Adaptive Optics Research Papers - Academia.edu (original) (raw)

An innovative concept of wavefront sensing for Rayleigh beacons is introduced along with an example of a possible wavefront sensor. This new approach does not require the gating technique to limit the useful range of the laser source and therefore looks simpler to implement than previous Rayleigh concepts, and may additionally allow more efficient use of the photons emitted by the Rayleigh beacon. Our technique is based upon an optical element in the focal plane area whose section does not change for the conjugation of different ranges from the telescope aperture, hence the name z-invariant. The wavefront sensor shown here is an example of this new class. It is a compact pupil-plane wavefront sensor and as such allows for a layer-oriented configuration. It is shown that its sensitivity, while higher than usual gating approaches, is far from the possible limits leading us to speculate that other z-invariant wavefront sensors can reach much larger efficiencies.

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- Adaptive Optics

The availability and performance of laser-based adaptive optics (AO) systems are strongly dependent on the power and quality of the laser beam before being projected to the sky. Frequent and time-consuming alignment procedures are usually required in the laser systems with free-space optics to optimize the beam. Despite these procedures, significant distortions of the laser beam have been observed during the first two years of operation of the Gemini South multi-conjugate adaptive optics system (GeMS). A beam shaping concept with two deformable mirrors is investigated in order to provide automated optimization of the laser quality for astronomical AO. This study aims at demonstrating the correction of quasi-static aberrations of the laser, in both amplitude and phase, testing a prototype of this two-deformable mirror concept on GeMS. The paper presents the results of the preparatory study before the experimental phase. An algorithm to control amplitude and phase correction, based on phase retrieval techniques, is presented with a novel unwrapping method. Its performance is assessed via numerical simulations, using aberrations measured at GeMS as reference. The results predict effective amplitude and phase correction of the laser distortions with about 120 actuators per mirror and a separation of 1.4 m between the mirrors. The spot size is estimated to be reduced by up to 15% thanks to the correction. In terms of AO noise level, this has the same benefit as increasing the photon flux by 40%.

- by Clementine Bechet
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- Inverse Problems, Adaptive Optics, Free Space Optics, Phase Retrieval

A Zernike decomposition of the pupil phase variance that is due to focus anisoplanatism for a sodium laser guide star is calculated, including the angular error variance associated with overall tilt. This quantity, or tilt focus anisoplanatism FA,tt 2 , is necessary in evaluating the usefulness of new techniques developed to measure the wave-front tilt from a sodium laser guide star. The analytical expression of FA,tt 2 in the case of refractiveindex fluctuation with a Kolmogorov power spectral density is given together with numerical results for two different C n Ј 2 (z) profiles. Numerical calculations show that tilt focus anisoplanatism may be a limitation when a sodium laser guide star is used to sense the overall wave-front tilt at visible wavelengths, especially for 8-m-class telescopes.

A joint sampling-time error and channel skew background calibration technique for time interleaved analog to digital converters (TI-ADC) is presented. The technique is aimed at applications in dual-polarization QPSK/QAM receivers for coherent optical communications at high data rates (e.g., 40Gb/s and beyond). Unlike previous proposals, the calibration algorithm introduced here is used to jointly compensate for samplingtime and channel skew errors. Estimates of the gradient of the mean squared error (MSE) or the bit error rate (BER) with respect to the sampling phases of the different signal lanes and interleaves are computed and used to iteratively minimize a cost function (i.e., MSE or BER). Computer simulations demonstrate the excellent behavior of the proposed compensation technique. The calibration algorithm can be implemented with minimal hardware requirements and with a slow clock. This allows power dissipation in a CMOS VLSI implementation to be minimized.

- by Benjamin Reyes
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- Computer Science, VLSI, Adaptive Optics, Calibration

The Heterodyne I w e n t (HIPI) for the Far-IniWed and Sub-millimeter Telescope (FIRST) requires local oscillators well into the terah-fiquency ran@. The mechanism to realize the local oscillators will involve synlhesbers, active multiplia chains (AMC's) with output fkquencies from 71-112.5 GIXZ, power amplifiers ta ampllfy the AMC signals, and chains of Schottky diode multipliers to aclum tenhertz frequencies. We will present the htest state-of-theeaxt results on 70-1 15 GI& Monolithk Mibxter-wave Integrated Circuit ("IC) power amptifief t.echolagy.

