller design. A distributed framework will be introduced in which each actuator has a separate processor that can communicate with a few direct neighbors. First, the DM will be modeled and shown to be compatible with the framework. Then, adaptive turbulence models that fit the framework will be shown to adequately capture the spatio-temporal behavior of the atmospheric disturbance, constituting a first step towards a distributed optimal control. Finally, the wavefront reconstruction step is fitted into the distributed framework such that the computational complexity for each processor increases only linearly with the telescope diameter.
Poster Session: Laser Guide Star and Multi-Conjugate AO Field Tests
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Laser guide star upgrade of Altair at Gemini North
Altair is the general-purpose Adaptive Optics bench installed on Gemini North that has operated successfully with Natural Guide Star (NGS) since 2003. The original design and fabrication included an additional WaveFront Sensor (WFS) to enable operation with Laser Guide Star (LGS). Altair has been recently upgraded and functional commissioning was performed between June and November 2005. The insertion of a dichroic beamsplitter in the NGS path allows to reflect the 589nm light to the LGS wavefront sensor and transmit the visible light of the NGS (or Tip-Tilt Guide star -TTGS-) to the tip-tilt-focus sensors. We will review the various modifications made for this dual operation, both in hardware and software, and describe the steps and results of the integration and testing phase on the sky.
Poster Session: Adaptive Optics Programs I
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Detection of atmospheric tip-tilt for lunar laser ranging
Yaoheng Xiong, Rui Guo
Atmospheric turbulence affects the direction and shape of laser beam on long distance laser ranging, especially on the Lunar Laser Ranging (LLR). That will result in decreasing the returned photon numbers on ground station when it performs the LLR. It is need to compensate turbulence effects in real-time on the LLR, first for the effects of atmospheric tip-tilt. In this paper, we present the experiment results at Yunnan Observatory 1.2m telescope that use a small area near the retro-reflector array on the moon surface as an expanded source to detect and compute the atmospheric tip-tilt signal in real time.
Enhancements and developments on NAOMI
Maarten Blanken, Chris Benn, Michiel van der Hoeven, et al.
The Nasmyth Adaptive Optics for Multi-purpose Instrumentation (NAOMI) on the William Herschel Telescope (WHT) has been developed recently into a common user AO (Adaptive Optics) instrument to accompany OASIS (Optically Adaptive System for Imaging Spectroscopy), a multi-slit spectrograph and INGRID (Isaac Newton Group Red Imaging Device) an Infrared detector. The most recent changes are the addition of an Atmospheric Dispersion Corrector (ADC) to be used for the optical wavelengths and a Dichroic Changer mechanism to select either a pass band or IR light for the Universal Science Ports (UPS). Future developments on NOAMI are planned as it is due to house the GLAS WFS (Ground Layer Adaptive optics System Wave Front Sensor), a wave front sensor for the future Laser Guide Star (LGS) system to be installed on the WHT in 2006. This paper describes the changes made with respect to the science ports and the changes to be made for the GLAS WFS; focusing on the GLAS WFS and the optical path and interface to the NAOMI adaptive optics system.
Low-order AO system in LAMOST
Xiangyan Yuan, Xiangqun Cui, Genrong Liu, et al.
The large sky area multi-object fiber spectroscopic telescope (LAMOST) is a special reflecting Schmidt telescope with its main optical axis on the meridian plane tilted by an angle of 25° to the horizontal. The clear aperture is 4m, working in optical band. The light path is 60m long when working in observing mode and it will be doubled if work in auto-collimation mode. So the image quality is affected clearly by the ground seeing and the dome seeing. In order to improve the seeing condition of the long light path, we enclosed the spherical primary and the focus unit in a tunnel enclosure and cooled the tunnel. This is an effective but passive method. Corresponding experiments and simulations show the main part of the aberrations caused by the ground seeing and dome seeing is slowly changed low order items such as tip-tilt, defocus, astigmatism, coma and spherical aberration. Thus we plan to develop the low-order AO system based on the low-cost 37-channel OKO deformable mirror for the telescope to better the ground seeing and the dome seeing, not aimed to reach diffraction limited image. This work is being carried on now.
GLAS: launching Rayleigh lasers from the WHT
Paul Jolley, Tom Gregory
The 4.2m William Herschel Telescope (WHT) at the Isaac Newton Group of Telescopes (ING) is due to have a new Rayleigh laser beacon installed. This will form part of the Ground-layer Laser Adaptive optic System (GLAS). GLAS will compliment the existing Nasmyth Adaptive Optic Multipurpose Instrument (NAOMI) currently in operation and allow for much greater sky coverage than before. A 30W laser will be launched from behind the secondary mirror of the WHT. To facilitate this, a support has been designed to mount the laser on the top end ring of the telescope. The mount is designed to give a gravity stable platform in a thermally stable environment. The mount required the use of astatic levers to help maintain alignment with the telescope. The laser beam is steered over the telescope vanes and into the Beam Launch Telescope (BLT) which is mounted behind the secondary mirror. The BLT then expands the beam and launches it to 20Km. Two wavefront sensors are used to correct the image. The laser guide star wavefront sensor (LGS WFS) uses a beamsplitter to pickoff the laser return at its wavelength. The existing NAOMI WFS is still used but is now the natural guide star wavefront sensor (NGS WFS) and corrects for tip/tilt. This paper will concentrate on describing the mechanical design and FEA of the laser up launch system (laser cradle and mount and the BLT). A very brief overview of the LGS WFS will be given for system completeness.
Multiconjugated adaptive optics for ELTs: an enhancement of the PIGS setup
ELTs depend on Adaptive Optics (AO) to reach the diffraction limit. To achieve sufficient sky coverage with AO, several Laser Guide Stars (LGS) will be needed, but the finite distance of the LGSs introduces optical problems which can not be solved easily, especially at telescopes with a diameter larger than 30m. PIGS (Pseudo Infinite Guide Star) is a novel sensing technique proposed to overcome some of these problems by using a slit mask and a reflective rod measuring in radial and azimuthal direction the wavefront abberations. The sensor was already demonstrated with a single LGS in laboratory and on sky. Currently we investigate the PIGS concept in a MCAO (Multi Conjugated Adaptive Optics) fashion by building a test setup in the laboratory. MCAO will solve the cone effect, one of the remaining problems with PIGS. The PIGS MCAO experiment goals on engineering problems and demonstration of the layer orientated concept with the PIGS technique. The PIGS concept and its extension to MCAO will be described and preliminary results presented.
Real time system for GLAS on WHT
The new ground layer adaptive optics system (GLAS) on the William Herschel Telescope (WHT) on La Palma will be based on the existing natural guide star adaptive optics system called NAOMI. A part of the new developments is a new control system for the tip-tilt mirror. Instead of the existing system, built around a custom built multiprocessor computer made of C40 DSPs, this system uses an ordinary PC machine and a Linux operating system. It is equipped with a high sensitivity L3 CCD camera with effective readout noise of nearly zero. The software design for the tip-tilt system is being completely redeveloped, in order to make a use of object oriented design which should facilitate easier integration with the rest of the observing system at the WHT. The modular design of the system allows incorporation of different centroiding and loop control methods. To test the system off-sky, we have built a laboratory bench using an artificial light source and a tip-tilt mirror. We present results of tip-tilt correction quality using different centroiding algorithms and different control loop methods at different light levels. This system will serve as a testing ground for a transition to a completely PC-based real-time control system.
