Showing papers on "Adaptive optics published in 1994"

Adaptive Optics in Astronomy

Abstract: Optical observations by ground‐based astronomers have long been limited by the distorting effects of the Earth's atmosphere. Primary mirrors have been polished to exquisite accuracy for telescopes with apertures as large as 10 meters, but at optical wavelengths these can deliver an angular resolution typically no better than that of a 25‐cm telescope, as atmospheric turbulence deforms the image on a millisecond time scale. One (highly expensive) approach to overcome this problem has been to loft instruments such as the Hubble Space Telescope above the atmosphere. Another approach, pursued by instrument builders in the astronomy community and their counterparts in the military, has been to design electro‐optical systems that measure and undo the effects of clear‐air turbulence in real time. (See figure 1.) A number of such adaptive optic devices have already been built and operated on large ground‐based telescopes, delivering near‐diffraction‐limited performance at infrared and visible wavelengths. With th...

First-order performance evaluation of adaptive-optics systems for atmospheric-turbulence compensation in extended-field-of-view astronomical telescopes.

Abstract: An approach is presented for evaluating the performance achieved by a closed-loop adaptive-optics system that is employed with an astronomical telescope. This method applies to systems incorporating one or several guide stars, a wave-front reconstruction algorithm that is equivalent to a matrix multiply, and one or several deformable mirrors that are optically conjugate to different ranges. System performance is evaluated in terms of residual mean-square phase distortion and the associated optical transfer function. This evaluation accounts for the effects of the atmospheric turbulence Cn2(h) and wind profiles, the wave-front sensor and deformable-mirror fitting error, the sensor noise, the control-system bandwidth, and the net anisoplanatism for a given constellation of natural and/or laser guide stars. Optimal wave-front reconstruction algorithms are derived that minimize the telescope’s field-of-view-averaged residual mean-square phase distortion. Numerical results are presented for adaptive-optics configurations incorporating a single guide star and a single deformable mirror, multiple guide stars and a single deformable mirror, or multiple guide stars and two deformable mirrors.

Ground-based imaging of extrasolar planets using adaptive optics

Abstract: The detection of extrasolar planets by direct imaging presents an extraordinary technical challenge. They must be identified against background light scattered from a star close by and about a billion times brighter. It has been supposed that a near-perfect space telescope would be required to avoid atmospheric blurring. But by using adaptive optics operating at fundamental performance limits, the new generation of large ground-based telescopes has the potential to detect planets orbiting nearby stars.

Two generations of laser-guide-star adaptive-optics experiments at the Starfire Optical Range

Abstract: We report results that were obtained with two generations (Generation I and Generation II) of a laser-guide-star adaptive-optics system that is capable of continuous compensation at 65-Hz (Generation I) and 130-Hz (Generation II) closed-loop bandwidths on a 1.5-m telescope. We used a copper-vapor laser that was focused at a 10-km range as the laser guide star and a range-gated Shack–Hartmann sensor to operate a continuous-facesheet deformable mirror that controlled either 149 or 241 actuators. We used a separate full-aperture sensor and a steering mirror to remove overall tilt. System performance was measured by imaging stars with a high-resolution CCD camera in a narrow spectral band that was centered at 0.88 μm, from which we computed point-spread functions, optical transfer functions, and Strehl ratios. Using the laser guide star, we achieved a FWHM image resolution of 0.13 arcsec and a Strehl ratio of 0.48. Using a natural guide star, we achieved a Strehl ratio of 0.64 at 0.13 arcsec FWHM resolution. We also obtained compensated images of the Trapezium region in Orion in H-α light, using only the laser guide star.

Analysis of multiconjugate adaptive optics.

Abstract: We investigate a method for widening the compensated field of view of an adaptive-optical telescope with multiple deformable mirrors and an array of artificial guide stars. An ensemble of wavefront sensor measurements, made with the individual guide stars in the array, is used to estimate the contribution of a region of the atmosphere to the cumulative phase distortion. Our analysis includes the effects of measurement noise, wavefront-sensor sampling, and reconstruction of the wave front from slope measurements. We performed numerical computations for an atmosphere consisting of two turbulent layers: one at 1% of the guide-star altitude with 90% of the total turbulence strength and one at 10% of the guide-star altitude with the remaining 10% of the total turbulence strength. If we assume that r0 = 0.15 m and that a photon-limited wave-front sensor detecting 50 photons/r0-sized subaperture is used, the results indicate that a 0.9-m-square telescope with a diagonal field of view of ~92 μrad ≈ 19 arcsec can use two deformable mirrors, four laser guide stars, and a natural tilt reference star to achieve an rms residual phase error that is <λ/7 over its entire field of view.

