Showing papers on "Adaptive optics published in 1990"

Atmospheric wavefront simulation using Zernike polynomials

Abstract: An algorithm is described that simulates atmospherically distorted wavefronts using a Zernike expansion of randomly weighted Karhunen- Loeve functions. Its performance is presented and analyzed, and the program is then used to forecast resulting structure function and Strehl resolution for adaptive optics systems.

Wavefront sensing and the irradiance transport equation

Abstract: Phase retrieval has recently been experimentally demonstrated, using the irradiance transport equation. We show that this equation provides a unified description for incoherent wavefront sensing methods, bringing new insights and opening new prospects in this area.

First diffraction-limited astronomical images with adaptive optics

Abstract: An adaptive optics prototype system has been tested at the 1.52 m telescope of the Observatoire de Haute Provence, resulting in diffraction-limited images at near infrared wavelengths (2.2 to 5 microns). This paper presents the first results and a short analysis, which demonstrate the considerable gain in resolution and sensitivity achieved by this technique. Single stars, close binary stars, and a satellite have been resolved. In some cases another star several arcseconds apart has been used as reference for the wavefront sensing.

Hartmann-Shack wavefront sensing and wavefront control algorithm

Abstract: The wavefront sensing and reconstruction algorithms needed for the control signals for the actuators of a deformable mirror when utilizing the Hartmann-Shack wavefront sensing method are discussed. The algorithms determine the actuator's use of the measured data of image displacement to control the mirror. An analysis is presented regarding the limitations inherent in various techniques of wavefront reconstruction. The direct control of subaperture wavefront gradients is discussed, in which a single step replaces the multistep process associated with wavefront reconstruction and decoupling. The single-step process is tested experimentally on a 19-element deformable mirror with subaperture dividing optics and an optical path for purposes of comparison. By increasing the number of fitting terms, coupling can be alleviated, and modal reconstruction is shown to prevent aliasing. Controlling gradients are found to make wavefront correcting more precise by modeling the response function between actuators and subapertures.

Adaptive optics for array telescopes using neural-network techniques

Abstract: IMAGES formed by ground-based telescopes are marred by atmospheric 'seeing9. The plane wavefront from an unresolved star is distorted by continually changing turbulent fluctuations in the air's refractive index. Diffraction-limited performance can in principle be recovered through the methods of adaptive optics, in which the instantaneous wavefront shape is sensed and corrected in real-time by deformable optics that cancel the distortion1,2. The highest resolution will be achieved when this technique is applied to multiple-telescope arrays. For such arrays, the biggest errors caused by seeing at infrared wavelengths are the variations in pathlength and wavefront tilt between array elements. We show here that these errors can be derived by an artificial neural network, given only a pair of simultaneous in-focus and out-of-focus images of a reference star formed at the combined focus of all the array elements. We have optimized a neural network appropriate for 2.2-μm wavelength imaging at the Multiple Mirror Telescope in Arizona. Corrections made by moving the beam-combining mirrors will largely recover the diffraction-limited profile, with a resolution of 0.06 arcsec.

Design and performance analysis of adaptive optical telescopes using lasing guide stars

Abstract: The use of adaptive optical systems using electrically deformable mirrors to compensate for turbulence effects is discussed. Since these systems require bright reference sources adjacent to the object of interest and can be used only to observe the brightest stars, artificial guide stars suitable for controlling an adaptive imaging system must be created in the upper atmosphere by using a laser to excite either Rayleigh backscattering in the stratosphere or resonance backscattering in the mesospheric sodium layer. The design requirements of a laser-guided adaptive telescope, as well as the expected imaging performance, are discussed in detail. It is shown that a 2-m ground-based laser-guided telescope can achieve imaging performance levels at visible wavelengths nearly matching the theoretical imaging performance of the Hubble Space Telescope (HST). The required lasers can be either bought off the shelf or built with current technology. The laser power requirement for the Rayleigh guide star approach is on the order of 82 W for zenith viewing when the atmospheric seeing cell diameter is 20 cm. For the same conditions the laser power requirement for the Na guide star approach is on the order of 14 W. Both systems will achieve near diffraction limited imaging with a Strehl ratio of approximately 0.67 and an angular resolution of approximately 0.07 arcsec for an observation wavelength of 0.5 mu m. >