- by Yvon Renotte
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- X Ray Astronomy, Dark Matter, X ray absorption spectroscopy, Adaptive Optics

The manufacturing of toric mirrors for the Very Large Telescope-Spectro-Polarimetric High-Contrast Exoplanet Research instrument (SPHERE) is based on Active Optics and stress polishing. This figuring technique allows minimizing mid and high spatial frequency errors on an aspherical surface by using spherical polishing with full size tools. In order to reach the tight precision required, the manufacturing error budget is described to optimize each parameter. Analytical calculations based on elasticity theory and finite element analysis lead to the mechanical design of the Zerodur blank to be warped during the stress polishing phase. Results on the larger (366 mm diameter) toric mirror are evaluated by interferometry. We obtain, as expected, a toric surface within specification at low, middle, and high spatial frequencies ranges.

- by Marc Ferrari
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- Mechanical Engineering, Adaptive Optics, Applied Optics, Optical physics

We present simulation results for 192 Gb/s (16 ×12) OTDM transmission with different time delays over standard fiber G.652. The optical 192 Gb/s to 12 Gb/s demultiplexing (OTDM) is used and the results compared with 160 Gb/s to 10 Gb/s OTDM demultiplexing. The observations show the feasibility of an error free 192 Gb/s OTDM transmission with a minimum time delay of 1 ps which yields BER as low as 3.38E-89 while the minimum BER obtained at 1 ps time delay for 160 Gb/s OTDM is 1.21E-244, which is practically error free.

- by yugnanda malhotra
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- Optical Switch, Adaptive Optics, Time Delay, Bit Error Rate

We discuss the effect of atmospheric dispersion on the performance of a mid-infrared adaptive optics assisted instrument on an extremely large telescope (ELT). Dispersion and atmospheric chromaticity is generally considered to be negligible in this wavelength regime. It is shown here, however, that with the much-reduced diffraction limit size on an ELT and the need for diffraction-limited performance, refractivity phenomena should be carefully considered in the design and operation of such an instrument. We include an overview of the theory of refractivity, and the influence of infrared resonances caused by the presence of water vapour and other constituents in the atmosphere. 'Traditional' atmospheric dispersion is likely to cause a loss of Strehl only at the shortest wavelengths (L-band). A more likely source of error is the difference in wavelengths at which the wavefront is sensed and corrected, leading to pointing offsets between wavefront sensor and science instrument that evolve with time over a long exposure. Infrared radiation is also subject to additional turbulence caused by the presence of water vapour in the atmosphere not seen by visible wavefront sensors, whose effect is poorly understood. We make use of information obtained at radio wavelengths to make a first-order estimate of its effect on the performance of a mid-IR ground-based instrument. The calculations in this paper are performed using parameters from two different sites, one 'standard good site' and one 'high and dry site' to illustrate the importance of the choice of site for an ELT.

- by Stefan Hippler
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- Water, Turbulence, Adaptive Optics, Dispersion

In vivo imaging of the retina on humans by means of adaptive optics can lead to a significant gain in resolution. We demonstrate the realization and use of a system made of a Shack-Hartmann wavefront sensor carefully matched to a 13-actuator bimorph deformable mirror sensor, operating at a closed loop frequency of 70 Hz at k ¼ 835 nm. Even with this simple but optimized system with 12 degrees of freedom, correcting only aberrations of moderate orders, we routinely and systematically obtain retinal images containing spatial information up to half the diffraction limit frequency of a dilated (7 mm) iris at a k ¼ 550 nm wavelength (1.6 lm diffraction spot size). Signal-to-noise ratio on the images is limited by eye safety constraints, but is sufficient to reach the high-frequency information on single, shortexposure (7 ms) images, which clearly show individual cones and capillary details. Correction is highly depending on proper centering of the eye, achieved with an active target. Focusing through the retina is possible with a reduced depth of focus. Variability of moderate order aberrations among dilated subjects has been observed. Using an image fitting algorithm, individual images are used to build a wider field corrected image of the retina ð%3°), possibly useful for diagnosis and microcirculation analysis.

- by D. Lafaille
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- Adaptive Optics, High Frequency, Spatial Information, In Vivo Imaging

We describe the performance of a bulk micromachined deformable mirror coated with a dielectric stack for adaptive optics applications with high power lasers. A reflectance of greater than 99.9% was measured and the mirror had a residual static aberration of less than 90 rim mis primarily in astigmatism. A thermally induced distortion of 71 nm R]VIS was observed for an incident intensity of 300 W/cm2 and an average power of 57W. The multi-layer coated deformable mirror survived over half a billion cycles without degradation and survived for 30 hours with 36 W cw 1064 nm laser light. In addition, the thermally induced distortion with 22 W of average laser power (350 W/cm2) was reduced from 88 nmto 31 nmrms.

