Showing papers on "Adaptive optics published in 1992"

Basic Wavefront Aberration Theory for Optical Metrology

Abstract: VIII. IX. X. XI. XII. Sign Conventions Aberration-Free Image Spherical Wavefront, Defocus, and Lateral Shift Angular, Transverse, and Longitudinal Aberration Seidel Aberrations A. Spherical Aberration B. Coma C. Astigmatism D. Field Curvature E. Distortion Zernike Polynomials Relationship between Zernike Polynomials and Third-Order Aberrations Peak-to-Valley and RMS Wavefront Aberration Strehl Ratio Chromatic Aberrations Aberrations Introduced by Plane Parallel Plates Aberrations of Simple Thin Lenses 2 4 9 12 15 18 22 24 26 28 28

Laser guide star in adaptive optics: the tilt determination problem.

Abstract: The use of laser beacons to create artificial reference stars for adaptive optics has been successfully demonstrated by several experiments (Fugate et al., 1991, Nat., 353; Primmerman et al., 1991, Nat., 353). Nevertheless, laser beacons do not allow the global wave-front tilt determination, which relies then on the availability of a natural reference star. Taking into account the photon noise and the anisoplanatic errors, we compare the performance of adaptive optics correction using natural or artificial (laser) reference star

Atmospheric Fluctuations: Empirical Structure Functions and Projected Performance of Future Instruments

Abstract: The extension of high quality astronomical observations towards larger apertures, adaptive optics, and infrared wavelengths leads to extrapolation of present knowledge of astronomical «seeing» by means of theoretical models, such as Kolmogorov turbulence combined with Taylor's «frozen atmosphere» swept past the observer by winds. Observations of path length fluctuations from a star to a two-telescope spatial interferometer at 11 μm wavelength, and also measurements of path length fluctuations 3 m above the ground by laser distance interferometers, show substantial deviations from such a model. Intermittent turbulence may be involved and relatively short outer scales are frequently indicated

Simple adaptive optical system

Abstract: A system for correcting wavefront distortion includes an eight channel deformable mirror 11 attached to a two-axis, tip/tilt mount 12. The deformable mirror 11 reflects an incoming telescopic wavefront 10 onto a beamsplitter 14. The beamsplitter 14 divides the reflected telescopic wavefront 13 into two separate beams 15, 20. The beam 20 reflected by the beamsplitter 14 may be used for experimentation and measurement. The beam 15 transmitted by the beamsplitter 14 is directed toward a modified Hartmann-Shack wavefront sensor 16. The modified Hartmann-Shack sensor 16 detects the slope of the wavefront at several locations across this transmitted beam 15 and provides analog signals 17 representing these slopes to a series of actuator drive circuits 18. The actuator drive circuits 18 provide excitation signals 19 to actuators 22, 30 on the deformable mirror 11 and the two-axis, tip/tilt mount 12. These actuators 22, 30 facilitate the reforming and repositioning of the deformable mirror 11 so as to correct any incoming wavefront 10 distortion.

Microlens arrays for Shack-Hartmann wavefront sensors

Abstract: Microlens arrays are a component of atmospheric wavefront sensors used recently with success by ground-based astronomers to obtain diffraction-limited images of stars in spite of wavefront disturbances introduced by the earth's atmosphere. Typical requirements for individual lenses are a size measured in tenths of millimeters, a sag measured in micrometers, and no room left between lenses, i.e., square or hexagonal lenses rather than circular lenses. Actual monolithic square microlens arrays up to 20 x 20 mm in size for 0.18- to 1.5-mm individual contiguous lenses of 0.0035-mm maximum sag are engraved in photoresist coatings by a two-axes rastering process. These arrays have been used for diurnal and nocturnal atmospheric wavefront measurements.