Laser-guide-star systems for astronomical applications

Abstract: With computational techniques developed in the investigation of high-energy laser-beam-control systems, a set of concise analytic models describing the essential properties of a laser-guide-star phase-conjugation system has been assembled. With the aid of these models an optimization strategy for mating adaptive optics to a 4-m-class optical telescope is evolved, and it is shown that such a system might be expected to improve the effective atmospheric seeing conditions by nearly a factor of 10 within the isoplanatic patch of the turbulence probe. For operation at visible wavelengths, a compensation system having ~300 actuators and a closed-loop bandwidth of 20 Hz is recommended. All the key hardware components have already been built and tested, with the exception of a suitable laser source for high-repetition-rate illumination of the Earth’s sodium layer.

Atmospheric-turbulence compensation by resonant optical backscattering from the sodium layer in the upper atmosphere

Abstract: A new concept is described for creating artificial beacons for atmospheric-turbulence compensation. A laser tuned to the sodium D2 line is used to excite resonant optical backscattering from the sodium layer in the upper atmosphere. The performance of such a system is compared with that expected from artificial beacons based on Rayleigh scattering in the lower atmosphere. It is found that sodium scattering has a more favorable scaling to large-aperture systems. Applications to the compensation of ground-based astronomical telescopes for atmospheric turbulence are described.

Adaptive optics for diffraction-limited infrared imaging with 8-m telescopes

Abstract: When equipped with adaptive optics, the coming generation of large 6–10-m telescopes can combine huge light grasp with very sharp images. We describe a specific design concept for recovery of diffraction-limited images in the 1.6- and the 2.2-μm atmospheric windows, yielding 0.05-arcsec resolution for an 8-m telescope. Our goal has been to achieve this performance routinely by not requiring above-average atmospheric conditions or the use of unusually bright nearby stars. Atmospheric blurring is sensed with a sodium laser beacon of a few watts. Image motion is sensed by starlight, with a quadrant detector that is sensitive to the broad infrared band in which photon flux is typically largest and the field star has been sharpened by laser-beacon correction that is shared with the science target. A detailed performance analysis shows that for typical conditions Strehl ratios of >25% are expected at 2.2 μm, with the probability of finding a sufficiently bright field star exceeding 50%.

Phase-diversity wave-front sensor for imaging systems.

Abstract: A phase-diversity wave-front sensor has been developed and tested at the Lockheed Palo Alto Research Labs (LPARL). The sensor consists of two CCD-array focal planes that record the best-focus image of an adaptive imaging system and an image that is defocused. This information is used to generate an object-independent function that is the input to a LPARL-developed neural network algorithm known as the General Regression Neural Network (GRNN). The GRNN algorithm calculates the wave-front errors that are present in the adaptive optics system. A control algorithm uses the calculated values to correct the errors in the optical system. Simulation studies and closed-loop experimental results are presented.

Accuracy analysis of a Hartmann-Shack wavefront sensor operated with a faint object

Abstract: A detailed analysis of the characteristics, regularities, and relationships of the centroiding errors of image spots caused by discrete and limited sampling, photon noise, and readout noise of the detector in a Hartmann-Shack wavefront sensor, wherein an image intensified charge-coupled device used as a photon detector is presented. The theoretical analysis and experimental results herein prove useful for optimum design and application of the sensor.

Liquid-crystal prisms for tip-tilt adaptive optics.

Abstract: Results from an electrically addressed liquid-crystal cell producing continuous phase profiles are presented. The adaptive deflection of a beam of light for use in a tip-tilt adaptive optics system is demonstrated. We compare the optical performance of liquid-crystal prisms with experimental data on atmospheric seeing at the William Herschel Telescope.