- by Supriyo Sinha
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- Adaptive Optics, Distortion, Gravity Waves, High Power

Visible Light Laser Guidestar Experiments (ViLLaGEs) is a new Micro-Electro Mechanical Systems (MEMS) based visible-wavelength adaptive optics (AO) testbed on the Nickel 1-meter telescope at Lick Observatory. Closed loop Natural Guide Star (NGS) experiments were successfully carried out during engineering during the fall of 2007. This is a major evolutionary step, signaling the movement of AO technologies into visible light with a MEMS mirror. With on-sky Strehls in I-band of greater than 20% during second light tests, the science possibilities have become evident. Described here is the advanced engineering used in the design and construction of the ViLLaGEs system, comparing it to the LickAO infrared system, and a discussion of Nickel dome infrastructural improvements necessary for this system. A significant portion of the engineering discussion revolves around the sizable effort that went towards eliminating flexure. Then, we detail upgrades to ViLLaGEs to make it a facility class instrument. These upgrades will focus on Nyquist sampling the diffraction limited point spread function during open loop operations, motorization and automation for technician level alignments, adding dithering capabilities and changes for near infrared science.

- by Daren Dillon
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- Engineering, Mechanism Design, Microelectromechanical systems, Near Infrared

In this paper, we show how the segmentation of an image into superpixels may be used as preprocessing paradigm to improve the accuracy of the optical flow estimation in an image sequence. Superpixels play the role of accurate support masks for the integration of the optical flow equation. We employ a variation of a recently proposed optical flow algorithm relying on local image properties that are taken into account only if the involved pixels belong to the same image segment. Experimental results show that the proposed optical flow estimation scheme significantly improves the accuracy of the estimated motion field with respect to other standard methods.

- by Theodosios Gkamas and +1
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- Computer Vision, Image segmentation, Adaptive Optics, Optical Imaging

The CAFADIS camera project has consisted in building a camera to measure wave-front phases and distances under different scenarios (from microns to kilometres), using highly specialised electronic technology, namely Graphics Processing Units (GPUs) and Field Programmable Gate Arrays (FPGAs). It is a passive method of depth extraction, it uses incoherent light (natural light). In this paper we will present our new developments.

- by Stanislav Gordeyev
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- Adaptive Optics

Multiconjugate adaptive optics ͑MCAO͒ systems with 10 4-10 5 degrees of freedom have been proposed for future giant telescopes. Using standard matrix methods to compute, optimize, and implement wavefront control algorithms for these systems is impractical, since the number of calculations required to compute and apply the reconstruction matrix scales respectively with the cube and the square of the number of adaptive optics degrees of freedom. We develop scalable open-loop iterative sparse matrix implementations of minimum variance wave-front reconstruction for telescope diameters up to 32 m with more than 10 4 actuators. The basic approach is the preconditioned conjugate gradient method with an efficient preconditioner, whose block structure is defined by the atmospheric turbulent layers very much like the layer-oriented MCAO algorithms of current interest. Two cost-effective preconditioners are investigated: a multigrid solver and a simpler block symmetric Gauss-Seidel ͑BSGS͒ sweep. Both options require off-line sparse Cholesky factorizations of the diagonal blocks of the matrix system. The cost to precompute these factors scales approximately as the three-halves power of the number of estimated phase grid points per atmospheric layer, and their average update rate is typically of the order of 10 Ϫ2 Hz, i.e., 4-5 orders of magnitude lower than the typical 10 3 Hz temporal sampling rate. All other computations scale almost linearly with the total number of estimated phase grid points. We present numerical simulation results to illustrate algorithm convergence. Convergence rates of both preconditioners are similar, regardless of measurement noise level, indicating that the layer-oriented BSGS sweep is as effective as the more elaborated multiresolution preconditioner.

- by Luc Gilles
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- Mechanical Engineering, Adaptive Optics, Numerical Simulation, Image Reconstruction

Optical wave-front propagation in a layered model for the atmosphere is analyzed by the use of diffraction theory, leading to a novel approach for utilizing artificial guide stars. Considering recent observations of layering in the atmospheric turbulence, the results of this paper indicate that, even for very large telescopes, a substantial enlargement of the compensated angular field of view is possible when two adaptive mirrors and four or five artificial guide stars are employed. The required number of guide stars increases as the thickness of the turbulent layers increases, converging to the conventional results at the limit of continuously turbulent atmosphere.

- by Joseph Shamir
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- Mechanical Engineering, Optics, Medical Imaging, Fiber Optics

Continuous-facesheet and segmented Boston Micromachines Corporations' (BMC) Micro-Electrical Mechanical Systems (MEMS) Deformable Mirrors (DM) have been tested for their response to high-power visible-wavelength laser light. The deformable mirrors, coated with either protected silver or bare aluminum, were subjected to a maximum of 2 Watt laser-light at a wavelength of 532 nanometers. The laser light was incident on a ~ 3.5×3.5 cm area for time periods from minutes to 7 continuous hours. Spot heating from the laser-light is measured to induce a local bulge in the surface of each DM. For the aluminum-coated continuous facesheet DM, the induced spot heating changes the surface figure by 16 nm rms. The silver-coated continuous-facesheet and segmented (spatial light modulator) DMs experience a 6 and 8 nm surface rms change in surface quality with the laser at 2 Watts. For spatial frequencies less than the actuator spacing (300 mm), the laser induced surface bulge is shown to be rem...