A photonic variable RF delay line for phased array antennas

Abstract: It is shown that by applying spatial frequency-dependent phase compensation in an optical heterodyne process a variable RF delay line can be synthesized over a prescribed frequency band. Experimental results which demonstrate the performance of the delay line with regard to both maximum delay and resolution over a broad bandwidth are presented. A spatially integrated optical system is proposed for control of phased array antennas. The integrated system provides mechanical stability essentially eliminates the drift problems associated with freespace optical systems, and can provide high-packing density. The approach uses a class of spatial light modulator known as a deformable mirror device and leads to a steerable arbitrary antenna radiation pattern of the true time-delay type. >

Adaptive Optics Revisited

Abstract: From the earliest days and nights of telescopic astronomy, atmospheric turbulence has been a serious detriment to optical performance. The new technology of adaptive optics can overcome this problem by compensating for the wavefront distortion that results from turbulence. The result will be large gains in resolving power and limiting magnitude, closely approaching the theoretical limit. In other words, telescopic images will be very significantly sharpened. Rapid and accelerating progress is being made today by several groups. Adaptive optics, together with the closely related technology of active optics, seems certain to be utilized in large astronomical telescopes of the future. This may entail significant changes in telescope design.

First diffraction-limited astronomical images with adaptive optics

Abstract: Les premieres observations astronomiques realisees avec un systeme d'optique adaptative, basees au sol, sont presentees

Optical distortion coefficients of high-power laser windows

Abstract: This paper concerns the problem of describing and evaluating thermal lensing phenomena that occur as a result of the absorption of laser light in solid windows. The aberration function expansion method is applied for deriving the two optical distortion coefficients x+ and xthat characterize the degradation in light intensity at the Gaussian focus of an initially diffraction-limited laser beam passing through a weakly absorbing stress-birefringent window. In a pulsed mode of operation, the concept of an effective optical distortion coefficient Xeff, which properly combines the coefficients x+ and x- in terms of potential impact on focal irradiances, then leads to the definition of a figure of merit for distortion. The theory and calculations presented in this and earlier papers provide simple analytical tools for predicting the optical performance of a windowmaterial candidate in a specific system's environment.

Estimation of the spatial coherence outer scale relevant to long baseline interferometry and imaging in optical astronomy

Abstract: A technique is proposed allowing estimation of the spatial coherence outer scale, from simultaneous measurements of Fried's parameter ro and of the variance of angle-of-arrival fluctuations. This optical parameter must be differentiated from the classical geophysical parameter called the outer scale of turbulence; its knowledge is important for long baseline interferometry and imaging in optical astronomy.

Atmospheric wavefront simulation and Zernike polynomials

Abstract: An algorithm is described which simulates atmospherically distorted wavefronts using a Zernike expansion of randomly weighted Karhunen-Loeve functions. Its performance are presented and analyzed thereafter. The program is then used to forecast resulting structure function and Strehl resolution for adaptive optics systems.

