Showing papers on "Adaptive optics published in 1995"

Wave-front temporal spectra in high-resolution imaging through turbulence

Abstract: A general formulation is given for the derivation of theoretical temporal power spectra of quantities related to turbulent wave-front phase. These temporal power spectra and their asymptotic power laws and cutoff frequencies are presented for various quantities of interest in the field of interferometry (differential piston), wave-front sensing (Shack–Hartmann and curvature sensor), adaptive optics (Zernike polynomials), and seeing monitoring (differential angle of arrival). We show that the differential piston spectrum has two cutoff frequencies and exhibits a very steep decrease at high frequencies. The curvature sensor is shown to be much less sensitive than the Shack–Hartmann sensor to the low temporal frequencies. A study of the Zernike temporal power spectra shows that their cutoff frequencies increase with the polynomial radial degree. Both single-layer and multilayer plane and spherical waves are considered. The effect of wind direction is also taken into account. We point out the influence of the cone effect on the temporal power spectra when Rayleigh or sodium laser guide stars are used for wave-front sensing. The cone effect results in a temporal decorrelation between natural and laser guide star wave fronts. Finally, we demonstrate that in adaptive optics systems low-order modes require higher servoloop bandwidths than do high-order modes in order for the residual variance to be balanced between the corrected modes. The same conclusion applies to fringe tracking in large telescope interferometers equipped with adaptive optics systems.

Better optical triangulation through spacetime analysis

Abstract: The standard methods for extracting range data from optical triangulation scanners are accurate only for planar objects of uniform reflectance illuminated by an incoherent source. Using these methods, curved surfaces, discontinuous surfaces, and surfaces of varying reflectance cause systematic distortions of the range data. Coherent light sources such as lasers introduce speckle artifacts that further degrade the data. We present a new ranging method based on analyzing the time evolution of the structured light reflections. Using our spacetime analysis, we can correct for each of these artifacts, thereby attaining significantly higher accuracy using existing technology. We present results that demonstrate the validity of our method using a commercial laser stripe triangulation scanner. >

High Dynamic Range Imaging Using a Deformable Mirror for Space Coronography

Abstract: The need for high dynamic range imaging is crucial in many astronomical fields, such as extra-solar planet direct detection, extra-galactic science and circumstellar imaging. Using a high quality coronograph, dynamic ranges of up to $10^{5}$ have been achieved. However the ultimate limitations of coronographs do not come from their optical performances, but from scattering due to imperfections in the optical surfaces of the collecting system. We propose to use a deformable mirror to correct these imperfections and decrease the scattering level in local regions called ``dark holes''. Using this technique will enable imaging of fields with dynamic ranges exceeding $10^8$. We show that the dark-hole algorithm results in a lower scattering level than simply minimizing the RMS figure error (maximum-strehl-ratio algorithm). The achievable scattering level inside the dark-hole region will depend on the number of mirror actuators, the surface quality of the telescope, the single-actuator influence function and the observing wavelength. We have simulated cases with a $37\times37$ deformable mirror using data from the {\it Hubble Space Telescope\/} optics without spherical aberrations and have demonstrated dark holes with rectangular and annular shapes. We also present a preliminary concept of a monolithic, fully integrated, high density deformable mirror which can be used for this type of space application.

Phase-retrieval analysis of pre- and post-repair Hubble Space Telescope images.

Abstract: Phase-retrieval measurements of point-spread functions from the pre- and post-repair Hubble Space Telescope are presented. The primary goal was to determine the aberrations present in the second wide-field and planetary camera (WFPC2) to align and validate its corrective optics. With both parametric model-fitting techniques and iterative (Gerchberg-Saxton) methods, accurate measurements have been obtained of the WFPC2 and Hubble Space Telescope optics, including improved maps of the zonal errors in the mirrors. Additional phase-retrieval results were obtained for the aberrated, prerepair cameras and the corrected faint-object camera. The information has been used to improve models produced by point-spread-function simulation programs. On the basis of the measurements a conic constant for the primary mirror of κ = -1.0144 has been derived.