- by Daren Dillon
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- Engineering, Materials Science, Microelectromechanical systems, Adaptive Optics

In free-space optical communication, the propagation of a laser beam through the atmosphere causes wavefront distortions that decrease the coupling efficiency (CE) from free space to single-mode fiber. This tremendously degrades the performance of the communication channel even in the case of weak turbulence regime. In this Letter, we demonstrate that a multi-actuator adaptive lens working in closed loop with a wavefront sensor can strongly reduce the effect of turbulence while reducing the system complexity with respect to correction systems using deformable mirrors or liquid crystal spatial light modulators. We obtain a three-fold increase in the CE in weak turbulence regime.

- by Hossein Saghafifar
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- Optics, Quantum Physics, Optical Fiber Communications, Adaptive Optics

Knowledge-based Systems and Expert Systems, in particular, are expensive to build and difficult to validate and debug because of their complexity and dynamism. Therefore, it is not easy for knowledge engineer and domain expert to identify the gaps and mistakes in knowledge base. Unit testing is unable to cover validation process at all stages, in many cases manual thorough review of decision process is needed. In this paper we spot main approaches to validation and verification issue and describe a component that helps to debug a knowledge base by visualising execution of rules that derive a particular result. This component is developed for Knowledge-based Systems built on Drools Platform 1 and we demonstrate application of this component in a knowledge-based engineering system for structural optical design.

- by Maxim Kolchin
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- Expert Systems, Adaptive Optics, Optical Imaging, Data Visualisation
- by ali rezania
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- Distributed Computing, Optical Switch, Adaptive Optics, Optical network

During the last two decades, optical stellar interferometry has become an important tool in astronomical investigations requiring spatial resolution well beyond that of traditional telescopes. This book, first published in 2006, was the first to be written on the subject. The authors provide an extended introduction discussing basic physical and atmospheric optics, which establishes the framework necessary to present the ideas and practice of interferometry as applied to the astronomical scene. They follow with an overview of historical, operational and planned interferometric observatories, and a selection of important astrophysical discoveries made with them. Finally, they present some as-yet untested ideas for instruments both on the ground and in space which may allow us to image details of planetary systems beyond our own.

- by S. Lipson
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- Algorithms, Crystallography, Adaptive Optics, Astrometry

Free-space laser communication has been demonstrated with application potential in many areas such as line-of-sight communications, satellite communications and the last mile solution in a fiber optics networking. Both 0.8 and 1.5 micron wavelengths are currently used in state-of-the-art free space laser communication systems; unfortunately the system performance is imposed by atmospheric turbulence. To reduce the atmospheric effect in free-space laser communication systems, several techniques have been used, such as adaptive optics, aperture averaging and multiple transmitters; however, significant improvement has not been achieved. Theoretically, the seeing effect may be released using a longer wavelength. In this paper, we present a 3.5 micron free-space laser communication system model and its system performance evaluation. A 3.5 micron propagation model based on MODTRAN simulation results in different weather patterns is presented first, and a propagation link budget system model is described after that. The propagation channel performance evaluation results are presented by means of bit error rate versus various propagation distances.

- by Edward Burlbaw
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- Engineering, Physics, Fiber Optics, Satellite Communications

This paper presents an approach to designing narrowband digital filters that are realizable using optical allpass building blocks. We describe a top-down design method by explicitly examining the derivation of an Infinite Impulse Response (IIR) architecture. Our result demonstrates a design that can achieve a 0.0025π passband edge while providing 60dB stopband attenuation. The design is aimed to reduce filter pole magnitudes, providing tolerance for waveguide losses and fabrication errors. The narrowband filter is based on the foundation of latticed allpass sections, which makes it naturally realizable using basic photonic components. Furthermore, analysis is performed on delay length variations that can result from the fabrication process.

- by truong nguyen
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- Mathematics, Computer Science, Photonics, Adaptive Optics

For the first time in history, astronomers can image the environments of nearby stars on scales approaching that of our solar system. New classes of astrophysical objects have been discovered including circumstellar debris disks, brown dwarfs, and super-Jupiter mass planets. These discoveries have galvanized intense public interest and have led to profound new insights into the formation and evolution of planetary systems such as our own. Among the key enabling technologies are adaptive optics (AO) and coronagraphy, which deliver the high contrast necessary for the discovery and characterization of faint stellar companions and circumstellar disks in the solar neighborhood. Among the AO systems available to astronomers, AEOS is unique because it delivers very high order wave front correction. The Lyot Project includes the construction and installation of an AEOS-optimized coronagraph that exploits the full astronomical potential of AEOS and represents a critical step toward the long-term goal of directly imaging and studying extrasolar planets (a.k.a. ``exoplanets''). We provide an update on the Project, whose coronagraph will be installed at AEOS in October 2003, and whose infrared camera saw first light in April 2003.

