Showing papers on "Adaptive optics published in 1991"

Principles of Adaptive Optics

Abstract: History and Background: Introduction. History. Physical Optics: Propagation with Aberrations. Imaging with Aberrations. Representing the Wavefront. Interference. Adaptive Optics Terms. Sources of Aberrations: Atmospheric Turbulence: Descriptions of Atmospheric Turbulence. Refractive Index Structure Constant. Turbulence Effects. Turbulence MTF. Thermal Blooming: Blooming Strength and Critical Power. Turbulence, Jitter, and Thermal Blooming. Non-atmospheric Sources: Optical Misalignments and Jitter. Thermally Induced Distortions of Optics. Manufacturing and Microerrors. Other Sources of Aberrations. Adaptive Optics Compensation: Phase Conjugation. Limitations of Phase Conjugation: Turbulence Spatial Error. Turbulence Temporal Error. Sensor Noise Limitations. Thermal Blooming Compensation. Artificial Guide Stars. Combining the Limitations. Linear Analysis of Random Wavefronts. Linear Analysis of Deterministic Wavefronts: Partial Phase Conjugation. Adaptive Optics Systems: Adaptive Optics Imaging Systems. Beam Propagation Systems: Local Loop Beam Cleanup Systems. Alternative Concepts. Pros and Cons of the Various Approaches. Unconventional Adaptive Optics: Nonlinear Optics. Elastic Photon Scattering, DFWM. Inelastic Photon Scattering. System Engineering. Wavefront Sensing: Directly Measuring Phase: The Non-uniqueness of the Diffraction Pattern. Determining Phase Information from Intensity. Modal and Zonal Sensing. Direct Wavefront Sensing--Modal: Importance of Wavefront Tilt. Measurement of Tilt. Focus Sensing. Modal Sensing of Higher-Order Aberrations. Direct Wavefront Sensing--Zonal: Interferometiric Wavefront Sensing. Hartman Wavefront Sensors. Curvature Sensing. Selecting a Method. Indirect Wavefront Sensing Methods: Multidither Adaptive Optics. Image Sharpening. Wavefront Sampling: Beamsplitters. Hole Gratings. Temporal Duplexing. Reflective Wedges. Diffraction Gratings. Hybrids. Sensitivities of Sampler Concepts. Detectors and Noise. WavefrontCorrection: Modal Tilt Correction. Modal Higher-Order Correction. Segmented Mirrors. Deformable Mirrors: Actuation Techniques. Actuator Influence Functions. Bimorph Corrector Mirrors. Membrane and Micromachine Mirrors. Edge Actuated Mirrors. Large Correcting Optics. Special Correction Devices: Liquid Crystal Phase Modulators. Spatial Light Modulators. Charged-large-array-mirrors. Reconstruction and Controls: Introduction. Single-Channel Linear Control: Fundamental Control Tools. Transfer Functions. Proportional Control. First- and Second-Order Lag. Feedback. Frequency Response of Control Systems. Digital Controls. Multivariate Adaptive Optics Controls: Solution of Linear Equations. Direct Wavefront Reconstruction: Phase from Wavefront Slopes. Modes from Wavefront Slopes. Phase from Wavefront Modes. Modes from Wavefront Modes. Zonal Corrector from Continuous Phase. Modal Corrector from Continuous Phase. Zonal Corrector from Modal Phase. Modal Correctors from Modal Phase. Indirect Reconstructions.Modal Corrector from Wavefront Modes. Zonal Corrector from Wavefront Slopes. Spatiotemporal Considerations. Subject Index.

Measurement of atmospheric wavefront distortion using scattered light from a laser guide-star

Abstract: THE possibility of using an artificial 'guide-star' to measure optical wavefront distortion caused by atmospheric turbulence has been discussed for some time1–4, but few experimental data are available5. Here we report experimental results demonstrating that atmospheric wavefront distortion can be measured by taking fast 'snapshots' of a guide-star formed by light scattered from a laser beam focused in the upper atmosphere. These results agree with a theoretical prediction6 that the mean-square wavefront error created by using an artificial guide-star at a finite distance rather than a real (infinitely distant) star is proportional to the five-thirds power of the telescope aperture. Using this understanding of the physics, we have demonstrated continuous, real-time atmospheric compensation of a 1.5-m telescope using a high-pulse-rate laser, pulse-synchronized wavefront sensor and deformable mirror, and have been able to resolve the 1.3-arcsecond binary star 53 ξ Ursa Majoris in an exposure time of only one second.