Optimizing closed-loop adaptive-optics performance with use of multiple control bandwidths

Abstract: The performance of a closed-loop adaptive-optics system may in principle be improved by selection of distinct and independently optimized control bandwidths for separate components, or modes, of the wave-front-distortion profile. We describe a method for synthesizing and optimizing a multiple-bandwidth adaptive-optics control system from performance estimates previously derived for single-bandwidth control systems operating over a range of bandwidths. The approach is applicable to adaptive-optics systems that use either one or several wave-front sensing beacons and also to systems that include multiple deformable mirrors for atmospheric-turbulence compensation across an extended field of view. Numerical results are presented for the case of an atmospheric-turbulence profile consisting of a single translating phase screen with Kolmogorov statistics, a Shack–Hartmann wave-front sensor with from 8 to 16 subapertures across the aperture of the telescope, and a continuous-face-sheet deformable mirror with actuators conjugate to the corners of the wave-front-sensor subapertures. The use of multiple control bandwidths significantly relaxes the wave-front-sensor noise level that is permitted for the adaptive-optics system to operate near the performance limit imposed by fitting error. Nearly all of this reduction is already achieved through the use of a control system that uses only two distinct bandwidths, one of which is the zero bandwidth.

Analysis of fundamental limits to artificial-guide-star adaptive-optics-system performance for astronomical imaging

Abstract: The concept of using the atmospheric backscatter from a pulsed laser as an artificial guide star (AGS) for an adaptive-optics system in an astronomical telescope is analyzed. The extent to which such an AGS can be used to provide the information that is needed for adaptive-optics-system compensation of a wave front that is distorted by propagation through the atmosphere is studied. Attention is directed to the effect of focus anisoplanatism, the measurement error that is introduced when the probe light from the AGS at a finite range travels a different path than does the light from an astronomical object at a larger (infinite) range. Because of focus anisoplanatism there is a residual wave-front error that the wave-front-distortion sensor, relying on the AGS, is unable to sense and for which the adaptive-optics system is therefore unable to compensate. This residual wave-front error has a mean-square value that takes the form σφFA2 = (D/d0)5/3, where d0 is an aperture-diameter-sized quantity that measures the magnitude of the effect of focus anisoplanatism. The value of d0 depends on the vertical distribution of the optical strength of turbulence, the optical wavelength of the imaging system, the zenith angle, and the backscatter altitude. An expression is given for d0, and sample results for d0 are presented. Analytic results are also developed for the effect of focus anisoplanatism on the Strehl definition (or normalized antenna gain) of the imaging system. Numerical results are presented for the normalized antenna gain for a wide variety of backscatter altitudes and for several vertical distributions of the optical strength of turbulence. These results indicate that the diameter dependence of the normalized antenna gain is fully expressed as a function of D/d0. The peak achievable antenna gain is approximately equal to 40% of that of a diffraction-limited system with an aperture diameter that is equal to d0, and the peak is achieved with an actual aperture diameter in the range of (7/6)d0 to (9/6)d0.

Wave-front correction by one or more synthetic beacons

Abstract: Adaptive optics has been used in a cooperative mode to measure the phase distortion of the light from a star and to correct its image with a deformable mirror. A wave-front sensor in the adaptive-optics system that measures the phase aberrations requires that an object be fairly bright for accurate performance of the measurement. The use of synthetic beacons provides a means of correcting the images of objects that are too dim to allow one to use their light to provide correction in a cooperative mode. Synthetic beacons at a finite distance do not provide a perfect correction in imaging an object at a greater distance. The error in making a correction with one or more beacons is analyzed. Analytical expressions that can be used to determine performance in a variety of geometries, with various beacon altitudes and numbers, are derived. This analysis is applied to 60-cm and 4-m systems.