Poster Session: Astronomical Results and Performance Characterization
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High contrast L' band adaptive optics imaging to detect extrasolar planets
We are carrying out a survey to search for giant extrasolar planets around nearby, moderate-age stars in the mid-infrared L' and M bands (3.8 and 4.8 microns, respectively), using the Clio camera with the adaptive optics system on the MMT telescope. To date we have observed 7 stars, of a total 50 planned, including GJ 450 (distance about 8.55pc, age about 1 billion years, no real companions detected), which we use as our example here. We report the methods we use to obtain extremely high contrast imaging in L', and the performance we have obtained. We find that the rotation of a celestial object over time with respect to a telescope tracking it with an altazimuth mount can be a powerful tool for subtracting telescope-related stellar halo artifacts and detecting planets near bright stars. We have carried out a thorough Monte Carlo simulation demonstrating our ability to detect planets as small as 6 Jupiter masses around GJ 450. The division of a science data set into two independent parts, with companions required to be detected on both in order to be recognized as real, played a crucial role in detecting companions in this simulation. We mention also our discovery of a previously unknown faint stellar companion to another of our survey targets, HD 133002. Followup is needed to confirm this as a physical companion, and to determine its physical properties.
PSF reconstruction for NAOS-CONICA
Adaptive optics (AO) allows one to derive the point spread function (PSF) simultaneously to the science image, which is a major advantage in post-processing tasks such as astrometry/photometry or deconvolution. Based on the algorithm of Veran et al. (1997), PSF reconstruction has been developed for four different AO systems so far: PUEO, ALFA, Lick-AO and Altair. A similar effort is undertaken for NAOS/VLT in a collaboration between the group PHASE (Onera and Observatoire de Paris/LESIA) and ESO. In this paper, we first introduce two new algorithms that prevent the use of the so-called "Uij functions" to: (1) avoid the storage of a large amount of data (for both new algorithms), (2) shorten the PSF reconstruction computation time (for one of the two) and (3) provide an estimation of the PSF variability (for the other one). We then identify and explain issues in the exploitation of real-time Shack-Hartmann (SH) data for PSF reconstruction, emphasising the large impact of thresholding in the accuracy of the phase residual estimation. Finally, we present the data provided by the NAOS real-time computer (RTC) to reconstruct PSF ((1) the data presently available, (2) two NAOS software modifications that would provide new data to increase the accuracy of the PSF reconstruction and (3) the tests of these modifications) and the PSF reconstruction algorithms we are developing for NAOS on that basis.
Real-time real-sky dual-conjugate adaptive optics experiment
The current status of a real-time real-sky dual-conjugate adaptive optics experiment is presented. This experiment is a follow-up on a lab experiment at Lund Observatory that demonstrated dual-conjugate adaptive optics on a static atmosphere. The setup is to be placed at Lund Observatory. This means that the setup will be available 24h a day and does not have to share time with other instruments. The optical design of the experiment is finalized. A siderostat will be used to track the guide object and all other optical components are placed on an optical table. A small telescope, 35 cm aperture, is used and following this a tip-tilt mirror and two deformable mirrors are placed. The wave-front sensor is a Shack-Hartmann sensor using a SciMeasure Li'l Joe CCD39 camera system. The maximum update rate of the setup will be 0.5 kHz and the control system will be running under Linux. The effective wavelength will be 750 nm. All components in the setup have been acquired and the completion of the setup is underway. Collaborating partners in this project are the Applied Optics Group at National University of Ireland, Galway and the Swedish Defense Research Agency.
Long exposure point spread function estimation from adaptive optics loop data: validation and results
Adaptive Optics systems have revolutionized ground based astronomy by providing real time correction for atmospheric aberrations. However, due to limited temporal and spatial bandwidth the correction provided is not perfect. With knowledge of the Point Spread Function correction can be further improved. The lack of point sources in Solar observations makes a direct measurement of the PSF impossible. We present a method to obtain a PSF estimate from the Adaptive Optics system loop telemetry. Using this method we can obtain a PSF for each captured AO corrected image and correct each image individually. We applied this method to a long time series of Solar data obtaining satisfactory results. Also in an attempt to validate this method we successfully observed the star Sirius with the Dunn Solar Telescope. The AO corrected star images provide a direct measurement of the PSF that can be compared to our estimates obtained from the AO telemetry data.
Poster Session: Real-Time Control Systems
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Testing FPGAs for real-time control of adaptive optics in giant telescopes
Real-time control has been clearly identified as a separate challenging field within Adaptive Optics, where a lot of computations have to be performed at kilohertz rate to properly actuate the mirror(s) before the input wavefront information has become obsolete. When considering giant telescopes, the number of guide stars, wavefront samples and actuators rises to a level where the amount of processing is far from being manageable by today's conventional processors and even from the expectations given by Moore's law for the next years. FPGA (Field Programmable Gate Arrays) technology has been proposed to overcome this problem by using its massively parallel nature and its superb speed. A complete laboratory test bench using only one FPGA was developed by our group [1], and now this paper summarizes the early results of a real telescope adaptive optics system based in the FPGA-only approach. The system has been installed in the OGS telescope at "Observatorio del Teide", Tenerife, Spain, showing that a complete system with 64 Shack-Hartmann microlenses and 37 actuators (plus tip-tilt mirror) can be implemented with a real time control completely contained within a Xilinx Virtex-4 LX25 FPGA. The wavefront sensor has been implemented using a PULNIX gigabit ethernet camera (714 frames per second), and an ANDOR IXON camera has been used for the evaluation of the overall correcting behavior.
Limitations of high performance real time parallel off-the-shelf computers for next generation AO systems
Taras Golota
Relatively inexpensive multiprocessor off-the-shelf computer systems have emerged as viable alternatives for high-performance, low-cost adaptive optics (AO) real-time computing. The performance of the AO systems depends on the number of active elements, sampling frequency and processing speed of the multiplication of matrices as a generic operation in this scientific application. Addressed questions are the limitations off-the-shelf computers for growing size AO systems at hight sampling frequency, matrix per vector multiplication performance and it's repeatability.
FPGA cluster for high-performance AO real-time control system
Whilst the high throughput and low latency requirements for the next generation AO real-time control systems have posed a significant challenge to von Neumann architecture processor systems, the Field Programmable Gate Array (FPGA) has emerged as a long term solution with high performance on throughput and excellent predictability on latency. Moreover, FPGA devices have highly capable programmable interfacing, which lead to more highly integrated system. Nevertheless, a single FPGA is still not enough: multiple FPGA devices need to be clustered to perform the required subaperture processing and the reconstruction computation. In an AO real-time control system, the memory bandwidth is often the bottleneck of the system, simply because a vast amount of supporting data, e.g. pixel calibration maps and the reconstruction matrix, need to be accessed within a short period. The cluster, as a general computing architecture, has excellent scalability in processing throughput, memory bandwidth, memory capacity, and communication bandwidth. Problems, such as task distribution, node communication, system verification, are discussed.