A simple low-order adaptive optics system for near-infrared applications

Abstract: It is shown that low-order wavefront compensation can significantly improve astronomical images over most of the sky. A novel approach to wavefront sensing and compensation is described. It is optimized for low-order correction and high efficiency. Computer-simulation results show it can achieve the desired performance, and preliminary laboratory tests demonstrate its feasibility.

Compensation of atmospheric optical distortion using a synthetic beacon

Abstract: ATMOSPHERIC turbulence limits the resolution of a ground-based astronomical telescope to much worse than the diffraction limit of the instrument. An adaptive-optics system, in which some part of the optical train of the telescope can be adjusted in real time, can compensate for atmospheric turbulence, but a beacon or guide-star is needed to allow measurement of the atmospheric optical distortion. It has been suggested1 that the measurement could be achieved by means of a synthetic beacon (also called an artificial beacon, and sometimes a laser guide-star) formed by atmospheric backscatter from a ground-based laser. We have performed an experiment to demonstrate atmospheric compensation using a synthetic beacon. With a pulsed dye laser to generate the beacon and a 241-channel adaptive-optics system to perform the phase correction, we obtained almost diffraction-limited resolution of star images in the visible part of the spectrum. Our results indicate that there are no technical barriers to atmospheric compensation using synthetic beacons at ground-based observatories.

Effects of turbulence-induced anisoplanatism on the imaging performance of adaptive-astronomical telescopes using laser guide stars

Abstract: The laser power requirement for an adaptive-astronomical telescope using laser guide stars is determined largely by the effects of turbulence-induced anisoplanatism. Owing to the relatively low altitude of laser guide stars and the small size of the isoplanatic angle at visible wavelengths, multiple guide stars are required for correcting large telescope apertures. The laser power requirements are proportional to the required number of guide stars. Using an analysis technique that takes into account the realistic characteristics of the wave-front sensor and deformable mirror, as well as the spherical nature of the wave front from the laser guide star, we present computational results that show how the imaging performance of a laser-guided adaptive telescope varies as a function of the number and height of the guide stars. The results are presented as a function of the isoplanatic angle θIP as defined by Fried [ J. Opt. Soc. Am.72, 52 ( 1982)]. A new parameter, the characteristic diameter of the largest telescope requiring a single guide star, is also introduced. This parameter is designated DIP and is related to the height of the guided star zg and the isoplanatic angle θIP by DIP = 2zgθIP. The effects of anisoplanatism on the design of a 2-m-diameter adaptive telescope using laser guide stars created in the mesospheric Na layer is considered. Using a Hufnagel Cn2 model, an isoplanatic angle of θIP = 1.64 arcsec (calculated for a value of ro = 20 cm), and zg = 92 km (the nominal height of the mesospheric Na layer), we find that three Na guide stars are required in order to achieve a rms wave-front error of approximately λ/10 across the telescope aperture.

Adaptive optics on a 3.6-m telescope : results and performance

Abstract: The results of the first observations made on a large (3.6-m) telescope with the adaptive optics prototype, named COME-ON, conceived to explore the application of this technique for the European Very Large Telescope, are presented. The wavefront is studied, after its correction by the servo-system, using a Zernike polynomial expansion. The analysis of uncorrected and corrected images in the near infrared (≤5 μm) leads to a detailed evaluation of the system performance in terms of improvement of angular resolution, reaching nearly the ideal diffraction profiles down to 2.2 μm, and Strehl ratio, approaching 0.6 to 0.8 at 3.8 μm

Limited degree-of-freedom adaptive optics and image reconstruction.

Abstract: The use of limited degree-of-freedom adaptive optics in conjunction with statistical averaging and a linear image reconstruction algorithm is addressed. Image reconstruction is traded for full predetection compensation. It is shown through analytic calculations that the average optical transfer function (OTF) is significant for high spatial frequencies in the case of imaging through atmospheric turbulence with an adaptive optics system composed of a Hartmann-type wave-front sensor and a deformable mirror possessing far fewer actuators than one per atmospheric coherence diameter (r(0)). Statistical averaging is used to overcome the effects of measurement noise and randomness in individual realizations of the OTF. The imaging concept and signal-to-noise considerations are presented.