Current trends in optics

Abstract: Atomic optics, S.M. Tan and D.R. Walls single atoms in cavities and traps, H. Walther meet a squeezed state and interfere in phase space, D. Krahmer et al can light be localized?, A. Lagendijk time-resolved laser induced breakdown spectroscopy, G. Lupkovics et al fractal optics, J. Uozumi and T. Asakura on the spatial parametric characterization of general light beams, R. Martinez-Herrero and P.M. Mejias to the unseen - vision in scattering media, E.P. Zege and I.L. Katsov backscattering through turbulence, A.S. Gurvich and A.N. Bogatorov why is the Fresnel transform so little known?, F. Gori Fourier curios, A.W. Lohmann the future of optical correlators, D. Casasent spectral hole burning and optical information processing, K.K. Rebane holographic storage re-visited, G.T. Sincerbox colour information in optical pattern recognition, M-J. Yzuel and J. Campos the optics of confocal microscopy, C.J.R. Sheppard diffraction unlimited optics, A. Lewis super-resolution in microscopy, V.P. Tychinsky and C.H.F. Velzel fringe analysis - anything new?, M. Kujawinska diagnosing the aberrations of the Hubble Space Telescope, J.R. Fienup laser beacon adaptive optics - boon or bust?, R.Q. Fugate

Image-spectrum signal-to-noise-ratio improvements by statistical frame selection for adaptive-optics imaging through atmospheric turbulence

Abstract: Adaptive-optics systems have been used to overcome some of the effects of atmospheric turbulence on large-aperture astronomical telescopes. However, the correction provided by adaptive optics cannot restore diffraction-limited performance, due to discretized spatial sampling of the wavefront, limited degrees of freedom in the adaptive-optics system, and wavefront sensor measurement noise. Field experience with adaptive-optics imaging systems making short-exposure image measurements has shown that some of the images are better than others in the sense that the better images have higher resolution. This is a natural consequence of the statistical nature of the compensated optical transfer function in an adaptive-optics telescope. Hybrid imaging techniques have been proposed that combine adaptive optics and postdetection image processing to improve the high-spatial-frequency information of images. Performance analyses of hybrid methods have been based on prior knowledge of the ensemble statistics of the underlying random process. Improved image-spectrum SNRs have been predicted, and in some cases experimentally demonstrated. In this paper we address the issue of selecting and processing the best images from a finite data set of compensated short-exposure images. Image sharpness measures are used to select the data subset to be processed. Comparison of the image-spectrum SNRs for the cases of processing the entire data set and processing only the selected subset of the data shows a broad range of practical cases where processing the selected subset results in superior SNR.

Wavefront reconstruction optics for use in a disk drive position measurement system

Abstract: An optical measurement system for use in a data recording disk drive includes wavefront reconstruction optics which correct and compensate for aberrations in laser beams reflected from radial and linear diffraction gratings. The reconstruction optics include two spherical lenses and two reflectors, one of each positioned on each side of a diffraction grating. Light directed from a laser beam toward the grating diffracts into a +1 order component directed at the first set of reconstruction optics and into a -1 order component directed at the second set of reconstruction optics. The reconstruction optics compensate for the variable yaw angle caused by a linear grating and for the optical wavefront aberrations caused by a radial grating. Use of the reconstruction optics permits an optical measurement system in a data recording disk drive to accurately measure the position of either a linear or rotary actuator and to generate a precise reference clock for use in timing the recording of servo information to the data recording disk.

Shearing interferometry for laser-guide-star atmospheric correction at large D/r 0

Abstract: Shearing interferometry offers a possible method to scale wave-front sensors to the large number of subapertures that will be required for correction of the new generation of large 6–10-m-class telescopes at visible wavelengths. We discuss static shearing interferometers, which are prototype wave-front sensors for use with laser guide stars at large values of D/r0. The dc interferometers utilize low-noise detector array technology and can be implemented with a variety of shear lengths to accommodate different atmospheric conditions. We discuss the optical design and the noise sensitivity of two versions of dc interferometer. Atmospheric tests of the interferometers for correcting a 500-subaperture adaptive system are presented, including fringe data obtained with ultraviolet and visible laser stars and compensated ultraviolet images of Vega obtained at our test facility in San Diego.

Effect of particulates on performance of optical communication in space and an adaptive method to minimize such effects.

Abstract: Decreased signal-to-noise ratio and maximum bit rate as well as increased in error probability in optical digital communication are caused by particulate light scatter in the atmosphere and in space. Two effects on propagation of laser pulses are described: spatial widening of the transmitted beam and attenuation of pulse radiant power. Based on these results a model for reliability of digital optical communication in a particulate-scattering environment is presented. Examples for practical communication systems are given. An adaptive method to improve and in some cases to make possible communication is suggested. Comparison and analysis of two models of communication systems for the particulate-scattering channel are presented: a transmitter with a high bit rate and a receiver with an avalanche photodiode and a transmitter with a variable bit rate and a new model for an adaptive circuit in the receiver. An improvement of more than 7 orders of magnitude in error probability under certain conditions is possible with the new adaptive system model.