Imaging through turbulence

Abstract: Introduction Overview of the Problem Area Historical Overview of Imaging Through Turbulence Overview of the Book Background: Fourier and Statistical Optics Fourier Optics Statistical Optics Turbulence Effects on Imaging Systems Index of Refraction Fluctuations in the Atmosphere Statistics of Index of Refraction Fluctuations Wave Propagation through Random Media First-Order Turbulence Effects on Incoherent Imaging Modal Expansions of Phase Perturbation Phase Screen Generation Speckle Imaging Techniques Introduction Overview of Speckle Imaging Speckle Interferometry Fourier Phase Estimation Techniques Image Reconstruction for Speckle Imaging Conclusion Adaptive Optical Imaging Systems Introduction Factors that Degrade AOI Systems Performance Adaptive Optical System Components and Models AOI System Performance Modeling Summary Hybrid Imaging Techniques Introduction Deconvolution from Wavefront Sensing Methods Involving Adaptive Optics Conclusion Index

Quantification of aero-optical phase distortion using the small-aperture beam technique

Abstract: We discuss the small-aperture beam technique, a new way of experimentally quantifying the instantaneous optical wave front distortions imposed by propagation through an optically active, turbulent flowfield. We lay out the theoretical basis for the technique and the relationship of the measured jitter of a small-aperture probe beam to optical path difference. A numerical simulation of a two-dimensional heated jet is used to explore the validity of using beam-jitter signals from multiple probe beams to obtain optical path difference in a flow region where eddy production constitutes the major character of the turbulent flowfield.

Use of power spectral density (PSD) functions in specifying optics for the National Ignition Facility

Abstract: In the second half of the 1990's, LLNL and others will be designing and beginning construction of the National Ignition Facility. This new laser will be capable of producing the worlds first controlled fusion ignition and burn, completing a vital milestone on the path of Fusion Energy. This facility will use more than 7,000 optical components, most of which have a rectangular aperture, which measure greater than 600 mm on the diagonal. In order to optimize the performance versus cost of the laser system, we have determined that specifications based on the Power Spectral Density (PSD) functions are the most effective for controlling mid-spatial wavelength errors. The draft optics specifications based on a combination of PSD and conventional roughness and P-V requirements are presented, with a discussion of their origins. The emphasis is on the application of a PSD function for transmitted wavefront optical specifications, and the benefits thereof. The PSD function is the most appropriate way to characterize transmitted wavefront errors with spatial frequencies ranging from several centimeters to a few hundred nanometers, with amplitudes in the (lambda) /100 regime. Such errors are commonly generated by cost effective, deterministic finishing technologies, and can be damaging to the laser, as well as causing unnecessary energy loss and inability to focus, in a high energy laser application. In addition, periodic errors can occur as a result of errors at other steps in the fabrication process, such as machine vibration in a fixed abrasive step, or material homogeneity ripple. The control of such errors will be essential to the construction of future high energy lasers.

Optimization and Performance of Adaptive Optics for Imaging Extrasolar Planets

Abstract: A recent study by Angel (1994) using simplified analytical models indicated the feasibility of imaging extrasolar planets from the ground, making use of adaptive optics correction of a large telescope We have performed detailed simulations of the method using computer codes that model propagation through atmospheric turbulence, adaptive correction, and broadband imaging We confirm that high-resolution correction at the limit of photon noise errors reduces the halo intensity to 10-6 of the peak star flux Our work shows how to avoid systematic errors Thus, we find that time delays between sensing the wave front and its detection lead to persistent structure in the stellar halo, which uncorrected would prevent rapid averaging of the residual halo speckle structure A local wave front reconstructor that extrapolates ahead in time has been devised to remove this problem We find the chromatic differences in wave front structure are small enough that the signal-to-noise ratio can be improved by wave front sensing and imaging in separate adjacent bands We verify that correction of amplitude scintillation is needed and the optimum level of clipping is derived A simulated image of a twin of the solar system at 8 pc is presented for the new 65 m telescope and the measured turbulence at the Multiple Mirror Telescope site The Jupiter twin shows up at the 5 σ level in a 5 hr integration

Adaptive wavefront shaping with liquid crystals

Abstract: The authors are interested in high resolution imaging systems, and in particular new techniques for wavefront sensing and correction.