Use of a neural network to control an adaptive optics system for an astronomical telescope

Abstract: ANGEL et al.1 recently showed how an artificial neural network could be used to measure optical phase distortion induced by atmospheric turbulence, and demonstrated by numerical simulation that such a system could be used to control the six 1.8-m mirrors of the Multiple Mirror Telescope by constantly adjusting them to compensate for atmospheric distortion of the image. The neural network estimates the phase distortion using two images of a reference star, or of a laser-produced guide star2, one image being at the best focus of the telescope while the other is intentionally out of focus. Here we report the successful test of a neural network with a real star. We applied a neural network to in- and out-of-focus images of Vega obtained with the 1.5-m single-mirror telescope at the Starfire Optical Range of the Air Force Phillips Laboratory near Albuquerque, New Mexico. The experimental results agree well with phase reconstructions obtained simultaneously with a conventional wave-front sensor.

Adaptive optics: a progress review

Abstract: Active and adaptive optics technology has emerged from the laboratory and is being applied to improve the performance of optical imaging and laser systems. In the last few years, development of both systems and components has accelerated. Many new concepts and devices have appeared, among which are high-performance deformable mirrors, new types of wavefront sensors, and more sophisticated wavefront processing algorithms. Equally important, a better understanding of the system design aspects of adaptive optics has been reached, particularly of the need for optimizing each system according to its application. For example, the dominant requirement in laser systems is to achieve a high Strehl ratio, whereas for ground-based astronomy the availability of guide stars is a major concern. Current developments in adaptive optics for ground-based astronomy include the use of IR wavelengths, partial wavefront compensation using natural guide stars, and the use of laser guide stars to allow all-sky coverage with full compensation at visible wavelengths. While progress to date has been impressive, much work remains to bring this technology into general use.

Fitting capability of deformable mirror

Abstract: Deformable mirror is the key element for adaptive optical wavefront correction. The number of actuators decides the complexity and cost of adaptive optical system. In this paper computer simulations of wavefront error for fitting different Zernike terms by deformable mirror with different number of actuators are presented. The arrangement of actuator and the influence function of mirror are discussed in respect of fitting error. The minimum number of actuators for fitting different Zernike orders of wavefront are given. Some optical experiments of fitting capability have been done with 19 and 37-element deformable mirrors and a Zygo interferometer.

Light propagation through the atmosphere and the properties of images formed by large ground-based telescopes

Abstract: A fresh look is taken at how light propagates through the atmosphere and how atmospheric turbulence affects images formed by large ground-based telescopes. Telescopes with fixed and adaptive optics are considered. The approach is based on a layered model of the atmosphere. It is shown that the atmosphere can be represented by an equivalent phase screen for the two quantities that determine most of the important image properties—the atmospheric modulation transfer function and the spectral correlation function. Techniques are described for measuring the parameters that define the equivalent phase screen. Expressions are given in terms of screen parameters for a number of image properties. Many of these properties are different from those in the conventional literature. Diffraction-limited cores in star images are discussed. An optimum wavelength at which resolution is maximized is also discussed. Resolution of the order of 0.05 arcsec is possible at this wavelength, but only if the telescope is near diffraction limited. The optimum wavelength can be used to produce maximum energy density at the focus of a ground-based laser beam directed at a target in space.

The New Ground‐Based Optical Telescopes

Abstract: We are in a period of rapid improvement in the capability of telescopes. Great strides are being taken to improve angular resolution, to increase sensitivity at all wavelengths and to make the most efficient use of wide‐field images. The technical challenges involved in such development are great, for not only do we need mirror surfaces much larger than what we have used before, but we require higher image quality. It has recently become clear that the best mountaintop observatory sites occasionally deliver images as sharp as 0.3 arcsecond, an image quality that many existing telescopes cannot maintain.

Iterative techniques to integrate different optical functions in a diffractive phase element

Abstract: Diffractive optics allows the incorporation of several optical functions, e.g., wave shaping and focusing, in one element. A method suitable to calculate a diffractive phase element with this feature is described. Coding and quantization effects are analyzed. As an example an array generator with integrated focal power is designed.

Space power by ground-based laser illumination

Abstract: Reducing energy storage requirements of space power systems by illuminating the photovoltaic arrays with a remotely located laser system is addressed. It is proposed that large lasers be located on cloud-free sites at one or more ground locations and that large lenses or mirrors with adaptive optical correction be used to reduce the beam spread due to diffraction or atmospheric turbulence. During the eclipse periods or lunar night, the lasers illuminate the solar arrays to a level sufficient to provide operating power. Two applications are discussed: illumination of geosynchronous orbit satellites and illumination of a moonbase power system. Issues for photovoltaic receivers for such a system are discussed. >

Apparatus for sensing wavefront aberration

Abstract: A wavefront aberration sensor includes a beam splitter, one or more aberration sensor modules and a photodetector for sensing total light power. The aberration sensor modules each provide two voltage outputs from a pair of photodetectors. Differences in the voltage pairs are normalized by the total light power to represent signed aberration amplitudes of phase aberrations present in an input optical beam. The aberration amplitudes may be combined in a digital computer to provide a reconstructed wavefront.