International Trends in Optics

Abstract: Integrated optics - OEICs or PICs?, H.Kogelnik quantum opto-electronics for optical processing, D.A.B.Miller optics in telecommunications - beyond transmission, P.W.E.Smith microoptics, K.Iga holographic optical elements for use with semiconductor lasers, H.P.Herzig and R.Dadliker fiber optic signal processing, B.Culshaw and I.Adonovic optical memories, Y.Tsunado what are photorefractives good for?, H.Rajbenbach and J.-P.Huignard adaptive interferometry - a new area of applications of photorefractive crystals, S.I.Stepanov water wave optics, J.J.Stamnes about the philosophies of diffraction, A.W.Lohmann the essential journals of optics, J.N.Howard optics in China - ancient and modern accomplishments, Z.-M.Zhang unusual optics - optical interconnects as learned from the eyes of (nocturnal insects), crayfish, shellfish and similar creatures, P.Greguss the opposition effect in volume and surface scattering, K.C.Dainty influence of source-correlations on spectra of radiated fields, E.Wolf quantum statistics and coherence of nonlinear optical processes, J.Perina one photon light-pulses versus attenuated classical light-pulses, A.Aspect and P.Grangier optical propagation through the atmosphere, A.Consortini are the fundamental principles of holography sufficient for the creation of new types of 3D cinematography and artificial intelligence?. Y.N.Denisyuk medical applications of holographic 3D display, J.Tsujiuchi Moire fringes and their applicatons, O.Bryngdahl breaking the boundaries of optical system design and construction , C.H.F.Velzel interferometry - what's new since Michelson?, P.Hariharan curreent trends in optical testing, D.Malacara adaptive optics, F.Merkel triple correlations and dispectra in high-resolution astonomical imaging. H.Weigelt phase retrieval imaging problems, J.R.Fienup blind deconvolution - recovering the seemingly irrecoverable!, R.H.T.Bates and H.Jiang pattern recognition, similarity, neural nets, and optics, H.H.Arsenault and Y.Sheng towards nonlinear optical processing, T.Szoplik and K.Chalasinska-Macukow new apects of optics for optical computing, V.Morozov digital optical computing, S.D. Smith computing - a joint venture for light and electricity?, P.Chavel.

Solar imaging with a segmented adaptive mirror

Abstract: A 19-segment adaptive-mirror system is currently being used on the Sacramento Peak 76-cm Tower Telescope to remove wave-front distortions resulting from atmospheric turbulence. The system has proven to be capable of substantially improving the quality of an image, at times achieving 0.33-arcsec resolution in visible wavelengths under 1–3-arcsec seeing conditions. An improvement in resolution seems to occur across a large field of view that is, at times, 30 arcsec in diameter.

Power spectrum and Fourier phase spectrum estimation by using fully and partially compensating adaptive optics and bispectrum postprocessing

Abstract: The use of predetection compensation for the effects of atmospheric turbulence combined with postdetection image processing for imaging applications with large telescopes is addressed. Full and partial predetection compensation with adaptive optics is implemented by varying the number of actuators in the deformable mirror. The theoretical expression for the single-frame power spectrum signal-to-noise ratio (SNR) is reevaluated for the compensated case to include the statistics of the compensated optical transfer function. Critical to this analysis is the observation that the compensated optical transfer function does not behave as a circularly complex Gaussian random variable except at high spatial frequencies. Results from a parametric study of performance are presented to demonstrate improvements in power spectrum estimation for both point sources and an extended object and improvements in the Fourier phase spectrum estimation for an extended object. Full compensation is shown to provide a large improvement in the power spectrum SNR over the uncompensated case, while successively smaller amounts of predetection compensation provide smaller improvements, until a low degree of compensation gives results essentially identical to those of the uncompensated case. Three regions of performance were found with respect to the object Fourier phase spectrum estimate obtained from bispectrum postprocessing: (1) the fully compensated case in which bispectrum postprocessing provides no improvement in the phase estimate over that obtained from a fully compensated long-exposure image, (2) a partially compensated regime in which applying bispectrum postprocessing to the compensated images provides a phase spectrum estimation superior to that of the uncompensated bispectrum case, and (3) a poorly compensated regime in which the results are essentially indistinguishable from those of the uncompensated case. Accurate simulations were used to obtain some parameters for the power spectrum SNR analysis and to obtain the Fourier phase spectrum results.