- by Anand Sivaramakrishnan
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- Adaptive Optics, Solar System, Infrared, Public Interest
- by Katherine Creath
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- Optics, Astrophysics, Adaptive Optics, Text

We introduce adaptive optics as a technique to improve images taken by ground-based telescopes through a turbulent blurring atmosphere. Adaptive optics rapidly senses the wavefront distortion referenced to either a natural or laser guidestar, and then applies an equal but opposite profile to an adaptive mirror. In this paper, we summarize the application of neural networks in adaptive optics. First, we report previous work on employing multi-layer perceptron neural networks and back-propagation to learn how to sense and reconstruct the wavefront. Second, we show how neural networks can be used to predict the wavefront, and compare the neural networks’ predictive power in the presence of noise to that of linear networks also trained with back-propagation. In our simulations, we find that the linear network predictors train faster, they have lower residual phase variance, and they are much more tolerant to noise than the non-linear neural network predictors, though both offer improvement over no prediction. We conclude with comments on how neural networks may evolve over the next few years as adaptive optics becomes a more routine tool on the new large astronomical telescopes.

- by Patrick McGuire
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- Back Propagation, Neural Network, Adaptive Optics, Scientific applications

A compact system for counting and time-tagging single photons is presented, based on a monolithic array sensor of 60 pixels able to detect single photons, namely the single-photon avalanche diode array (SPADA). First, the working principle and performance of the single-photon detector pixel is detailed, with particular attention paid to monolithic array integration. Then the electronics needed to quench each pixel after avalanche ignition, namely the active-quenching circuit (AQC) is discussed, since the features of this quenching electronics dramatically affect the operating conditions of the detector, hence its actual performance. The discussion then focuses on integration of the SPADA system into Astrophysics applications such as adaptive optics, fast-transient imaging and atmospheric layer sensing. The whole electronics necessary to control SPADA operating conditions and temperature is also described, together with the complete opto-mechanics used to focus the telescope pupil onto the detector. Finally, experimental results are reported.

- by Federico Baronti
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- Quantum Physics, Nanotechnology, Adaptive Optics, Modern

The proposed Extremely Large Telescopes (ELTs), including Giant Magellan Telescope (GMT), are limited by optical distortions introduced by the Earth's atmosphere. These distortions will be partially compensated by the use of Adaptive Optics (AO), optomechanical systems that can measure the incoming wavefronts of light from celestial objects and rapidly apply an appropriate optical correction using active optical elements, typically deformable mirrors. In this work, we will report the results of the characterization at the focal plane of wavefront images given by different Zernike modes.

- by Marcelo Leigui and +1
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- Adaptive Optics

Orbital debris in low Earth orbit (LEO) are now sufficiently dense that the use of LEO space is threatened by runaway collisional cascading. A problem predicted more than thirty years ago, the threat from debris larger than about 1 cm demands serious attention. A promising proposed solution uses a high power pulsed laser system on the Earth to make plasma jets on the objects, slowing them slightly, and causing them to re-enter and burn up in the atmosphere. In this paper, we reassess this approach in light of recent advances in low-cost, light-weight modular design for large mirrors, calculations of laser-induced orbit changes and in design of repetitive, multi-kilojoule lasers, that build on inertial fusion research. These advances now suggest that laser orbital debris removal (LODR) is the most cost-effective way to mitigate the debris problem. No other solutions have been proposed that address the whole problem of large and small debris. A LODR system will have multiple uses beyond debris removal. International cooperation will be essential for building and operating such a system.

- by bogdan marcovici
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- Mechanical Engineering, Aerospace Engineering, International Cooperation, Space
- by David Pepper
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- Image Processing, Adaptive Optics, Wave propagation, Associative Memory

High resolution wavefront sensors are devices with a great practical interest since they are becoming a key part in an increasing number of applications like extreme Adaptive Optics. We describe the optical differentiation wavefront sensor, consisting of an amplitude mask placed at the intermediate focal plane of a 4-f setup. This sensor offers the advantages of high resolution and adjustable dynamic range. Furthermore, it can work with polychromatic light sources. In this paper we show that, even in adverse low-light-level conditions, its SNR compares quite well to that corresponding to the Hartmann-Shack sensor.