Bimorph adaptive mirrors and curvature sensing.

Abstract: The applicability of wave-front correction by means of a bimorph mirror in conjunction with a curvature sensor is described. We use Zernike polynomials to describe the quality of the atmospheric-turbulence correction analytically. The match is limited by boundary conditions of the mirror and by the discreteness of the electrodes. The correction is limited by coupling of lower- and higher-order Zernike polynomials and necessitates an interfacing computer between the wave-front sensor and the bimorph mirror.

Design, layout, and early results of a feasibility experiment for sodium-layer laser-guide-star adaptive optics

Abstract: We describe the design and the early results of a feasibility experiment for sodium-layer laser-guide-star adaptive optics. Copper-vapor-laser-pumped dye lasers from Lawrence Livermore National Laboratory’s Atomic Vapor Laser Isotope Separation program are used to create the guide star. The laser beam is projected upward from a beam director that is located ~5 m from a 0.5-m telescope and forms an irradiance spot ~2 m in diameter at the atmospheric-sodium layer (at an altitude of 95 km). The laser guide star is approximately fifth magnitude and is visible to the naked eye at the top of the Rayleigh-scattered laser beam. To date, we have made photometric measurements and open-loop wave-front-sensor measurements of the laser guide star. We give an overview of the experiment’s design and the laser systems, describe the experimental setup, show preliminary photometric and open-loop wave-front-sensor data on the guide star, and present predictions of closed-loop adaptive-optics performance based on these experimental data. The long-term goal of this effort is to develop laser guide stars and adaptive optics for use with large astronomical telescopes.

Applications for space power by laser transmission

Abstract: A new method for providing power to space consists of using high-power cw lasers on the ground to beam power to photovoltaic receivers in space. Such large lasers could be located at cloud-free sites at one or more ground locations, and use large mirrors with adaptive optical correction to reduce the beam spread due to diffraction or atmospheric turbulence. This can result in lower requirements for battery storage, due to continuous illumination of arrays even during periods of shadow by the Earth, and higher power output, due to the higher efficiency of photovoltaic arrays under laser illumination compared to solar and the ability to achieve higher intensities of illumination. Applications include providing power for satellites during eclipse, providing power to resurrect satellites which are failing due to solar array degradation, powering orbital transfer vehicles or lunar transfer shuttles, and providing night power to a solar array on the moon.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Nonisoplanatic imaging by phase diversity.

Abstract: Phase diversity allows one to use multiple images with known phase changes such as defocus to learn the optical characteristics of an imaging medium and to estimate the unknown object It is shown that the method can be extended to the case in which the optical system is nonisoplanatic; thus it can recover both the extended object and the angle-dependent, aberrating phase of the medium The technique is a timely extension of the conventional phase-diversity concept and could be used to solve the isoplanatic patch problem of adaptive optics, to determine a spatially varying point-spread function in image restoration, and to image through a single-fiber optic

Sodium-layer laser-guide-star experimental results

Abstract: We describe a series of experiments to characterize the sodium-layer guide star that was formed with the high-power laser developed for the Lawrence Livermore National Laboratory Atomic Vapor Laser Isotope Separation program An emission spot size of 30 m was measured, with an implied laser irradiance spot diameter of 20 m The rms spot motion at the higher laser powers, with active beam-pointing control, was less than 05 arcsec and had little effect on the observed spot size under these conditions We measured the resonant backscatter from the sodium layer as a function of laser power to obtain a saturation curve With a transmitted power of 1100 W and an atmospheric transmission of 06, the irradiance from the guide star at the ground was 10 (photons/cm2)/ms, corresponding to a visual magnitude of 51 The implications for the performance of wave-front sensors with a laser guide star of this magnitude and resulting closed-loop adaptive-optics performance are discussed

Adaptive optics at the University of Hawaii I: current performance at the telescope