Atmospheric turbulence and the resolution limits of large ground-based telescopes

Abstract: Conventional theory of imaging through the atmosphere is based on two main assumptions: (1) atmospheric turbulence is assumed to follow a Kolmogorov spectrum and (2) the outer scale, Lo, is assumed to be much larger than any telescope. There are numerous reports in the literature, however, of image properties that are not consistent with this theory—for example, cores in star images and lack of expected image motion. In almost every case these reports are consistent with a smaller value of Lo. There is also evidence of smaller Lo from other, more direct sources such as balloonborne temperature probes and long-baseline interferometry. If Lo is smaller than previously thought, as is suggested here, many long-held ideas about imaging with ground-based telescopes will have to be modified. A much more favorable picture emerges, especially at near-infrared wavelengths. At these wavelengths, resolution in the range 0.03–0.1 arcsec should be routinely attainable with 4–10-m telescopes, even though seeing at visible wavelengths is only 1 arcsec. To attain such high levels of resolution, telescopes must be built to diffraction-limited standards rather than to the currently accepted standards, which fall well short of this limit. Recent images obtained at 2.2 μm with the 4-m Kitt Peak telescope show that very high resolution (0.1 arcsec) is attainable. The images also show that telescope aberrations prevent even higher resolution (0.05 arcsec). A further benefit of a smaller Lo is that the isoplanatic angle of the atmosphere at near-infrared wavelengths is likely to be much larger than previously thought. Thus much wider angular regions are available from which to select suitably bright stars for guiding and tracking. A small Lo also means that ground-based infrared laser beams may be focused to diffraction-limited accuracy on targets in space without necessarily having to use wave-front compensation.

Deformable mirrors: design fundamentals, key performance specifications, and parametric trades

Abstract: A wide variety of deformable mirror structures have been studied for wavefront correction since the advent of adaptive optics nearly two decades ago. These structures generally fall into two categories: (1) segmented facesheet and (2) continuous facesheet. The segmented mirror technology features independently activated mirror elements controlled in the piston, tip, and tilt modes. The continuous facesheet designs use discrete electroceramic or electrostatic displacement actuators arranged in either an axial or bimorph position to bend the continuous facesheet. In addition there are two methods of correction: (1) zonal control and (2) modal control. The basic mirror types are discussed and analyzed in terms of wavefront correction capabilities. Curve fitting characteristics are explained in terms of the optical influence function and mirror meshing functions. The continuous facesheet deformable mirror is used as a model to develop basic design equations which are used for parametric trades.

Adaptive optics using the electrooptic effect

Abstract: An adaptive electrooptical lens system for use in optical data storage systems, optical phased arrays, laser or other optical projectors, and raster scanning devices, and the like. The invention provides an electrooptical means for scanning an optical beam or moving an optical storage or retrieval point. Beam movement is achieved electrooptically, by changing the index of refraction of an electrooptical material by controlling electric fields applied thereto. A plurality of electrodes are disposed on one surface of the electrooptic material and a ground electrode is disposed on the other. The electrodes are adapted to apply electric fields derived from a voltage source to the electrooptic material that selectively changes its index of refraction and provides for a predetermined index of refraction profile along at least one dimension thereof, thus forming a lens. By appropriately forming the electrode pattern and properly controlling the voltages applied thereto, differing lens shapes may be formed. Since the response times of the electrooptic materials employed in the present invention are on the order of nanoseconds (10-9 sec) or less, the intrinsic response frequency of the lens system is 109 Hz or more. The present invention thus increases the data storage and retrieval capacity of optical systems in which it is employed.

Linear reconstruction of compensated images: theory and experimental results.

Abstract: Linear image reconstruction techniques are proposed for postprocessing astronomical images measured with compensated imaging systems. Linear techniques use averaging to overcome the effects of noise and deconvolution to remove system effects. Experimental results from compensated image measurements of four single stars and one binary star at visible wavelengths are reported for the first time, to our knowledge, and a previously derived analytic expression relating the statistics of the compensated optical transfer function to the compensated image spectrum signal-to-noise ratio is verified. The performance of deconvolution on a bright binary star with angular subtense previously estimated to be 0.52 arcsec (2.52 μrad) is demonstrated.

Phase variance and Strehl ratio in adaptive optics

Abstract: Some correction schemes in adaptive optics are performed with the use of the phase reconstructed from measured gradients. For these cases it can be shown that the minimum phase-variance estimator leads to the maximum Strehl ratio, that is, to the maximum peak intensity in the far field.