- by Manuel Cagigal
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- Adaptive Optics, Optical physics, High Resolution, Low Light

Radar and optical remote sensing data are used in a unified algorithm to estimate forest variables. The study site is the H. J. Andrews experimental forest in Oregon, which has significant topography and several mature and old-growth conifer stands with biomass values sometimes exceeding 1000 tons/ha. Polarimetric multifrequency Airborne Synthetic Aperture Radar (AIRSAR) backscatter, interferometric C-band Topographic Synthetic Aperture Radar (TOPSAR) coherence, and multispectral Landsat Thematic Mapper (TM) digital numbers are used in a regression analysis that relates them to forest variable measurements on the ground. Parametric expressions are derived and used to estimate the same variables(s) at other locations from the combination of AIRSAR and TM data. It is shown that the estimation accuracy is significantly improved when the radar and optical data are used in combination compared to estimating the same variable from a single data type alone.

- by Steven Acker
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- Geophysics, Algorithms, Forestry, Remote Sensing

Adaptive optics (AO) systems are used to increase the spatial resolution achieved by ground-based telescopes, which are limited by the atmospheric motion of air layers above them. Therefore, the real cut-off frequency is extended closer to the theoretical diffraction limit of the telescope thus allowing more high-frequency information from the object to be present in the image. Nevertheless, although the goal of image reconstruction and deconvolution algorithms is basically the same (i.e., to recover a “real” diffracted limit image, free of noise, from the object), and since the correction of AO is not complete (i.e., the effective cut-off frequency achieved by AO is still below the theoretical diffraction limit), the simultaneous use of such deconvolution algorithms over dataset acquired with AO is possible and desirable to further enhance their contrast. On the other hand, multiresolution tools like the wavelet transform (WT) have been historically introduced into multiple deconvo...

- by Roberto Gallé
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- Geography, Adaptive Optics, Deconvolution

A 32 × 32 microelectricalmechanical systems mirror is controlled in a closed-loop adaptive optics test bed with a spatially filtered wavefront sensor (WFS), Fourier transform wavefront reconstruction, and calibration of references with a high-precision interferometer. When correcting the inherent aberration of the mirror, 0:7 nm rms phase error in the controllable band is achieved. When correcting an etched phase plate with atmospheric statistics, a dark hole 10 3 deeper than the uncontrollable phase is produced in the phase power spectral density. Compensation of the mirror's influence function is done with a Fourier filter, which results in improved loop convergence. Use of the spatial filter is shown to reduce the gain variability of the WFS in a quadcell configuration.

- by Sandrine Thomas
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- Mechanical Engineering, Adaptive Optics, Applied Optics, Optical physics

This paper presents a self-calibration technique for the removal of measurement errors caused by thermal gradients in thermopile-based infrared thermometry, particularly when measuring low temperatures. Applications for this self-calibration method include low-temperature measurement in the food industry and infrared thermometers for remote temperature monitoring in cold climates. The self-calibration technique reported in this paper is shown to reduce the measurement error to within ±1 • C within 5 s of an extreme thermal shock, compared with an uncompensated thermometer that does not recover until the thermal gradient is removed. The root-mean-square temperature noise for the duration of the thermal shock test is less than 0.2 • C. This technique is the subject of a patent application and can be applied to any infrared thermometer utilizing a thermopile, regardless of the thermopile size and geometry.

- by Gary Fuller and +1
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- Adaptive Optics, Measurement Error, Root-Mean Square Error, Temperature measurement

Laser guide star adaptive optics and interferometry are currently revolutionizing ground-based near-IR astronomy, as demonstrated at various large telescopes. The Large Binocular Telescope from the beginning included adaptive optics in the telescope design. With the deformable secondary mirrors and a suite of instruments taking advantage of the AO capabilities, the LBT will play an important role in addressing major scientific questions. Extending from a natural guide star based system, towards a laser guide stars will multiply the number of targets that can be observed. In this paper we present the laser guide star and wavefront sensor program as currently being planned for the LBT. This program will provide a multi Rayleigh guide star constellation for wide field ground layer correction taking advantage of the multi object spectrograph and imager LUCIFER in a first step. The already foreseen upgrade path will deliver an on axis diffraction limited mode with LGS AO based on tomography or additional sodium guide stars to even further enhance the scientific use of the LBT including the interferometric capabilities.

- by Guido Brusa
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- Adaptive Optics

Maximum-likelihood (ML) estimation in wavefront sensing requires careful attention to all noise sources and all factors that influence the sensor data. We present detailed probability density functions for the output of the image detector in a wavefront sensor, conditional not only on wavefront parameters but also on various nuisance parameters. Practical ways of dealing with nuisance parameters are described, and final expressions for likelihoods and Fisher information matrices are derived. The theory is illustrated by discussing Shack-Hartmann sensors, and computational requirements are discussed. Simulation results show that ML estimation can significantly increase the dynamic range of a Shack-Hartmann sensor with four detectors and that it can reduce the residual wavefront error when compared with traditional methods.