Feedback controlled optics with wavefront compensation

Abstract: The sensitivity model of a complex optical system obtained by linear ray tracing is used to compute a control gain matrix by imposing the mathematical condition for minimizing the total wavefront error at the optical system's exit pupil. The most recent deformations or error states of the controlled segments or optical surfaces of the system are then assembled as an error vector, and the error vector is transformed by the control gain matrix to produce the exact control variables which will minimize the total wavefront error at the exit pupil of the optical system. These exact control variables are then applied to the actuators controlling the various optical surfaces in the system, causing the immediate reduction in total wavefront error observed at the exit pupil of the optical system.

Neural network adaptive optics for the Multiple Mirror Telescope

Abstract: The MMT consists of six comounted 1.8 m telescopes from which the light is brought to a combined coherent focus. Atmospheric turbulence spoils the MMT diffraction-limited beam profile, which would otherwise have a central peak of 0.06 arcsec FWHM, at 2 microns wavelength. At this wavelength, the adaptive correction of the tilt and path difference of each telescope beam is sufficient to recover diffraction-limited angular resolution. Computer simulations have shown that these tilts and pistons can be derived by an artificial neural network, given only a simultaneous pair of in-focus and out-of-focus images of a reference star formed at the combined focus of all the array elements. We describe such an adaptive optics system for the MMT, as well as some successful tests of neural network wavefront sensing on images, and initial real-time tests of the adaptive system at the telescope; attention is given to a demonstration of the adaptive stabilization of the mean phase errors between two mirrors which resulted in stable fringes with 0.1 arcsec resolution.

Actuator influence functions of active mirrors

Abstract: A method for calculating the actuator influence functions of an active mirror is described. The method is based on a model of the mirror as a thin plate attached by actuators to a very stiff reaction structure. Arbitrary actuator layouts can be handled. The mirror deflections calculated agree well with the results of a finite element analysis of the structure.

High sensitivity-wide dynamic range optical tilt sensor

Abstract: A new and improved adaptive optics wavefront tilt sensor having both very high sensitivity and wide dynamic range is presented. In accordance with this invention, distortion is induced in each subaperture beam by passing a high f-number beam through a field lens having large amounts of spherical aberration. The distortion provides transition from a wide dynamic range, low sensitivity wavefront tilt measurement to a narrow dynamic range, high sensitivity wavefront tilt measurement. This beam is then impinged on a single photodector which provides at least one electrical signal indicative of centroid position.

Correction of phase distortions in a nonlinear interferometer with an optical feedback loop

Abstract: The feasibility of compensation of phase distortions in a nonlinear interferometer with a purely optical feedback loop was demonstrated experimentally. It was found that phase switching waves appeared in the process of establishment of steady-state conditions.

Prototype high speed optical delay line for stellar interferometry

Abstract: The long baselines of the next-generation ground-based optical stellar interferometers require optical delay lines which can maintain nm-level path-length accuracy while moving at high speeds. NASA-JPL is currently designing delay lines to meet these requirements. The design is an enhanced version of the Mark III delay line, with the following key features: hardened, large diameter wheels, rather than recirculating ball bearings, to reduce mechanical noise; a friction-drive cart which bears the cable-dragging forces, and drives the optics cart through a force connection only; a balanced PZT assembly to enable high-bandwidth path-length control; and a precision aligned flexural suspension for the optics assembly to minimize bearing noise feedthrough. The delay line is fully programmable in position and velocity, and the system is controlled with four cascaded software feedback loops. Preliminary performance is a jitter in any 5 ms window of less than 10 nm rms for delay rates of up to 28 mm/s; total jitter is less than 10 nm rms for delay rates up to 20 mm/s.