Moonbase night power by laser illumination

Abstract: Moonbase solar-power concepts must somehow address the energy storage problem posed by the 354-hour lunar night. Attention is presently given to the feasibility of laser-array illumination of a lunar base, using technology that is projected to be available in the near term. Beam-spreading due to atmospheric distortions could be reduced through the use of adaptive optics to compensate for atmospheric turbulence.

First Results of an On-Line Adaptive Optics System with Atmospheric Wavefront Sensing by an Artificial Neural Network

Abstract: The first results from an adaptive optics system operating on-line at the telescope with the wavefront aberration sensed by a trained artificial neural network are presented. Star images were formed at 2.2 μm wavelength by two coherently phased apertures of the Multiple Mirror Telescope (MMT), and analyzed by the neural net. The net derives wavefront parameters in a few milliseconds, and the system performance is fast enough that the aberration is nearly frozen during the time needed to make a correction. With the servo loop in operation, the corrected image shows significant power at the diffraction limit of 0.1″

High-resolution ground-based coronagraphy using image-motion compensation.

Abstract: The first results of a new approach to ground-based stellar coronagraphy are reported. A coronagraph has been equipped with an image-motion compensation system for the stabilization of the telescope field, permitting both improved image resolution and contrast at optical wavelengths. By stopping the telescope aperture D to ~ 4 r0, where r0 is Fried’s parameter, the maximum attainable resolution gain factor of 2.2 was achieved. Gains measured for D/r0 > 14 were below the theoretical value of 1.3 theory and were indicative of centroid anisoplanatism, a small spatial coherence outer scale, or both. These effects are also evidenced by diminished power at low frequencies in the power spectrum of image motion over the full telescope aperture. A comparison of stabilized and unstabilized images shows that this coronagraph may detect circumstellar objects 2 magnitudes fainter than those detectable with a conventional coronagraph.

Speckle imaging of satellites at the U.S. Air Force Maui Optical Station

Abstract: Results are presented from a series of experiments in which the U.S. Air Force Maui Optical Station’s 1.6-m telescope and a bare CCD speckle camera system were used to image satellites at distances of up to 1000 km. A brief overview of the image reconstruction algorithms is presented. The choice of the experiment site and various imaging parameters are described. Power spectra and power spectral signal-to-noise ratio curves that result from imaging several point stars are compared with theory. Reconstructed images of several binary stars are shown as a base-line assessment of our technique. High-quality image reconstructions of an Earth-satellite, the Hubble Space Telescope, are presented. The results confirm that speckle imaging techniques can be used with a bare CCD imaging system to provide a powerful and flexible method for imaging objects of moderate magnitude.

Two-stage optics : high-acuity performance from low-acuity optical systems

Abstract: The concept of two-stage optics, developed under a program to enhance the performance, lower the cost, and increase the reliability of the 20-m Large Deployable Telescope, is examined. The concept permits the large primary mirror to remain as deployed or as space-assembled, with phasing and subsequent control of the system done by a small fully assembled optical active element placed at an exit pupil. The technique is being applied to correction of the fabrication/testing error in the Hubble Space Telescope primary mirror. The advantages offered by this concept for very large space telescopes are discussed.

Overcoming perspective elongation effects in laser-guide-star-aided adaptive optics.

Abstract: Tracking the instantaneous position of the laser guide star removes its perspective elongation thus allowing an increased separation of the laser transmitter from its telescope.

Experimental demonstration of a Rayleigh-scattered laser guide star at 351 nm.

Abstract: A commercially manufactured excimer laser was used to project a focused laser beam into the atmosphere to altitudes in the range of 10 to 30 km. Rayleigh-backscattered light from a slice of the illuminated column, when imaged in a ground-based telescope, produced an image that is essentially identical to that of a natural star. This experiment demonstrates the ease of producing bright and sharply focused laser guide stars at 351 nm. Such laser guide stars are ideal for guiding astronomical telescopes equipped with adaptive optics.