FPGA developments for the SPARTA project: Part 2
The European Southern Observatory (ESO) and Durham University's Centre for Advanced Instrumentation (CfAI) continue to progress the design of a next generation Adaptive Optics (AO) Real-Time Control System (RTCS). This common flexible platform, labelled SPARTA 'Standard Platform for Adaptive optics Real-Time Applications' will control the AO systems for a set of 2nd generation VLT instrumentation, and will scale to implement the initial AO systems for the European Extremely Large Telescope (E-ELT). Durham has used Field Programmable Gate Arrays (FPGA) to design a front-end Wavefront Sensor (WFS) Processing Unit (WPU) for SPARTA. FPGA devices have been used to alleviate the highly parallel computationally intensive WPS processing task from system processors to increase the obtainable control loop frequency and reduce the computational latency in the control system. The FPGA device reduces WFS frames to gradient vectors before passing the data to the system processors. The FPGA allows the processors to deal with other tasks such as wavefront reconstruction, telemetry and real-time data recording, allowing for more complex adaptive control algorithms to be executed. Durham has design, coded, implemented and tested a FPGA core incorporating the VITA 17.1 standard serial Front Panel Data Port (sFPDP) protocol to allow a data transfer rate of 2.5Gbps-1 from the WFS Controller to the SPARTA platform. This paper overviews the SPARTA WPU requirements and design, the sFPDP FPGA Core and a description of the platform's implementation phase.
Poster Session: Natural Guide Star AO Field Tests
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Application of 1.2m adaptive optics system for astronomy
Yaoheng Xiong, Xiang Chang, Rui Guo, et al.
A 61-actuator adaptive optical system has been upgraded at Yunnan observatory 1.2m telescope since 2004, and is only one visible wavelength AO system in China now. Considering its relative small diameter and angular resolution, the main purpose of this AO system, besides the high resolution astronomical imaging, is for long distance laser ranging. This paper describes these AO system performances and its observation results, and the possible application on lunar laser ranging.
Poster Session: Laser Systems, Facilities, and Concepts
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Second generation laser traffic control: algorithm changes supporting Mauna Kea, La Palma, and future multi-telescope laser sites
Douglas Summers, Nikolaos Apostolakos, René Rutten, et al.
A Laser Traffic Control System (LTCS) for laser beam avoidance has been in use at the W. M. Keck observatory on Mauna Kea since 2002. Subsequent LTCS installations have occurred at Gemini North (2003), and at the William Herschel Telescope on La Palma, Canary Islands (2005). Gemini North laser tests in 2005 necessitated algorithm changes to provide support for multiple laser configurations. Operational differences for how laser-telescope priority resolutions occur on La Palma vs. Mauna Kea necessitated algorithm changes to address more generic specification of priority rules, collision event queries, and better display feedback. A joint collaboration between the W. M. Keck observatory and the Isaac Newton Group, to install the LTCS at La Palma and enhance its priority processing algorithm and display functions, occurred in 2005. The changes made should be sufficient to support LTCS software implementations at many different sites, current and future, where multiple laser/telescope configurations are planned. This paper will describe the algorithm changes, review outstanding issues, and describe planned development activities supporting a broader use potential to include sites with ELTs.
Transmission characteristics of high-power 589-nm laser beam in photonic crystal fiber
Meguru Ito, Yutaka Hayano, Norihito Saito, et al.
We are developing Laser Guide Star Adaptive Optics (LGSAO) system for Subaru Telescope at Hawaii, Mauna Kea. We achieved an all-solid-state 589.159 nm laser in sum-frequency generation. Output power at 589.159 nm reached 4W in quasi-continuous-wave operation. To relay the laser beam from laser location to laser launching telescope, we used an optical fiber because the optical fiber relay is more flexible and easier than mirror train. However, nonlinear scattering effect, especially stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), will happen when the inputted laser power increases, i.e., intensity at the fiber core exceed each threshold. In order to raise the threshold levels of each nonlinear scattering, we adopt photonic crystal fiber (PCF). Because the PCF can be made larger core than usual step index fiber (SIF), one can reduce the intensity in the core. We inputted the high power laser into the PCF whose mode field diameter (MFD) is 14 μm and the SIF whose MFD is 5 μm, and measured the transmission characteristics of them. In the case of the SIF, the SRS was happen when we inputted 2 W. On the other hand, the SRS and the SBS were not induced in the PCF even for an input power of 4 W. We also investigated polarization of the laser beam transmitting through the PCF. Because of the fact that the backscattering efficiency of exciting the sodium layer with a narrowband laser is dependent on the polarization state of the incident beam, we tried to control the polarization of the laser beam transmitted the PCF. We constructed the system which can control the polarization of input laser and measure the output polarization. The PCF showed to be able to assume as a double refraction optical device, and we found that the output polarization is controllable by injecting beam with appropriate polarization through the PCF. However, the Laser Guide Star made by the beam passed through the PCF had same brightness as the state of the polarization.
589 nm sum-frequency generation laser for the LGS/AO of Subaru Telescope
Yoshihiko Saito, Yutaka Hayano, Norihito Saito, et al.
We developed a high power and high beam quality 589 nm coherent light source by sum-frequency generation in order to utilize it as a laser guide star at the Subaru telescope. The sum-frequency generation is a nonlinear frequency conversion in which two mode-locked Nd:YAG lasers oscillating at 1064 and 1319 nm mix in a nonlinear crystal to generate a wave at the sum frequency. We achieved the qualities required for the laser guide star. The power of laser is reached to 4.5 W mixing 15.65 W at 1064 nm and 4.99 W at 1319 nm when the wavelength is adjusted to 589.159 nm. The wavelength is controllable in accuracy of 0.1 pm from 589.060 and 589.170 nm. The stability of the power holds within 1.3% during seven hours operation. The transverse mode of the beam is the TEM00 and M2 of the beam is smaller than 1.2. We achieved these qualities by the following technical sources; (1) simple construction of the oscillator for high beam quality, (2) synchronization of mode-locked pulses at 1064 and 1319 nm by the control of phase difference between two radio frequencies fed to acousto-optic mode lockers, (3) precise tunability of wavelength and spectral band width, and (4) proper selection of nonlinear optical crystal. We report in this paper how we built up each technical source and how we combined those.
The laser guide star facility for Subaru Telescope
Yutaka Hayano, Yoshihiko Saito, Meguru Ito, et al.
The purpose of this paper is to report on the current status of developing the new laser guide star (LGS) facility for the Subaru LGS adaptive optics (AO) system. Since two major R&D items, the 4W-class sum-frequency generating laser1 and the large-area-core photonic crystal fiber2, have been successfully cleared, we are almost ready to install the LGS facility to the Subaru Telescope. Also we report the result for LGS generation in Japan.