- by David Lara
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- Adaptive Optics, Probability, Stochastic processes, Software

Conventional adaptive optics systems using a single wavefront corrector suffer from a limited field of view. Multi-conjugate adaptive optics use two or more corrector to improve off-axis correction. We describe an experimental system which simulates dual-layer turbulence, and present results using a single corrector showing anisoplanatic effects. Future experiments using a second corrector are also discussed.

- by Colin Dunlop
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- Adaptive Optics, Field of View

CSP Ganged Heliostat Technologies
Investigations in a Tensile Based Non-imaging System
Abstract:
Background
Concentrating Solar Power (CSP) and specifically Power Tower or Beam Down type systems achieve high levels of solar concentration and efficiency. Collecting fields comprise a large fraction of the system installation and maintenance costs. Technological advances promise economically competitive solar power. Skysun, LLC proposes a ganged heliostat to significantly reduce these costs.
Method
Typically, a heliostat requires one each of the following: mirror module, support structure, dual axis drive, post/pedestal and foundation. Each of Skysun, LLC’s heliostats require: mirror module, reduced support structure and a single axis drive, eliminating the need for a pedestal and foundation for each heliostat. The ganged heliostat consists of two cables supporting a plurality of single-axis actuated heliostats. The cables act both as a supporting structure and as a translator of focusing motions to the many heliostats. The cables terminate to an actuated rotational member supported by a substantial post. Cable tension may be variable. This configuration reduces the ratio of posts and foundations to heliostats, and eliminates dual axis drive actuators, substituting single axis actuators instead. The ganged heliostat may be rotated to the vertical for ease of robotic cleaning and water reclamation. The ganged heliostat may also be inverted, with the reflective surface downward, to protect against weather events such as hail. Finally, the ganged heliostat may be secured to protect against high wind conditions.
The reflective surface, which can be deformed by cable and heliostat orientation, provides an efficient means to form a large concave collecting surface laying principally in the horizontal. Reflected incident rays, being non-normal, suffer from astigmatism. Novel deformations of the reflective surface eliminate astigmatic aberration. A toric - shaped deformation of the reflective surface reduces the size of a chosen astigmatic focus, yielding higher concentration. Latitudinal and longitudinal deformations maintain focus upon a fixed receiver. In the ideal, the astigmatic focus is reduced to a point.
Conclusion
The goal of this paper is to outline the relatively inexpensive methods utilized by Skysun’s ganged heliostat prototype and how the methodology may be scaled up. Skysun, LLC proposes a ganged heliostat to significantly reduce collecting field costs to $75/m2 installed.
Key words: solar concentrating CSP Power Tower Beam Down heliostat astigmatism deformation ganged

- by Jim Clair
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- Photonics, Adaptive Optics, Solar trackers and concentrators, CSP

We present a novel catadioptric-stereo rig consisting of a coaxially-aligned perspective camera and two spherical mirrors with distinct radii in a "folded" configuration. We recover a nearly-spherical dense depth panorama (360 • x153 • ) by fusing depth from optical flow and stereo. We observe that for motion in a horizontal plane, optical flow and stereo generate nearly complementary distributions of depth resolution. While optical flow provides strong depth cues in the periphery and near the poles of the view-sphere, stereo generates reliable depth in a narrow band about the equator. We exploit this principle by modeling the depth resolution of optical flow and stereo in order to fuse them probabilistically in a spherical panorama. To aid the designer in achieving a desired field-of-view and resolution, we derive a linearized model of the rig in terms of three parameters (radii of the two mirrors plus axial separation from their centers). We analyze the error due to the violation of the Single Viewpoint (SVP) constraint and formulate additional constraints on the design to minimize the error. Performance is evaluated through simulation and with a real prototype by computing dense spherical panoramas in cluttered indoor settings.