First tests of the Cassegrain adaptive optics system of the Mount Wilson 100-in telescope

Abstract: In January 1994, we began construction of a modern adaptive optics system for the newly refurbished 100-inch telescope. The design philosophy of the adaptive optics system is to achieve a working system in the visible in a short time at relatively low cost. This means wavefront sensing with natural guide stars and implementation at the bent Cassegrain focus of the telescope. The system has an integrated wavefront sensor and finder camera, and is automated for one-person operation. It uses off-the-shelf components where possible. The deformable mirror, which has 241 actuators, is on loan from the U.S. Air Force. The use of an existing mirror imposes constraints that have driven some of the design considerations. The system is operating at the telescope, with early results described below.

Advanced Modeling of Micromirror Devices

Abstract: The flexure-beam micromirror device (FBMD) is a phase only piston style spatial light modulator demonstrating properties which can be used for phase adaptive corrective optics. This paper presents a complete study of a square FBMD, from advanced model development through final device testing and model verification. The model relates the electrical and mechanical properties of the device by equating the electrostatic force of a parallel-plate capacitor with the counter-acting spring force of the device's support flexures. The capacitor solution is derived via the Schwartz-Christoffel transformation such that the final solution accounts for non-ideal electric fields. The complete model describes the behavior of any piston-style device, given its design geometry and material properties. It includes operational parameters such as drive frequency and temperature, as well as fringing effects, mirror surface deformations, and cross-talk from neighboring devices. The steps taken to develop this model can be applied to other micromirrors, such as the cantilever and torsion-beam designs, to produce an advanced model for any given device. The micromirror devices studied in this paper were commercially fabricated in a surface micromachining process. A microscope-based laser interferometer is used to test the device in which a beam reflected from the device modulates a fixed reference beam. The mirror displacement is determined from the relative phase which generates a continuous set of data for each selected position on the mirror surface. Plots of this data describe the localized deflection as a function of drive voltage.

Adaptive Multilayer Optical Neural Network with Optical Thresholding

Abstract: An adaptive multilayer optical neural network with all-optical forward propagation including optical thresholding by a liquid crystal light valve (LCLV) is described. It has a large number of modifiable optical interconnections that are implemented by liquid crystal television screens, and it has a modular structure enabling the cascading of layers, each layer with its own light source. Sigmoid fits to response curves of four LCLVs are evaluated and their suitability as optical thresholding functions is examined on the basis of neural network simulations.

Adaptive optics wave measurement and correction system

Abstract: An adaptive optics system which simultaneously measures the phase function and corrects the phase distortion of a wavefront using a liquid crystal device placed in an interferometer. The liquid crystal device inserts the conjugate phase function in the path and obtains a null in the interference pattern.

Binary adaptive optics: atmospheric wave-front correction with a half-wave phase shifter

Abstract: We describe a binary approach to adaptive wave-front correction, especially suitable for narrow band applications, which would be simpler than conventional adaptive technology. Appropriate parts of the aberrant wave front are phase retarded by half a wavelength to ensure that none of the image-forming rays add together destructively. Simulations for monochromatic light show that the residual wave-front errors, in the absence of other errors, would result in Strehl ratios of ~40% with diffraction-limited widths at visible wavelengths. We simulate the imaging performance of such a system and describe a possible implementation that uses a ferroelectric liquid-crystal spatial light modulator.