Continuous facesheet low voltage deformable mirrors

Abstract: Litton/Itek Optical Systems has been a leader in the development of deformable mirrors for use in high quality optical systems since 1973. The monolithic piezoelectric mirror (MPM), which was introduced in 1974, has been the quality product against which all other visible wavelength compensators have been compared. The stacked actuator deformable mirror (SADM), first delivered in 1980, set new performance standards for infrared systems. These devices continue to maintain high quality large stroke performance in the field. In 1981 Itek began the development of a new concept in deformable mirrors, the low voltage electrodisplacive mirror (LVEM). Itek teamed with Bell Aerospace in 1984 to develop a cooled silicon electrodisplacive mirror (CSEM) technology. The CSEM development emphasized extending the LVEM technology to high energy laser systems applications by incorporating a silicon multiport pin-fin heat exchanger into the mirror structure. Recently, in 1988, Itek began developing a cost efficient cooled mirror technology compatible with moderate flux levels and quantity production. The LVEM uncooled technology has matured and represents the current state of the art in deformable mirror performance. Upon its maturity, the CSEM technology will provide both high performance and cost efficient cooled deformable mirrors for the needs of the high energy laser community.

Adaptive optics: interaction matrix measurements and real time control algorithms for the COME-ON project

Abstract: The objectives of the COME-ON project are to experiment a 19 actuator adaptive optics system planned to be used on a 4 m diameter telescope This project has been worked out by 4 teams from: - Laserdot (Aerospatiale) - Office National d'Etudes et de Recherches Aerospatiales - Observatoire de Paris - European Southern Observatory (ESO) The adaptive system is made of the following major components: - a 19 actuator adaptive mirror - a Shack-Hartmann wavefront sensor and its real time slope processor - a 68020 Motorola based real time control processor In the present paper we relate the works done for: - The interaction matrix identification This matrix links the slope measurements and the controls of the adaptive mirror A process to determine this matrix by a direct measurement method is described ; considerations on the matrix inversion and control mode cancellation are examined - The control algorithm performing the real time computations needed to control the 19 actuators is described Some results of the laboratory and observatory tests are given Same paper has been presented during the SPIE's 90 Symposium on Astronomical Telescopes and Instrumentation for the 21st Century in February 1990

Segmented mirrors for atmospheric compensation

Abstract: An optical wavefront propagating through the atmosphere will be perturbed by local variations in the refractive index ofthe atmospheric gases. When accumulated over long optical path distances they will impart a spatial and temporally random distortion to the wavefront. These distortions have a characteristic spatial coherence length r0 and an atmospheric decorrelation time T0. In directed energy applications, atmospheric distortions can reduce the peak target energy densities of larger diameter laser beams by orders of magnitude. The problem is not solved through the use of larger apertures; once the aperture size increases beyond one or two r0, the far-field spot remains constant in size. Hence, for large aperture systems, the overall performance is set by the spatial coherence of the atmosphere and not by the system's exit pupil.

Adaptive optics prototype system for infrared astronomy: I. System description

Abstract: This paper is a presentation of the so-called COME-ON adaptive optics prototype system developed jointly by four European institutions. This system has been tested on the 1.52m telescope of the Observatoire de Haute Provence on October 12 to 23 and November 13 to 24, 1989. Diffration-limited infrared imaging has been achieved during these first tests. The adaptive optics system consists of a 19 actuator deformable mirror and a Hartmann-Shack type wavefront sensor. In this instrument the wavefront sensing is performed at visible wavelengths while the correction is performed for near infrared imaging (1 .2 to 5 .tm). Specialized computers drive the deformable mirror and a tip-tilt mirror. The bandwidth of the servo-loop is 9 Hz at 0 dB point in open-loop. The results obtained with this instrument will be very useful for the design of the future adaptive optics system for the ESO Very Large Telescope (VLT).

Development Of Mesospheric Sodium Laser Beacon For Atmospheric Adaptive Optics

Abstract: A solid-state source of long pulse length, sodium-resonance radiation was developed for the purpose of generating an artificial star in the earth's mesospheric sodium layer. This radiation is generated by sum-frequency mixing the output of a 1.064 micron Nd:YAG laser with the output of a 1.319-micron Nd:YAG laser. By operating these lasers at wavelengths very close to the peak of their tuning curves, it is possible to match the wavelength of the sum-frequency radiation to that of the sodium D2 adsorption wavelength. Two pulsed laser systems were constructed, one producing as much as 0.6 J of sodium resonance radiation at a 10-Hz repetition rate and another producing as much as 24 mJ at a 840-Hz repetition rate. In both laser systems, the 1.06-micron and 1.32-micron Nd:YAG lasers are configured as mode-locked master oscillators followed by power amplifiers. Other aspects of this project are presented.