Continuously operated spatial light modulator apparatus and method for adaptive optics

Abstract: A spatial light modulator that is particularly suited for adaptive optics systems includes a charge transfer plate in which accumulated charge is continuously drained from the charge transfer pins, allowing the modulator to be operated in a continuous mode with a very rapid speed of response. Charge is drained through RC circuits that consist of inherent or discrete resistors and inherent capacitances associated with the pins. A lenslet array focuses an input beam onto the curvature bases of pixels in an associated deformable mirror, thereby obtaining reflections from the mirror with generally flat wavefronts and a greatly increased optical efficiency; a companion lenslet array and mirror combination outside the SLM compensates pixel inversions produced by the first lenslet array. The charge transfer plate is made gas impervious by fabricating the charge transfer pins as thermally migrated conductors, and conductive electric shield pins are preferably also provided to shield the charge transfer pins from each other.

Modeling of the image observed through a turbulent atmosphere

Abstract: Computer simulations of a wavefront distorted by atmospheric turbulence are considered focusing on large scale and dynamic simulations. Attention is also given to wavefront sensor and wavefront corrector simulations. Computer modeling of adaptive optical systems takes into account an optical wave field in the plane of receiving aperture, a wavefront distortion sensor, a wavefront distortion corrector, and quantum fluctuation of the optical wave intensity.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical metrology for two large highly aspheric telescope mirrors

Abstract: We describe a relatively simple, but highly effective, approach to the system design and alignment of an all-refractive Offner null corrector and phase-measuring Shack cube interferometer. In addition we outline procedures for fabricating and testing the optical components. Allowable errors for all parameters are determined by a tolerance analysis that separates axisymmetric and residual figure errors. An open construction optics frame provides a high degree of metering flexibility by incorporating simple kinematic mounts that provide adjustment of each lens while also allowing the lens to be removed and replaced with < 2-μm absolute repeatability. Nonaxisymmetric alignment errors are removed by rotating the optics on a high-precision bearing. Axial spacings are measured with contact transducers attached to both ends of an Invar metering rod. Two completed systems have guided the stressed-lap polishing of 1.8-mf/1.0 and 3.5-m f/1.5 aspheric mirrors.

Making Stars to See Stars: DOD Adaptive Optics Work is Declassified

Abstract: Since the 1970s, researchers working first for the Defense Advanced Research Projects Agency and later under the aegis of the Strategic Defense Initiative have been developing and testing adaptive optics systems—systems for nullifying the effects of atmospheric turbulence on light that passes through it. In May 1991, to the delight of the astronomical community, much of the work became declassified. Adaptive optics works by measuring the distorting effects of the atmosphere on the light from a guide star and adjusting a deformable mirror to conjugate these effects. In particular, the declassified research involves the use of laser beams to create one or more artificial beaconsin the sky to act as guide stars.

The time-domain analysis simulation for advanced tracking (TASAT) approaches to compensated imaging

Abstract: The TASAT code is a complete end-to-end system simulation of tracking and pointing systems. In its ground-based laser (GBL) system configuration, the code will treat tracking and imaging systems that look through atmospheric turbulence at high fidelity. In particular, the effects of atmospheric turbulence, adaptive optics servo lag, anisoplanatism, and deformable mirror (DM) fitting error are treated with a novel time average point-spread function (PSF). Tracking and imaging sensors are treated as a sequence of physical effects, including allocation of the image energy to pixels with finite dead bands, sensor noise effects, and integrating sensor dwell with analog-to-digital conversion. We treat the physics of atmospheric turbulence and adaptive optics in some detail in this paper, and discuss results for variations in system bandwidth, actuator spacing, atmospheric coherence length, and anisoplanatism effects. The DM is treated as a spatial and temporal transfer function acting on Kolmogorov atmospheric turbulence. We develop the time average PSF using the residual atmospheric structure function. Finally, we use the convolution of the PSF with realistic satellite imagery to assess tracking and imaging performance.

Scalable aspheric corrective mirror for end-pumped solid-state lasers.

Abstract: A procedure is presented to design an aspheric corrective mirror to remove the effects of thermally induced optical aberrations in end-pumped solid-state lasers. The design is based on solving the inverse problem of bending a thin plate of variable thickness; i.e., given the plate deflection profile a thickness profile must be calculated by solving the differential equation for bending. The advantage of this type of aberration correction is the fact that it can be scaled to different pump powers during operation while still matching the aspheric profile in question. Guidelines for fabrication of the mirror are also presented.