The Rayleigh technical demonstrator: a novel concepts platform
As part of a collaboration between Durham University and ESO, an experimental platform is presented whose purpose is for testing generic laser-based wavefront sensors (WFS) for adaptive optics. The Rayleigh Technical Demonstrator (RTD) has been designed to allow a laser launch and Rayleigh back-scatter collection by installing components solely on a Nasmyth platform of the William Herschel Telescope, La Palma. The aim is to provide a WFS testing port within the RTD, permitting new WFS concepts to be tested rapidly in conjunction. This means that the RTD only requires small modifications for each concept to be tested. The second goal is to permit near-contemporaneous comparison of WFS data with that from a tomographic WFS. Currently, the RTD is planned to trial with three "cone effect-free" WFS concepts as part of the CALDO project. Presented here is an overview of the RTD design with detailed information on novel components and design choices.
Multi-watt 589-nm Na D2-line generation via frequency doubling of a Raman fiber amplifier: a source for LGS-assisted AO
L. Taylor, Y. Feng, D. Bonaccini Calia, et al.
We develop a novel solid state fibre laser system, AFIRE, for the purposes of laser guidestar (LGS) assisted adaptive optics (AO), based on the second harmonic generation (SHG) from a high-power (P1178 ~25W) CW narrowband (Δυ < 3GHz) Raman fibre amplifier developed by IPF. We present what we believe to be the highest power, narrowband single-pass CW 589nm SHG result reported to date, P589 ~ 4.2W from P1178 ~ 19W (ηVIS > 22%). We demonstrate our understanding of the arising absorption-induced thermal effects (namely, dephasing and degradation of the conversion), offer predictions towards higher powers and conversion levels, and show that our current results are essentially pump-power limited. We are confident of the scalability of both the IR and visible parts of our system, to these higher output powers and conversion efficiencies.
Design of a narrow band 589 nm laser by direct Raman shift in single mode fiber
We present the design of a 0.5 GHz linewidth 589 nm master laser by direct Raman shift from a commercially available solid state green laser, done in a single mode fiber. The laser design would be a breakthrough because it would allow a large number of currently unavailable lasing wavelengths to be reached in solid state devices. Our purpose is to produce 0.2-0.5 W CW at 589 nm, to seed a Raman amplifier scheme and achieve 10-15 W CW output at 589 nm. Our simulations show that > 0.5 W may be obtained with a 10 W 532 nm pump laser and sets of appropriately steep Fiber Bragg Gratings which resonate only the Raman Stokes.
Ultra-low loss hollow-core photonic crystal fibers at 589 nm for LGS-assisted AO
We are developing a hollow-core photonic crystal fiber to be used as a beam relay in pulsed sodium laser guide star systems with a target maximum attenuation of 30 dB/km, 20 dB/km as goal. In order to optimize the fiber geometry, we numerically model the light propagation using a finite-element solver that allows us to simultaneously optimize several structural parameters.
Resonant electro-optical modulator for high laser-power single-mode fiber relay
W. Hackenberg, M. Zaehringer, A. Silber
We describe the design and performance of a resonant electro-optical modulator, based on stoichiometric lithium tantalate, capable of handling high optical powers and providing a large modulation depth. This phase modulator is part of the single-mode fiber relay used in the ESO VLT Laser Guide Star Facility for adaptive optics to transport 589-nm laser light from the PARSEC dye laser installed in the laser clean room to the laser launch telescope. The purpose of the phase modulator is to broaden the single-frequency PARSEC laser linewidth before the laser light is injected into the single-mode relay fiber. By this the power handling capability of the single-mode fiber is increased by a factor of 3 or 10 W of in-coupled power at 589 nm, while maintaining the excitation efficiency of the mesospheric sodium D2 transition.
Laser guide star projection for large telescopes
Perspective imaging of laser guide stars leads to their elongation. This effect is significant in future large telescopes, where many such beacons are necessary. Solutions to these problems include mechanical or electronic movement at the sensing devices, optical separation of the reference sources and more. Another solution is to shift the burden of analysis to the lasers and create a pattern which suffers less from perspective elongation when scattered back onto the detectors. Finally, different analysis methods can then be employed to solve for the wave front tomographically from this projected pattern.
Poster Session: Wavefront Sensors and Detectors
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New micro-lithography techniques for the manufacture of wavefront sensors for the FALCON concept: ADONF
FALCON is an original concept for next generation instrumentation at ESO VLT or at future ELTs. It is a multi-objects integral field spectrograph with multiple integral field units (IFU) performing adaptive optics correction in order to reach spatial and spectral resolution ideally suited for distant galaxy studies. The resolutions required for the VLT are typically 0.15 - 0.25 arcsec and R>=5000 in the 0.8-1.8 μm wavelength range. The studied galaxies are very faint objects that cannot be directly used to perform wavefront sensing. Thus, we use at least three Wave-Front Sensors (WFS) per IFU to sense the wavefront of stars located around the galaxy, and the on-axis wavefront from the galaxy will be deduced from the off-axis measurements by atmospheric tomography, and then corrected thanks to an adaptive optics (AO) system within each IFU. Since the WFS is ideally located directly in the focal plane of the telescope, this implies to develop miniaturized devices for the wavefront sensing. Our approach is based on a Shack Hartmann principle and - instead of using a bulky detector behind - we plan to use a miniaturized system including fibers able to transport the light from the focal plane of the microlens array towards a place where the bulk issue is less critical. We draw up the main specifications of this miniaturized system and we present the characteristics of elements manufactured by using new microlithography techniques.
MCP-Medipix2 hybrid detector for AO wavefront sensors
John Vallerga, Jason McPhate, Bettina Mikulec, et al.
A hybrid optical detector being developed at Berkeley has most of the attributes desired for the next generation AO wavefront sensors. The detector consists of proximity focused MCPs read out by a multi-pixel application specific integrated circuit (ASIC) chip developed at CERN ("Medipix2") with individual pixels that amplify, discriminate and count input events. The detector has 256 x 256 pixels, zero readout noise (photon counting) and can be read out at 1kHz frame rates. We will report on the progress achieved after two years of our three year development effort for this detector technology funded as part of the Adaptive Optics Development Program managed by the National Optical Astronomy Observatory. Details on the first vacuum tube constructed with a Medipix2 ASIC along with the fast kHz parallel electronic readout are presented. We also describe a new hybrid detector design based on HgCdTe APD arrays coupled to a Medipix2 readout that could bring zero readout noise at high frame rates to the near IR regime.
Poster Session: Multi-Conjugate AO and Control System Lab Tests
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Point spread function reconstruction from Woofer-Tweeter adaptive optics bench
This paper describes a model-based and experimental evaluation of a point spread function (PSF) reconstruction technique for a Dual Deformable Mirror (DM) Woofer-Tweeter (W/T) Adaptive Optics (AO) system. In the W/T architecture, the Woofer is a low-order-high-stroke DM, and it is used to compensate for the low-frequency-high-amplitude effects introduced by the atmospheric turbulence. The Tweeter is a high-order-low-stroke DM that is used to compensate for the high-frequency-low-amplitude effects introduced by the atmospheric turbulence. The research concept of having Dual DMs allows the W/T AO system to have a high degree of correction of large amplitude wavefront distortion. The role of the UVic AO bench is to demonstrate the closed-loop wavefront control feasibility for a W/T AO concept to be used on the science instruments of the Thirty Meter Telescope (TMT).