Variations on a Hartmann theme

Abstract: We explore modifications of the classical Hartmann wavefront sensing technique that can be used to improve its accuracy, dynamic range, and spatial resolution. We describe a differential sensor with variable sensitivity. We review the use of various possible Hartmann masks and discuss their interferometric properties. We propose the use of Fourier analysis and show its relationship to moire methods. We finally envisage the possibility of mapping both the slope and the total curvature (Laplacian) of the wavefront with the same setup.

Polar-coordinate laser writer for binary optics fabrication

Abstract: A Laser Writer system for recording centro-symmetric patterns in photoresist has been developed as an alternative method for binary optics mask and component fabrica.- tion.This system is capable of generating binary amplitude patterns with linewidths below 1 tm and with a positional accuracy of less than 0.ljim on up to 3 in. diameter planar substrates. The measured wavefront error and diffraction efficiency of a directwrite two-phase-level F/10 lens confirm that high quality components can be fabricated quickly, easily, and at low cost.

Two-dimensional OCP wavefront sensor employing one-dimensional optical detection

Abstract: An improved two-dimensional optical centroid processor ("OCP") wavefront sensor is described in which a scanning mirror 146 scans the wavefront of input light 130 to provide a scanned beam 148 (i.e. a portion of the wavefront). The scanned beam 148 is focused on an OCP 168 filter comprising four rows of mask cells. The filtered light emanating from the cells impinges on one-dimensional photodetector devices 192, 198, 202, 206 (i.e. photodetector rows). Thus, the scanning of the wavefront by the scanning mirror provides two-dimensional measurement while employing one-dimensional photodetector devices. Each of the photodetector devices provides an output to a ratio calculator 201 which is used to correct the tilt in the wavefront by driving a deformable mirror (not shown).

Fabrication of large optical surfaces using a combination of polishing and mirror bending

Abstract: In order to reduce polishing costs and correct unexpected errors in fabrication and polishing, the support of very large optics can be actively enlisted in telescope mirror optical figure adjustment. A set of leaf springs is used by the Keck Ten-Meter Telescope to apply moments about the pivots of the mirror mosaics' whiffletree support. The springs successfully reduce the polished rms surface error by a factor of 6 to 15, while reducing the 80-percent enclosed energy diameter by a factor of 2.5-6.0. Additional current limitations on figure improvement include the difficulties of polishing higher spatial frequencies and predicting warping during mirror fabrication.

Testing of telescope optics: a new approach

Abstract: Results recently obtained for the use of the curvature-sensing method as a substitute for slope sensing in optical wavefront reconstruction, using long-exposure CCD images of the beam cross-section on either side of the telescope focal plane. A program based on the solution to the Poisson equation is then applied in order to reconstruct the wavefront. Relative to the existing Hartmann sensing methods, curvature-sensing yields sensitivity comparable to that of the Shack-Hartmann test. Additional optics and reference plane-based calibration are obviated. Tests of the new method on an 88-inch Ritchey-Chretien telescope have yielded a map of residual wavefront errors as a solution of the Poisson equation.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Adaptive optics wavefront corrector using addressable liquid-crystal retarders: II

Abstract: We present recent results of our investigations into the feasibility of a wavefront corrector for adaptive optics, using nematic liquid crystal material. In particular, we are aiming at a small, flexible adaptive optics module, to be located at the focal plane of existing and new-generation telescopes in order to improve imaging quality. We address some of the specific problems investigated, concerning the theoretical behavior of the LC corrector, its comparison with other state-of-the-art wavefront correctors, modal and zonal, the speed and slew rate of the actuators, and a real-time capacitive servo loop scheme controlling the actuator retardance.