Design of a linear quadratic Gaussian controller for an adaptive optics system

Abstract: The authors consider the control of a phase-correcting ground-based adaptive optical telescope. They present the design of a linear quadratic Gaussian controller for generating actuator voltages to a deformable mirror, given a set of slope measurements from a wavefront sensor. A truth model is developed based on theoretical developments and actual test data. The resulting state-space model has as its states the Zernike coefficients associated with the atmospheric turbulence and the deformable mirror. The reflected image can be described by combining the Zernike coefficients. As a result of the approach taken, modal control is accomplished as opposed to zonal control since the coupling of actuator influence functions is taken into account in developing the design models. The results of covariance and Monte Carlo analysis demonstrate the limits on performance imposed by the Kalman filter. Computer simulations predict that the controller presented can reduce phase distortion to an r.m.s. value of approximately 0.1 wavelengths. >

Infrared imaging of extrasolar planets

Abstract: An optical system for direct detection, in the infrared, of planets orbiting other stars is described. The proposed system consists of a large aperture (about 16 m) space-based telescope to which is attached a specialized imaging instrument containing a set of optical signal processing elements to suppress diffracted light from the central star. Starlight suppression is accomplished using coronagraphic apodization combined with rotational shearing interferometry. The possibility of designing the large telescope aperture to be of a deployable, multiarm configuration is examined, and it is shown that there is some sacrifice in performance relative to a filled, circular aperture.

Active correction of a thermal lens in a solid-state laser. I. Metal mirror with a controlled curvature of the central region of the reflecting surface

Abstract: A metal active mirror with the curvature of its central region variable within wide limits was proposed, made, and investigated. Promising applications of such a mirror in a solid-state laser resonator for correction of the thermal lens effects were demonstrated.

High-bandwidth control for low-area-density deformable mirrors

Abstract: The progress to date and essential features of approaches to optics and controlled structures technology (CST) are reviewed. The CST framework is suggested as a means of gaining new insight on deformable optical surface control. Within the CST approach, control bandwidths may extend into the frequency range of structural vibration modes, enabling possible reduction in area density and improvements in performance.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Imaging performance analysis of adaptive optical telescopes using laser guide stars

Abstract: The use of laser guide stars in conjunction with adaptive optical telescopes offers the possibility of nearly diffraction-limited imaging performance from large, ground-based telescopes. We investigate the expected imaging performance of an adaptive telescope, using laser guide stars created in the mesospheric sodium (Na) layer. A 2–3-m class telescope is analyzed for the case of a single, on-axis guide star at an altitude of 92 km (the nominal height of the mesospheric Na layer). We analyze an annular telescope pupil with ∼15 wave-front sensor subapertures and mirror actuators spanning the pupil diameter. The imaging performance is quantified in terms of the pupil-averaged rms wave-front error, the optical transfer function, the point spread function, the Strehl ratio, and finally the angular resolution. The performance analysis takes into account the degradation caused by the limitation of the wave-front sensor as well as the deformable mirror. These limitations include the finite spacing and size of the wave-front sensor subapertures and the spacing and influence function of the mirror actuators. The effects of anisoplanatism and shot noise are also included in the analysis. The results of the investigation indicate that a 3-m adaptive telescope using a single Na guide star is capable of achieving a Strehl ratio of 0.57 and an angular resolution nearly matching that of diffraction-limited performance (0.05 arcsec). This performance is achieved assuming that r0 = 20 cm and a 5-W laser is used to create the guide star. The effect of variations in seeing conditions and guide star brightness is also investigated.

Design and performance optimization of fiber optic adaptive filters.

Abstract: There is a great need for easy-to-fabricate and versatile fiber optic signal processing systems in which optical fibers are used for the delay and storage of wideband guided lightwave signals. We describe the design of the least-mean-square algorithm-based fiber optic adaptive filters for processing guided lightwave signals in real time. Fiber optic adaptive filters can learn to change their parameters or to process a set of characteristics of the input signal. In our realization we employ as few electronic devices as possible and use optical computation to utilize the advantages of optics in the processing speed, parallelism, and interconnection. Many schemes for optical adaptive filtering of electronic signals are available in the literature. The new optical adaptive filters described in this paper are for optical processing of guided lightwave signals, not electronic signals. We analyzed the convergence or learning characteristics of the adaptive filtering process as a function of the filter parameters and the fiber optic hardware errors. From this analysis we found that the effects of the optical round-off errors and noise can be reduced, and the learning speed can be comparatively increased in our design through an optimal selection of the filter parameters. A general knowledge of the fiber optic hardware, the statistics of the lightwave signal, and the desired goal of the adaptive processing are enough for this optimum selection of the parameters. Detailed computer simulations validate the theoretical results of performance optimization.