Scientific goals for the MMT's multilaser-guided adaptive optics
The MMT's five Rayleigh laser guide star system has successfully demonstrated open loop wavefront sensing for both ground-layer and laser tomography adaptive optics (AO). Closed loop correction is expected for the first time in the autumn of 2006. The program is moving into its second phase: construction of a permanent facility to feed AO instruments now used with the telescope's existing natural star AO system. The new facility will preserve the thermal cleanliness afforded by the system's adaptive secondary mirror. With the present laser power of 4 W in each of the Rayleigh beacons, we will first offer ground-layer correction over a 2 arcmin field in J, H, and K bands, with expected image quality routinely 0.2 arcsec or better. Later, we will also offer imaging and spectroscopy from 1.5 to 4.8 μm with a tomographically corrected diffraction limited beam. The development of these techniques will lead to a facility all-sky capability at the MMT for both ground-layer and diffraction-limited imaging, and will be a critical advance in the tools necessary for extremely large telescopes of the future, particularly the Giant Magellan Telescope. We describe the present state of system development, planned progress to completion, and highlight the early scientific applications.
Poster Session: Analytical Studies
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Spatial frequency domain model for adaptive optics compensation of segmented mirror misalignments and figure errors
In addition to their essential function of providing atmospheric turbulence compensation, astronomical Adaptive Optical (AO) systems also supplement the role of active optics (aO) by providing some additional correction of the wavefront aberrations introduced by mirror mounting, alignment, thermal distortion and/or fabrication errors. This feature is particularly desirable for segmented mirror telescopes such as the Thirty Meter Telescope (TMT), but wavefront discontinuities across segment boundaries are challenging to properly sense and correct. In this paper we describe a fast, analytical, frequency domain model which may be used to study and quantify the above effects, and discuss a range of sample results obtained to support the development of the top-level requirements for the TMT primary mirror. In general, AO compensation of mirror segment piston errors is not particulary useful unless the deformable mirror (DM) interactuator spacing is equivalent to no more than one-half of a mirror segment diameter (when both of these dimensions are expressed in the same pupil plane). Effective AO compensation of mirror segment tip/tilt errors, or low order segment figure errors such as astigmatism, typically requires 3-4 DM actuators per mirror segment. These results illustrate the importance of quantifying and minimizing uncorrectable telescope wavefront errors when developing performance predictions for adaptive optical systems.
Comparison of tip-tilt controllers using the STRAP wavefront sensor in the W.M. Keck Observatory laser guide star adaptive optics system
The W. M. Keck Observatory adaptive optics (AO) system uses the STRAP wavefront sensor to sense tip-tilt using a natural guidestar. The higher-order wavefront sensing can be done using light from either a laser guidestar (LGS) or a natural guidestar (NGS). The tip-tilt guidestar can be as bright as 10th or as faint as 19th magnitude. In both cases, as high a control bandwidth as possible is desired. Thus, it is of interest to determine the potential of various control algorithms over a wide range of signal-to-noise ratios (SNRs). This paper compares two control algorithms using a set of tip-tilt data taken with the STRAP wavefront sensor (a set of four avalanche diodes arranged as a quad cell). The two algorithms are the standard integral control and a minimum variance (LQG) control designed using the power spectral density (PSD) of the data. The bandwidths of the integral control and the minimum variance control are adjusted to produce the least RMS residual wavefront error. The controllers are compared for SNRs representative of the expected range of guidestars.
Constraining the GLAO parameter space with turbulence profile models
Nature provides reduced turbulence at intermediate altitudes and as a result the mean image quality of a ground layer adaptive optics (GLAO) corrected field degrades slowly with increasing diameter. If this function has a shallow slope all the way out to the maximum seeing limited field of view of the telescope then surveys at that telescope may significantly benefit from GLAO. Using published optical turbulence profile models for Cerro Pachon we show that the GLAO gains require that the full seeing limited field of view be corrected for two example telescopes, each attempting two example science cases. We also show that using only four sodium laser guide stars the very wide GLAO system with optimal actuator and subaperture pitch will not be affected by servo lag and wavefront sensor noise.
Evaluation of ground layer adaptive optics for the wide field optical spectrograph on the TMT
We arrive at a Ground Layer Adaptive Optics (GLAO) design that offers true seeing-improved performance and operation for the red and infrared wavelengths. The design requires an adaptive secondary (AM2) and that the sodium Laser Guide Star (LGS) launch telescope be able to steer four of the beams to 8.5 arcminutes off-axis. When provided with this, the proposed design is potentially the simplest, lowest cost design that can take the form of an upgrade. This is seen as a significant advantage over designs that would build an adaptive mirror into each of the four arms of WFOS. We show that the performance penalty for using one mirror instead of four to correct the entire 81 square arcminute WFOS field is minor.
Poster Session: MEMS and Deformable Mirror Modeling
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Fitting error analysis for the VLT deformable secondary mirror
The present work reports the results of fitting error analysis for the Deformable Secondary Mirrors (DSM) of the Very Large Telescope (VLT). The analysis is performed in terms of residual rms wave-front error, PSF profile, requested actuator position and force stroke. A comparison with an ideal Karhunen-Loeve and Zernike wave-front correctors is also shown. The analysis is based on influence functions simulated by FEA.
Poster Session: Pyramid and Layer-Oriented Wavefront Sensing
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Integration, testing, and laboratory characterization of the mid-high layer wavefront sensor for LINC-NIRVANA
The Mid-High Wavefront Sensors (MHWS) are components of the adaptive optics system of LINC-NIRVANA, the Fizeau interferometer that will be mounted at the LBT. These sensors, one for each telescope arm, will measure the atmospheric turbulence in the high altitude layers, using up to 8 reference stars in a 2 arcmin Field of View, and they will be coupled with two Ground Layer WFSs that will measure the lower part of the atmospheric turbulence using up to 12 stars over an annular Field of View from 2 to 6 arcmin in diameter. We will describe the opto-mechanical layout of the MHWS and the Assembly, Integration and Test (AIT) phase of the first sensor in the laboratory of the Bologna Observatory.
Pyramid wavefront sensing: theory and component technology development at LAO
Pyramid wavefront sensors offer an alternative to traditional Hartmann sensing for wavefront measurement in astronomical adaptive optics systems. The Pyramid sensor has been described as a slope sensor with potential sensitivity gains over the Shack Hartmann sensor, but in actuality seems to exhibit traits of both a slope sensor and a direct phase sensor. The original configuration, utilizing glass pyramids and modulation techniques, is difficult to implement. We present results of laboratory experiments using a Pyramid sensor that utilizes a micro-optic lenslet array in place of a glass pyramid, and does not require modulation. A group of four lenslets forms both the pyramid knife-edge and the pupil reimaging functions. The lenslet array is fabricated using a technique that pays careful attention to the quality of the edges and corners of the lenslets. The devices we have tested show less than 1 micron edge and corner imperfections, making them some of the sharpest edges available. We finish by comparing our results to theoretical wave optic predictions which clearly show the dual nature of the sensor.