Single-pass and double-pass propagation through complex paraxial optical systems

Abstract: A generalized form of spectral representation theory is developed and used with the ABCD formulation of the Huygens–Fresnel integral for studying optical wave propagation through a random medium in the presence of any complex paraxial optical system that can be characterized by an ABCD ray matrix. Formal expressions are developed for the basic optical field moments and various related second-order statistical quantities in terms of three fundamental moments of the first- and second-order complex phase perturbations. Special propagation environments include line-of-sight propagation, single-pass propagation through arbitrary ABCD optical systems, and double-pass propagation through the same random medium in the presence of an ABCD optical system. For illustrative purposes the method is used in the development of expressions for the mean and the normalized variance of the irradiance associated with the Fourier-transform-plane geometry of a lens and the enhanced backscatter effect (EBS) associated with irradiance and phase fluctuations of a reflected Gaussian-beam wave from a Gaussian mirror. The EBS analysis accounts for both finite size and finite focal length of the mirror.

Stellar Optical Interferometry in the 1990s

Abstract: The unaided eye has an angular resolution of about 1 arcminute. From the invention of the telescope in the 17th century to the middle of the 1970s, astronomers improved on this resolution by two orders of magnitude by building bigger telescopes and putting them at good sites. Even at good sites, however, atmospheric turbulence limits the resolution at visible and infrared wavelengths to 1 arcsec or a little better. In the past 20 years, a further factor‐of‐ten improvement has come with two developments that deal with the atmosphere: “speckle interferometry,” in which the blurred image is frozen in a short exposure and the image is reconstructed from many exposures, and adaptive optics, in which the effects of the atmosphere are sensed, then corrected with a defbrmable mirror, before the image is recorded. (See Laird A. Thompson's article in PHYSICS TODAY, December 1994, page 24.)

Adaptive optics and the outer scale of turbulence

Abstract: The outer-scale influence on spatial and temporal characteristics of turbulence-induced wave-front distortions is discussed. The calculation results for the image centroid and the Zernike modes are presented. Two methods of the outer-scale estimation based on tilt and defocus and on image centroid measurements are suggested. The application of the results obtained to adaptive optics problems is considered. Finally, a possibility of prediction of the wave-front statistical characteristics is discussed.

Optical simulation of phase-distorted imaging systems: nonlinear and adaptive optics approach

Abstract: We discuss a technique based on nonlinear and adaptive optics for simulation of phase-distortion effects in imaging systems This technique uses a nonlinear two-dimensional optical feedback system to produce a controllable spatially and temporally varying chaotic intensity distribution The intensity pattern is converted into a spatially varying thin phase screen using an optically addressed spatial phase modulator A chaotic phase distortion is then introduced into an imaging system's output image by propagation through the phase screen A deformable mirror with computer control is used for simulation of large-scale phase distortions

Adaptive optical image compensation experiments on stellar objects

Abstract: The resolution of a large telescope is limited by various kinds of dynamic wavefront disturbance such as atmospheric turbulence. This has been a problem since the invention of the telescope. An approach for solving the problem is provided by adaptive optical techniques. Compensating for stellar objects in real time is, however, very difficult, because only limited light energy can be used and the temporal-spatial characteristics of dynamic disturbances place very high demands on system specifications. This paper reports the first experiment of a 21-element adaptive optical system for stellar objects on the Yunnan Observatory 1.2-m telescope and presents the improvements of the system for low-light-level stellar objects. The experimental results show that the adaptive optical system can work satisfactorily when the photon counts are as low as 93 per subaperture per integrating time. The experiments show that the effects of adaptive optics in reducing the divergence and wandering of the image spot, in improving resolution, and in increasing image contrast are significant.

Derivation of preliminary specifications for transmitted wavefront and surface roughness for large optics used in inertial confinement fusion