Time-resolved wavefront measurements and analyses for a pulsed, nuclear-reactor-pumped laser gain region

Abstract: The wavefront error across a laser aperture is often the limitingfactor in laser resonator design and performance. For long-pulse, side-pumped lasers, such as some nuclear-reactor-pumped or flashlamp-pumpedlasers, significant transverse index gradients can develop and couple tothe laser resonator modes. To design a laser resonator for efficient powerextraction, a spatially and temporally resolved determination of the index variation is required. High-speed photography using a fastframing camera with a modification of the Hartmann technique is applied to measure the index field and the resulting wave front error in the gain region of a pulsed,nuclear-reactor-pumped laser. With moderate initial gas pressures, the index fields and the wavefront errors are found to have roughly parabolicshapes except near the side walls. These results are in reasonable agree-ment with the predictions of a gasdynamic model, which describes thegas motion induced by the spatial nonuniformity of the pumping.

Adaptive coherent optical receiver array

Abstract: A linear four element adaptive heterodyne optical receiver array employing semiconductor lasers is described. The array adaptively adjusts the phase of each subaperture IF signal to correct for wavefront tilt and distortion and non-uniform optical and RF path delays across the array by phase-locking each channel to a reference channel.

NSO/SP adaptive optics program

Abstract: The current status of the adaptive optics system for the National Solar Observatory (NSO) 75-cm aperture, evacuated solar telescope at Sunspot, New Mexico, is described. This system is interfaced to a birefringent filter and two solar spectrographs. The optical system provides for the High Altitude Observatory Advanced Stokes Polarimeter (ASP) that measures solar vector magnetic fields. Recently the optical design has been modified to accommodate a 19-segment adaptive mirror and its 19-segment quad-cell tracker.

Adaptive optics system performance approximations for atmospheric turbulence correction

Abstract: Analysis of adaptive optics system behavior often can be reduced to a few approximations and scaling laws. For atmospheric turbulence correction, the deformable mirror (DM) fitting error is most often used to determine a priori the interactuator spacing and the total number of correction zones required. This paper examines the mirror fitting error in terms of its most commonly used exponential form. The explicit constant in the error term is dependent on deformable mirror influence function shape and actuator geometry. The method of least squares fitting of discrete influence functions to the turbulent wavefront is compared to the linear spatial filtering approximation of system performance. The author finds that the spatial filtering method overestimates the correctability of the adaptive optics system by a small amount. By evaluating fitting error for a number of DM configurations, actuator geometries, and influence functions, fitting error constants verify some earlier investigations. Limitations of the approximations and scaling laws are evaluated and compared to wave optics ground-to-space propagations. Results include determination of a multiplicative conversion factor of 1.06 that should be applied to the actuator spacing whenever the spatial filtering method is used for adaptive optics system performance analysis.

Innovative approach to next-generation telescope design

Abstract: The new Phased Array Mirror, Extendible Large Aperture (PAMELA) technology can be applied to innovative designs for large astronomical telescopes and laser beam directors. Filled primary apertures with diameters exceeding 15 m will be capable of diffraction-limited imaging of objects at visible wavelengths. The same aperture can function as a nearly perfect conventional telescope primary mirror when objects or reference stars are too faint for adaptive compensation of atmospheric turbulence. This performance will be accomplished by means of a dual-mode control system which utilizes either local figure sensing to yield a nearly perfect wide-field, highly segmented mirror or a fully adaptive wavefront compensation system to drive the mirror segments to the phase conjugate positions to correct for atmospheric turbulence within the isoplanatic angle restriction. The active optics sensing for local figure control can be accomplished either by using differential height and tilt sensors on each segment or by using an optical interferometer at the center of curvature of the primary mirror. This paper presents a design concept for a 10-m telescope with a spherical primary mirror composed of about 10,000 10-cm diameter hexagonal segments. Principles of operation are described, and estimates of performance are given.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.