Poster Session: Adaptive Optics Programs II
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Performance of the deformable mirror for Subaru LGSAO
Shin Oya, Aurelien Bouvier, Olivier Guyon, et al.
The performance of a deformable mirror with 188 electrodes is reported in this paper. The deformable mirror has been manufactured by CILAS for a new adaptive optics system at Subaru Telescope equipped with laser-guide-star. The type of deformable mirror is bimorph PZT with the blank diameter of 130 mm (beam size 90 mm).
An application of active optics to x-ray imaging: X-mas (x-ray milli arc-second) project
Masahiro Tsujimoto, Shunji Kitamoto, Yohsuke Ohkubo, et al.
We report the current status of the "X-mas" (X-ray milli-arcsecond) project. X-mas is an application of the AO technology to the X-ray optics, aiming to obtain high-resolution defraction-limited X-ray images. Our X-ray telescope employs the Newton optics with a paraboloid primary and a 31-element deformable secondary mirrors. The aperture of the primary mirror is 80 millimeters with the focal length of 2 meters. Multi-layer coating of the mirrors by silicon and molybdenum realizes a large reflectivity of ~60% for the primary and 30-50% for the secondary mirror at 13.5 nm, which enables us to construct a normal incidence optics at this wavelength. We use a laser guide source and a wave front sensor to optimize the form of the secondary deformable mirror for the purpose of offsetting the large-scale figure errors in the X-ray optics. A back-side illumination X-ray CCD detector manufactured by Hamamatsu Photonics is used for X-ray detections. We have assembled all these elements and started to accumulate data. Closed-loop AO is in operation for the laser guide source. Likely X-ray images are obtained through the telescope. The results in 2005-2006 are presented.
MEMs-based speckle spectrometer
A. I. Sheinis, L. Nigra, M. Q. Kuhlen
We describe a new concept for a MEMS-based active spatial filter for astronomical spectroscopy. The goal of this device is to allow the use of a diffraction-limited spectrometer on a seeing limited observation at improved throughput over a comparable seeing-limited spectrometer, thus reducing the size and cost of the spectrometer by a factor proportional to r0/D (For the case of a 10 meter telescope this size reduction will be approximately a factor of 25 to 50). We use a fiber-based integral field unit (IFU) that incorporates an active MEMS mirror array to feed an astronomical spectrograph. A fast camera is used in parallel to sense speckle images at a spatial resolution of λ/D and at a temporal frequency greater than that of atmospheric fluctuations. The MEMS mirror-array is used as an active shutter to feed speckle images above a preset intensity threshold to the spectrometer, thereby increasing the signal-to-noise ratio (SNR) of the spectrogram. Preliminary calculations suggests an SNR improvement of a factor of about 1.4. Computer simulations have shown an SNR improvement of 1.1, but have not yet fully explored the parameter space.
Poster Session: AO Lab Tests
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A reflective Gaussian coronagraph for ExAO: laboratory performance
We report laboratory results of a coronagraphic testbed to assess the intensity reduction differences between a "Gaussian" tapered focal plane coronagraphic mask and a classical hard-edged "Top Hat" function mask at Extreme Adaptive Optics (ExAO) Strehl ratios of ~94%. However, unlike a traditional coronagraph design, we insert a reflective focal plane mask at 45o to the optical axis and used a "spot of Arago blocker" (axicon stop) before a final image in order to block additional mask edge-diffracted light. The testbed simulates the optical train of ground-based telescopes (in particular the 8.1m Gemini North telescope) and includes one spider vane and different mask radii (r= 1.9λ/D, 3.7λ/D, 7.4λ/D) and two types of reflective focal plane masks (hard-edged "Top Hat" and "Gaussian" tapered profiles). In order to investigate the performance of these competing coronagraphic designs with regard to extra-solar planet detection sensitivity, we utilize the simulation of realistic extra-solar planet populations (Nielsen et al. 2006). With an appropriate translation of our laboratory results to expected telescope performance, a "Gaussian" tapered mask radius of 3.7λ/D with an axicon stop performs best (highest planet detection sensitivity). For a full survey with this optimal design, the simulation predicts ~30% more planets detected compared to a similar sized "Top Hat" function mask with an axicon stop. Using the best design, the "point contrast ratio" between the stellar PSF peak and the coronagraphic PSF at 10λ/D (0.4" in H band if D = 8.1m) is 1.4 x 106. This is ~10 times higher than a classical Lyot "Top Hat" coronagraph.
HORATIO: the Leiden high-order adaptive optics testbed
The current generation of Adaptive Optics systems has lead to an improvement in resolution and contrast of up to one to two orders of magnitude for 8-10 meter class telescopes. With the upcoming generation of Extremely Large Telescopes, AO has the potential for an even larger gain. But the scaling of the current AO systems to ELTs will not be trivial; both in AO hardware as well as in AO control, techniques need to be developed which can cope with the large number of sub-apertures. The Leiden High Order AO Testbed was developed for the implementation and testing of innovative ELT-compatible AO systems. In the current setup, the system is centered around a Liquid Crystal Spatial Light Modulator, a Shack-Hartmann wavefront sensor and a PC based RTC. Initially, the system will be used to develop and test efficient high-order AO algorithms under laboratory conditions, but is sufficiently flexible to allow for fast replacement of all components for the testing of different hardware components. In this paper we will present the design, building and testing of HORATIO.
MANU-CHAO: a laboratory ground-layer adaptive optics experiment
We present a laboratory setup of a Ground-Layer Adaptive Optics system. This system is a scaled-down version of the MCAO system of MAD (a MCAO system for the VLT) / LINC-NIRVANA (a Fizeau Imager for the LBT) and measures the wavefront aberrations with 4 pyramids in a layer-oriented fashion with optical co-addition. The laboratory setup contains besides the wavefront-sensing unit a telescope-simulator, a dynamic turbulence generator and a Deformable Mirror for the wavefront correction. We describe the overall system and its single components, open- and closed-loop measurements of the characteristics of a system working in GLAO mode and first results when using a Kalman filter for the control of the wavefront reconstruction process.
Poster Session: Innovative Deformable Mirror Technologies
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Selecting the electromagnetic actuator of the ELT primary mirror
Ciro Del Vecchio, Fabrizio Marignetti, Armando Riccardi, et al.
The segmented, adaptive primary mirror of the ELT is the most delicate optical component of the telescope. Its full-adaptive operational modes require an high bandwidth actuation system, able to provide large and precise stroke motions. As the core component of the actuation system hardware, the electromagnetic device has to be accurately designed. This paper depicts the preliminary electromagnetic study of the actuator. After a discussion of the chosen concept, the design process and the computational approach are illustrated. The goal of the study is the definition of the basic geometrical and physical parameters which allow the minimization of the power dissipated to deliver the proper actuation force, in order to reduce the local overheating in the telescope optical path.