Abstract: In preparation for beginning the design of the National Ignition Facility (NIF) in the United States and the Laser Mega-Joule (LMJ) in France, we are in the process of deriving new specifications for the large optics required for these facilities. They are currently being evaluated through modeling and experimentation. These specifications will be ready for general release by the end of the year. Traditionally, specifications for transmitted wavefront and surface roughness of large ICF optics have been based on parameters which were easily measured during the early 1980s, such as peak-to-valley wavefront error (PV) and root-mean- square (rms) surface roughness, as well as wavefront gradients in terms of waves per cm. While this was convenient from a fabrication perspective, since the specifications could be easily interpreted by fabricators in terms which were understood and conventionally measurable, it did not accurately reflect the requirements of the laser system. In some cases, optics which were not adequate for a given application which was particularly sensitive to periodic errors, were fabricated acceptably in terms of the optics specifications. For the NIF and LMJ laser systems, we have availed ourselves of advances in metrology and interferometry and an enhanced understanding of laser system performance to derive specifications which are based on power spectral densities (PSDs). Such requirements can more accurately reflect the requirements of the laser system for minimizing the amplitude of mid- and high-spatial frequency surface and transmitted wavefront errors, while not over constraining the fabrication in terms of low spatial frequencies, such as residual coma or astigmatism, which are typically of a very large amplitude compared to periodic errors. In order to study the effect of changes in individual component tolerances, it is most useful to have a model capable of simulating real behavior. The basis of this model is discussed in this paper, outlining the general approach to the 'theoretical' study of ICF optics specifications, and an indication of the type of specification to be expected is shown, based upon existing ICF laser optics. The problem of specifying optics for high energy lasers is more difficult than for 'classical' optical systems for many reasons, which are discussed as well.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Sparse matrix wave-front estimators for adaptive-optics systems for large ground-based telescopes

Abstract: A closed-loop adaptive-optics servo system is an iterative optical–digital processor that can be characterized by a first-order difference equation. This identification leads to a new class of wave-front estimators, some of which are extremely sparse and permit real-time subaperture intensity weighting.

Dynamic modeling and identification of an adaptive optics system

Abstract: Multi-input, multi-output (MIMO) control of modern adaptive optics systems requires careful modeling of the coupling between actuator influence functions and the relevant dynamics of the mirror control loop. In this paper, we present a novel approach to modeling and identification of adaptive optics systems, and demonstrate the procedure in the context of an experimental adaptive optics system. The results illustrate that the modeling captures the relevant inter-actuator couplings as well as the important MIMO dynamics of the adaptive optics system. Furthermore, the modeling approach significantly reduces the complexity of identification for the adaptive optics system.

Model of an adaptive optical system controlled by a neural network

Abstract: An adaptive optical system (AOS) with a feedback loop closed via a feedforward neural network (NN) is considered. The vector of the wavefront corrector control signals is computed by the network from two vectors of the intensity moments measured in two near-field planes by two matrix photodetectors. The NN is trained with a back-propagation algorithm to predict the vector of adaptative mirror signals from the mea- sured intensity vectors. During training, the network forms an optimal

WIYN telescope active optics system

Abstract: In this paper we describe the active optics system of the WIYN 3.5 meter telescope put into operation in the spring of 1995 on Kitt Peak, Arizona. The active optics system provides real time collimation of the telescope and optical figure control of the primary mirror. The individual subsystems are first described. These include the wavefront curvature sensing technique, the support and articulation of the secondary mirror, the control of the primary mirror figure and rigid body motion, and the mechanics and electronics used for controlling and monitoring the optics. Algorithms for the complete loop are then discussed. This involves mapping coma terms used to actively correct the collimation, while residual phase errors are corrected by active control of the forces supporting the primary mirror. In the next section we compare two operational modes: open loop using mapped collimation and optical figure corrections, and closed loop using feedback from the wavefront sensor directly. Finally, preliminary stellar images obtained with the actively controlled telescope are presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Adaptive optics for in-orbit aberration correction: spherical aberration feasibility study

Abstract: We investigate the feasibility of using an adaptive mirror for in-orbit aberration corrections. The advantage of an in situ aberration correction of optical components in the space environment is that the mirror shape can be adjusted in an iterative fashion until the best image is obtained. Using the actuator spacing, corresponding to one half of the Nyquist frequency, the Strehl ratio of the corrected wave front improves to 0.95 when the mirror is fabricated with 6.5 waves of spherical aberration. The Strehl ratio decreases to 0.86 when the number of actuators is reduced by a factor of 4, in a two-dimensional adaptive optics model.