- by Carlos A Jaramillo
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- Adaptive Optics, Optical Imaging, Linear Model, Optical Flow

This paper presents results from a high spatial resolution survey of 33 main-belt asteroids with diameters >40 km using the Keck II Adaptive Optics (AO) facility. Five of these () were confirmed to have satellite. Assuming the same albedo as the primary, these moonlets are relatively small (∼5% of the primary size) suggesting that they are fragments captured after a disruptive collision of a parent body or captured ejecta due to an impact. For each asteroid, we have estimated the minimum size of a moonlet that can positively detected within the Hill sphere of the system by estimating and modeling a 2-σ detection profile: in average on the data set, a moonlet located at 2/100 × R Hill (1/4 × R Hill ) with a diameter larger than 6 km (4 km) would have been unambiguously seen. The apparent size and shape of each asteroid was estimated after deconvolution using a new algorithm called AIDA. The mean diameter for the majority of asteroids is in good agreement with IRAS radiometric measurements, though for asteroids with a D < 200 km, it is underestimated on average by 6-8%. Most asteroids had a size ratio that was very close to those determined by lightcurve measurements. One observation of 104 Klymene suggests it has a bifurcated shape. The bi-lobed shape of 121 Hermione described in Marchis et al. [Marchis, F., Hestroffer, D., Descamps, P., Berthier, J., Laver, C., de Pater, I., 2005c. Icarus 178, 450-464] was confirmed after deconvolution. The ratio of contact binaries in our survey, which is limited to asteroids larger than 40 km, is surprisingly high (∼6%), suggesting that a non-single configuration is common in the main-belt. Several asteroids have been analyzed with lightcurve inversions. We compared lightcurve inversion models for plane-of-sky predictions with the observed images (9 Metis, 52 Europa, 87 Sylvia, 130 Elektra, 192 Nausikaa, and 423 Diotima, 511 Davida). The AO images allowed us to determine a unique photometric mirror pole solution, which is normally ambiguous for asteroids moving close to the plane of the ecliptic (e.g., 192 Nausikaa and 52 Europa). The photometric inversion models agree well with the AO images, thus confirming the validity of both the lightcurve inversion method and the AO image reduction technique.

- by Erik Hom
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- Geochemistry, Geophysics, Image Processing, Adaptive Optics

Wavelength-division multiplexing passive optical access network (WDM-PON) has been considered as a promising next generation access network solution. We demonstrate and analyze the transmission performances and power budget issues of full duplex single feeder-fiber WDM-PON scheme. A 10-Gb/s differential quadrature phase shift keying(DQPSK) downstream and remodulated ON/OFF keying(OOK) upstream data signals are transmitted over 25 km standard single mode fiber. Simulation results show error free transmission having adequate power margins in both downstream and upstream transmission, which prove the applicability of the proposed scheme in future passive optical access networks. The power budget confines both the PON splitting ratio and the distance between the Optical Line Terminal (OLT) and Optical network Unit (ONU). Index Terms-Wavelength division multiplexing, Passive optical access network, Power budget, Optical line terminal, Optical network unit.

We experimentally demonstrate an intensity modulated and direct detection optical OFDM with variable bit rate from 5 Gb/s to 9 Gb/s using BPSK format. A fast processing based on the Hartley transform is performed with low complexity DSP, achieving the same performance as 4QAM FFT-based processing. Using the same bandwidth occupancy as required for 5 Gb/s, the bit rate can be increased up to 80% and transmitted over 25 km SSMF, by reducing the guard band and adopting an optimized transceiver design with additional overhead, including half-length training symbols and cyclic extension.

- by Ioannis Tomkos
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- Adaptive Optics, OFDM, Optical physics, Fiber Optic

The transmission of multiple wireless signals over optical links has attained a great research interest nowadays. In case where optical fibers are difficult to be deployed, or installation cost is prohibited, optical wireless systems provide an efficient alternative means. In this paper, we consider the WiMAX (IEEE802.16) standard and construct a simple but adequate scenario to investigate radio signal transmission over terrestrial optical wireless channels. An appropriate system architecture is adopted and a channel model, which entails some of the most critical impairments of the optical channel, i.e., attenuation, turbulence, pointing error effects, as well as of the RF channel, i.e., path loss, shadowing, and Rayleigh fading, is taken into account. The overall link budget and a closed-form of the outage probability of the system are deduced. Several analytical results are depicted using a realistic set of parameter values, to lend a helpful insight to the performance of the proposed architecture.

- by Dimitris Varoutas
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- Turbulence, Adaptive Optics, Probability, System Architecture

The present work focuses on the main phenomenological features of stone cleaning by lasers. These are the removal rate, cleaning degree, and chromatic appearance of the treated surface associated with different conservation problems and laser parameters. A set of three different outdoor stone conservation problems were investigated here. The measurement of the ablation rates were carried out on encrusted stone artifacts and two sets of standards in order to derive general behaviors through repeatable measurements. The analysis of the irradiation tests provided quantification of the different efficiencies, cleaning degree, and chromatic appearances associated with the fundamental harmonic of Q-switching, long Q-switching, and short free-running Nd:YAG lasers (1064 nm), as well as with the second harmonic of Q-switching sources (532 nm). PACS numbers: 89.90.+n, 42.62.Cf, 44.10.+i

- by Vadim Parfenov and +4
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- Adaptive Optics, Interferometry, Holography, Optical physics and laser technology