Active polishing of a 2mm thin shell for large adaptive secondary mirrors
Large adaptive secondary mirrors are a promising solution for the next generation of high order adaptive optics systems and are under development in all major observatories. One of the key points of these systems is the manufacturing of the large glass thin shell, often convex aspheric, used as the 'mirror surface' and attached to the actuators. Due to the very tight surface quality specifications for high order or extreme adaptive optics systems (XAO), one of the major challenges is to avoid all high spatial frequencies errors during the manufacturing of these extremely thin convex hyperbolic shells. In order to achieve the required surface quality, we present an active optics technique based on elasticity theory and mirror polishing under constraints, allowing to easily generate highly aspheric optics, using only full size tools (spherical or flat), and avoiding such high spatial frequencies defects. The proposed original process for thinning, smoothing and polishing a large (1.1m) thin (2mm) shell has been modelled using finite elements method. The feasibility of this process is demonstrated. Results in terms of load optimization, evolution of stresses and constraints within the shell and expected surface optical quality are presented.
Optical figuring specifications for thin shells to be used in adaptive telescope mirrors
The present work describes the guidelines to define the optical figuring specifications for optical manufacturing of thin shells in terms of figuring error power spectrum (and related rms vs scale distributon) to be used in adaptive optics correctors with force actuators like Deformable Secondary Mirrors (DSM). In particular the numerical example for a thin shell for a VLT DSM is considered.
Thermal deformable mirror
Carlos A. Caicedo, Allan Wirth, Greg LaRosa, et al.
Beam correction of high power lasers often requires large wavefront deformations with very slow rates of change. Adaptive Optics Associates, Inc. (AOA) has developed a new type of deformable mirror in which thermal expansion of tubes control the mirror's figure. This paper is the subject of a patent disclosure. Such a mirror can produce large stroke at rates compatible with thermal distortions in high power lasers. This paper discusses the design of this device including range of deformation, ways to measure the temperature of the tubes, and algorithms to control mirror shape while compensating for the non-symmetrical response of thermal expansion and contraction. The construction of a prototype device and the associated control electronics and software is also covered. Photographs of the working device are shown.
Poster Session: Wavefront Reconstruction Theory
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Post-coronagraph wavefront sensing for the TMT Planet Formation Imager
G. Vasisht, I. J. Crossfield, P. J. Dumont, et al.
Direct detection of exo-planets from the ground may be feasible with the advent of extreme-adaptive optics (ExAO) on large telescopes. A major hurdle to achieving high contrasts behind a star suppression system (10-8/hr-1/2) at small angular separations, is the "speckle noise" due to residual atmospheric and telescope-based quasistatic amplitude and phase errors at mid-spatial frequencies. We examine the potential of a post-coronagraphic, interferometric wavefront sensor to sense and adaptively correct just such errors. Pupil and focal plane sensors are considered and the merits and drawbacks of each scheme are outlined. It is not inconceivable to implement both schemes or even a hybrid scheme within a single instrument to significantly improve its scientific capabilities. This work was carried out in context of the proposed Planet Formation Imager instrument for Thirty Meter Telescope (TMT) project.
Full wave front reconstruction in the Fourier domain
E. N. Ribak, Y. Carmon, A. Talmi, et al.
Recently we developed and tested different algorithms for wave front reconstruction from dense Hartmann-Shack patterns. All depend on the recognition of a main frequency in the patterns, whose distortion from wave aberrations can be construed as slight phase changes in the pattern. An alternative description of these aberrations is a slight frequency change in Fourier domain. The slopes can thus be found by demodulation in either the image or the Fourier domain. These slopes can then be integrated in the Fourier domain again for the wave front itself. For smooth slopes both demodulation and integration can be performed in the Fourier domain. In addition, commands for the adaptive optics loop can be taken directly in the Fourier domain, saving on processing time. We modeled and tested these algorithms thoroughly in simulation and in laboratory experiments on two separate adaptive optics systems.
Poster Session: Turbulence, Anisoplanatism, and Sky Coverage
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The MPIA multipurpose laboratory atmospheric turbulence simulator MAPS
We built an optical system that emulates the optical characteristics of an 8m-class telescope like the VLT and that contains rotating glass plates phase screens to generate realistic atmosphere-like optical turbulence. Together with an array of single mode fibers fed from white light sources to simulate various stellar configurations, we can investigate the behavior of different single or multi-conjugate adaptive optics setups. In this paper we present the characteristics of phase screens etched on glass plates surfaces obtained from Silios Technologies.
Beyond conventional G-SCIDAR: the ground-layer in high vertical resolution
Sebastian E. Egner, Elena Masciadri, Dan McKenna, et al.
By using the SCIDAR instrument at the VATT on the top of Mt. Graham and a very wide binary star with a separtion of 35", the vertical structure of the turbulence in the first few hundred meters above the telescope was measured. When using such a binary and analysing the cross-correlation images, a vertical resolution for the turbulence profile of a few tens of meters can be achieved near the ground. This permits to determine the inner structure and the wind sheer of the single turbulent layers inside the ground-layer. We present the principles and the data-reduction process of this method and show first results obtained with this method at Mt. Graham. As an application, we estimate the fraction of the turbulence between the dome of the VATT and the primary mirror of the LBT.
G-SCIDAR measurements on Mt. Graham: recent results
Sebastian E. Egner, Elena Masciadri, Dan McKenna, et al.
We present recent results of the atmospheric turbulence measured with a Generalized SCIDAR at Mt. Graham, running for 16 nights in 2004 and 2005 at the focus of the VATT Telescope. The principle of the data reduction process is shown, as well as the validation of the obtained results. From the reduced C2N and wind-speed profiles, together with an estimate for the dome-seeing, the astroclimatic parameters such as seeing ε, isoplanatic angle υ0 and wavefront coherence time τ0 are calculated. We obtained median values for ε (0.67"±0.17"), υ0 (2.71"±1.11") and τ0 (3.63msec ± 1.66msec), which indicate that Mt. Graham is as an astronomical site comparable to the best ones in the world. As an application, the calculated C2N profiles were used together with layer-transfer functions for a MCAO system to estimate the optimal conjugated heights of the DMs for the MCAO system of LINC-NIRVANA.
Full characterization of the turbulence generator MAPS for MCAO
In the framework of the MCAO Demonstrator MAD, ESO has developed a turbulence generator called MAPS to emulate a 3D evolving Paranal atmosphere. An all-optical solution has been chosen to produce the turbulence: a set of rotating refractive Phase Screens in which are imprinted patterns distorting the WF as the atmosphere would. In order to characterize the turbulence produced by MAPS, we have used several independent techniques (static WF measurements, long-exposure PSF, AO closed loop voltages) from the WF sensing wavelengths to the near-IR imaging ones. Such a characterization is essential to a further comparison between the lab results and the future on-sky measurements. This analysis required to study the link between the different parameters generally used to quantify the turbulence, in particular the Fried parameter r0, the seeing and the FWHM of long-exposure images. The conclusions are that the outer scale L0 has a preponderant influence on those values. This study will be completed by future on-sky measurements allowing to establish how realistic is the turbulence produced by the Phase Screens in MAPS.