Abstract: The experimental adaptive optics system, currently developed at the University of Hawaii, is now equipped with a VME-based control system, and a high sensitivity wave-front sensor. The sensor uses an array of 13 photon-counting avalanche photodiodes which enable the system to work with faint reference or `guide' sources, as faint as magnitude 15. Results of the first successful observing runs are described here.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Rapid evaluation of d 0 : the effective diameter of a laser-guide-star adaptive-optics system

Abstract: The Kolmogorov theory is used to illustrate that the mean-square wave-front error E2, which results when the wave-front distortion associated with an artificial guide-star reference is used to compensate a telescope aperture of diameter D, for imaging an object at infinity is given by E2 = (D/d0)5/3. The quantity d0 is a measure of the effective diameter of the compensated imaging system (i.e., a telescope with a diameter equal to d0 will have 1 rad of rms wave-front error) and is expressed as an integral over the Cn2 profile. The Cn2-weighting function is expressed in terms of hypergeometric functions whose series representation converges very rapidly. (Typically only a few terms are required). As a result d0 can be evaluated quite quickly on a microcomputer or a scientific calculator. In this study the quantity d0 is evaluated for six Cn2 profiles of interest, illustrating the importance of including the altitude weighting in the theoretical formulation of d0. In addition, this study illustrates the importance of removing the piston and the tilt from the wave-front distortion when assessing the performance of an imaging system.

Adaptive optics for astronomy

Abstract: Preface. Part I: The Problematics of Adaptive Optics. Introduction to Wave Optics C.A. Haniff. Atmospheric Turbulence Optical Effects: Understanding the Adaptive Optics Implications D.L. Fried. Atmospheric Adaptive Optics Applications V.P. Lukin. Modelling Atmospheric Turbulence Effects on Ground-Based Telescope Systems L.W. Bradford, S.M. Flatte, C.E. Max. Servo-Loop Analysis for Adaptive Optics M. Demerle, P.Y. Madec, G. Rousset. The Problematics of Adaptive Optics Design F. Roddier. Part II: Critical Components and Systems. Wavefront Sensing G. Rousset. Optimization of Wavefront Sensors for the Highest Accuracy and Sensitivity J.R.P. Angel. Deformable Mirrors E.N. Ribak. Astronomical Reference Sources F. Rigaut. Sodium-Layer Laser Guide Stars H.W. Friedman. Synthetic Beacons for Atmospheric Compensation D.P. Greenwood, R.R. Parenti. Sodium Beacon Used for Adaptive Optics on the Multiple Mirror Telescope B.J. Carter, et al. Evaluating the Performance of Adaptive Optical Telescopes B.M. Welsh, M.C. Roggemann. Integrated Adaptive Optics Systems E.H. Richardson. Stray Radiation Issues in Astronomical Systems with Adaptive Optics S.M. Pompea. Active Compensation of Global Wavefront Tilts E. Ballesteros, T. Viera, F. Lorenzo, M. Reyes, J.A. Bonet, C. Martin. Part III: Astrophysics with Adaptive Optics. The Impact of Adaptive Optics on Focal Plane Instrumentation S.T. Ridgway. Prospects for Alternative Approaches to Adaptive Optics A.H. Greenaway. Adaptive Optics for Long Baseline Optical Interferometry J.-M. Mariotti. Astrophysics with Adaptive Optics: Results and Challenges P. Lena.Index.

Experiment system of thin-mirror active optics

Abstract: In this system, the experiment mirror has 500 mm aperture and 6 mm thickness. There are 58 actuators and three fixed points in it. A Shack-Hartmann test apparatus is used for the measurement of wavefront aberration. In this apparatus an ingenious equivalent of lenslet array is used. All image points formed by it appear very clear theoretical diffraction pattern. And a CCD from a TV camera is used. Like European Southern Observatory, we use quasi-Zernike polynomial to fit the wavefront aberration for correcting. But in our work correction is to the whole wavefront aberration (except lateral focus and longitudinal focus). In our work, another important character is that the damp least square method is used for determining the forces. The correction results are the root mean square of wavefront aberration about 0.02 - 0.04 micrometers . A circle including measuring and correcting the wavefront aberration takes about 3.3 minutes. A more precise algorithm proposed by us is used for calculating the wavefront aberration for checking.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.