Poster Session: Simulation Studies and Tools
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Simulations of ground-layer adaptive optics for extremely large telescopes
Visa A. Korkiakoski, Miska Le Louarn, Christophe Vérinaud
Simulations of various GLAO systems for telescopes up to 32 meter are presented. We have generated point spread functions of several GLAO configurations using ESO's parallel simulation tool and compared the results with analytically computed PSFs. A good agreement is observed when simulating bright guide stars (both LGSs and NGSs) although the analytical approach does not consider for instance wavefront sensor errors or servo lag. Other more realistic configurations (asymmetrically located and dim guide stars) are investigated and the need for advanced control methods in such cases are considered.
Modeling a GLAO system for the Gemini Observatory
Ground layer adaptive optics (GLAO) can significantly decrease the size of the point spread function (PSF) and increase the energy concentration of PSFs over a large field of view at visible and near-infrared wavelengths. This improvement can be realized using a single, relatively low-order deformable mirror (DM) to correct the wavefront errors from low altitude turbulence. Here we present GLAO modeling results from a feasibility study performed for the Gemini Observatory. Using five separate analytic and Monte Carlo models to provide simulations over the large available parameter space, we have completed a number of trade studies exploring the impact of changing field of view, number and geometry of laser guide stars, DM conjugate altitude and DM actuator density on the GLAO performance measured over a range of scientific wavelengths and turbulence profiles.
Poster Session: Innovative Wavefront Sensing Concepts
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Multiple guide star tomography demonstration at Palomar Observatory
We have built and field tested a multiple guide star tomograph with four Shack-Hartmann wavefront sensors. We predict the wavefront on the fourth sensor channel estimated using wavefront information from the other three channels using synchronously recorded data. This system helps in the design of wavefront sensors for future extremely large telescopes that will use multi conjugate adaptive optics and multi object adaptive optics. Different wavefront prediction algorithms are being tested with the data obtained. We describe the system, its current capabilities and some preliminary results.
Interferometric wavefront sensors for extreme adaptive optics on the Thirty Meter Telescope
Direct observation of extrasolar Jovian planets will enable detailed investigation and understanding of the formation of these planet populations and also of their relative abundance. Future large telescopes, such as the Thirty Meter Telescope(TMT), will enable the study of such planet populations at relatively small working distances from the parent star. We present an analysis of an extreme adaptive optics system utilizing a self-referencing phase-shifting interferometer as the primary wave-front sensor. A module of the adaptive optics system consists of a conventional Shack-Hartmann wave-front sensor to provide the initial start-up of the adaptive optics system, thereby placing a significant amount of energy into the core of the point spread function which will act as the reference for the primary interferometric wave-front sensor. The interferometric-based wave-front sensor is shown to provide a significant improvement in the achievable contrast ratio compared with conventional adaptive optics systems containing Shack-Hartmann wave-front sensors.
Eliminating perspective elongation for LGS based AO systems at ELTs
Stephan Kellner, Roberto Ragazzoni, Emiliano Diolaiti, et al.
Laser Guide Star Adaptive Optics systems become in the moment reality at several 8...10m class telescope facilities. At these aperture diameters all effects, arising from the finite distance and vertical extend of the artificial excited guide star can still be neglected for the wavefront sensing process. This is changing completely when the telescope diameter become further increased as it is the case of next generation Extremely Large Telescopes. Effects such as perspective elongation become dominant and impact negatively the wavefront sensing process. This will result in severe quality restrictions for next generation Adaptive Optics systems. In this paper we want to introduce and explain in detail a novel kind of wavefront sensing techniques, optimized to overcome perspective elongation. Its basic idea is to project the generated LGS virtually to infinity. We will give a theoretical introduction and some background information about the proposed sensing concept and introduce a practical solution for a potential optical setup. The latter is based on inverting the so called Bessel Beam concept, producing diffraction less beam over a certain finite range. Furthermore we will characterize the new sensor type, compare the senor efficiency to alternative approaches and discuss finally both its advantages and disadvantages.
Poster Session: Calibration and Control Topics
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Implementation of modal optimization system of Subaru-188 adaptive optics
Masayuki Hattori, Taras Golota, Guyon Olivier, et al.
Subaru AO-188 is a curvature adaptive optics system with 188 elements. It has been developed by NAOJ (National Astronomical Observatory of Japan) in recent years, as the upgrade from the existing 36-element AO system currently in operation at Subaru telescope. In this upgrade, the control scheme is also changed from zonal control to modal control. This paper presents development and implementation of the modal optimization system for this new AO-188. Also, we will introduce some special features and attempt in our implementation, such as consideration of resonance of deformable mirror at the lower order modes, and extension of the scheme for the optimization of the magnitude of membrane mirror in wave front sensor. Those are simple but shall be useful enhancement for the better performance to the conservative configuration with conventional modal control, and possibly useful in other extended operation modes or control schemes recently in research and development as well.
LQ control design for adaptive optics systems based on MIMO identified model
In this paper, a novel procedure to design optimal controllers for adaptive optics systems is proposed. The most important feature of this procedure is that it does not require a complex and long tuning procedure like the standard PI controllers and that it is based on real data, i.e. does not require an a priori model for the plant. For this reason, we use system identification methods which, starting from real input-output time series, are able to build a discrete-time equivalent model for the whole plant. Since the plant is Multi-Input Multi-Output (MIMO), the identification methods are the one referred in the literature as Subspace Identification Methods (SID). They provide a state-space model with a deterministic part and a stochastic part. The first one is, in our case, composed by the cascade of the discrete-time approximation of the transfer function of all subsystems, whereas the latter takes into account the measurement errors and, more important, the atmospheric turbulence. In particular the stochastic part does not provide the statistics of the atmosphere, but explains how the wave-front sensor sees the turbulence. The optimal linear-quadratic controller is based on the deterministic part of the identified model and minimizes the variance of the wave-front phase error. Of course, minimizing the error variance is the same as to maximizing the Strehl ratio. Our approach is an extension of the SISO-class controllers, based on the interaction matrix/control matrix, to the dynamical MIMO-class case. In this manner, the dynamical interaction between actuators and sub-apertures is not lost.
Modified correlation tracker algorithm for tip-tilt correction system and project ANGARA on the Big Solar Vacuum Telescope
Vladimir P. Lukin, Leonid V. Antoshkin, Nina N. Botygina, et al.
The possibility of applying adaptive correction to ground-based solar astronomy is considered. Several experimental systems for image stabilization are described along with the results of their tests. As a result of the installation of the first order adaptive-optics system, the Big Solar Vacuum Telescope (BSVT) acquired the new quality. Different ways of development of an adaptive correction to be used in the BSVT of the Baikal Astrophysical Observatory are discussed.