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Showing papers on "Adaptive optics published in 1997"


Journal ArticleDOI
TL;DR: A fundus camera equipped with adaptive optics is constructed that provides unprecedented resolution, allowing the imaging of microscopic structures the size of single cells in the living human retina.
Abstract: Even when corrected with the best spectacles or contact lenses, normal human eyes still suffer from monochromatic aberrations that blur vision when the pupil is large. We have successfully corrected these aberrations using adaptive optics, providing normal eyes with supernormal optical quality. Contrast sensitivity to fine spatial patterns was increased when observers viewed stimuli through adaptive optics. The eye's aberrations also limit the resolution of images of the retina, a limit that has existed since the invention of the ophthalmoscope. We have constructed a fundus camera equipped with adaptive optics that provides unprecedented resolution, allowing the imaging of microscopic structures the size of single cells in the living human retina.

1,456 citations


Journal ArticleDOI
TL;DR: It is demonstrated that adaptive wave-front correction by a parallel stochastic perturbation technique can significantly improve image quality.
Abstract: We describe an adaptive wave-front control technique based on a parallel stochastic perturbation method that can be applied to a general class of adaptive-optical system. The efficiency of this approach is analyzed numerically and experimentally by use of a white-light adaptive-imaging system with an extended source. To create and compensate for static phase distortions, we use 127-element liquid-crystal phase modulators. Results demonstrate that adaptive wave-front correction by a parallel-perturbation technique can significantly improve image quality.

304 citations


Patent
21 Nov 1997
TL;DR: In this paper, a system and method for objective measurement and correction of focusing optical systems comprising optics disposed in the path of the beam (18) which directs the beam through the focusing optical system, e.g., and eye (120) and focuses the beam at its rear portion (122).
Abstract: A system and method for objective measurement and correction of focusing optical systems comprising optics disposed in the path of the beam (18) which directs the beam through the focusing optical system, e.g., and eye (120) and focuses the beam at its rear portion (122). The beam is diffusely reflected back and a wavefront analyzer (26) is disposed in the path of the wavefront projected from the optics and calculates the distortions as an estimate of aberrations of the focusing optical system.

303 citations


Journal ArticleDOI
TL;DR: In this article, a method to estimate the PSF related to each image, using data from the adaptive optics control computer, namely, the wavefront sensor measurements and the commands to the deformable mirror, accumulated in synchronization with the acquisition, is presented.
Abstract: Astronomical images obtained with adaptive optics systems can be enhanced by using image restoration techniques. However, this usually requires an accurate knowledge of the system point-spread function (PSF) which is variable in time. We present a method to estimate the PSF related to each image, using data from the adaptive optics control computer, namely, the wave-front sensor measurements and the commands to the deformable mirror, accumulated in synchronization with the acquisition. This method requires no extra observing time and has been successfully tested on PUEO, the Canada–France–Hawaii Telescope adaptive optics system. With this system, accurate PSF estimations could be achieved for guide stars of magnitude 13 or brighter.

178 citations


Journal ArticleDOI
12 Sep 1997-Science
TL;DR: In this article, a sodium-layer laser guide star beacon with high-order adaptive optics at Lick Observatory (Mount Hamilton, California) produced a 2.4 intensity increase and 2.2 decrease in full width at half maximum for an astronomical point source, compared with image motion compensation alone.
Abstract: A sodium-layer laser guide star beacon with high-order adaptive optics at Lick Observatory (Mount Hamilton, California) produced a factor of 2.4 intensity increase and a factor of 2 decrease in full width at half maximum for an astronomical point source, compared with image motion compensation alone. The image full widths at half maximum were identical for laser and natural guide stars (0.3 arc second). The Strehl ratio with the laser guide star was 65 percent of that with a natural guide star. This technique should allow ground-based telescopes to attain the diffraction limit, by correcting for atmospheric distortions.

173 citations


Journal ArticleDOI
TL;DR: In this paper, the authors describe the development of a new type of micromachined device designed for use in correcting optical aberrations, a nine-element continuous deformable mirror was fabricated using surface micromechining.
Abstract: The authors describe the development of a new type of micromachined device designed for use in correcting optical aberrations. A nine-element continuous deformable mirror was fabricated using surface micromachining. The electromechanical behavior of the deformable mirror was measured. A finite-difference model for predicting the mirror deflections was developed. In addition, novel fabrication techniques were developed to permit the production of nearly planar mirror surfaces.

140 citations


Journal ArticleDOI
TL;DR: The potential of wavefront shaping with liquid-crystals for modulating the eye's refractive state was evaluated and spatial undersampling of the wavefront retardation function by the discrete array of liquid crystal cells was traced.
Abstract: We evaluated the potential of wavefront shaping with liquid-crystals for modulating the eye's refractive state. A spatial light modulator with 127 liquid crystals cells was imaged in the entrance pupil of the eye and programmed to induce prismatic, spherical, and astigmatic refractive changes. Psychophysical evaluation of these optical effects was in agreement with expectations for prisms up to approximately 0.08 D and for lenses up to approximately 1.5 D. These maximum dioptric values represent wavefront retardation of about 3 to 4 wavelengths of 584 nm light across a 3-mm diameter pupil. Optical aliasing of high-power prisms was traced to spatial undersampling of the wavefront retardation function by the discrete array of liquid crystal cells. Undersampling may also be the factor which limits the useful dioptric range of the technique.

107 citations


Journal ArticleDOI
TL;DR: The Beamlet is a single-beam prototype of future multibeam megajoule-class Nd:glass laser drivers for inertial confinement fusion that uses a multipass main amplifier, adaptive optics, and efficient, high-fluence frequency conversion to the third harmonic.
Abstract: The Beamlet is a single-beam prototype of future multibeam megajoule-class Nd:glass laser drivers for inertial confinement fusion. It uses a multipass main amplifier, adaptive optics, and efficient, high-fluence frequency conversion to the third harmonic. The Beamlet amplifier contains Brewster-angle glass slabs with a clear aperture of 39 cm × 39 cm and a full-aperture plasma-electrode Pockels cell switch. It has been successfully tested over a range of pulse lengths from 1–10 ns up to energies at 1.053 µm of 5.8 kJ at 1 ns and 17.3 kJ at 10 ns. A 39-actuator deformable mirror corrects the beam quality to a Strehl ratio of as much as 0.4. The 1.053-µm output has been converted to the third harmonic at efficiencies as high as 80% and fluences as high as 8.7 J/cm2 for 3-ns pulses.

106 citations


Journal ArticleDOI
TL;DR: In this paper, the first results from a real-time closed-loop adaptive optics system incorporating a liquid crystal based corrector device and a Shack-Hartmann wavefront sensor are presented.

70 citations


Journal ArticleDOI
TL;DR: In this paper, the authors discuss turbulence and image-detection statistics, describe the fundamentals of methods for overcoming turbulence effects, and provide representative performance results for both hardware-and image-processing-based methods.
Abstract: Atmospheric turbulence profoundly limits the angular resolution of astronomical telescopes working at visible and near-infrared wavelengths. In fact, the angular resolution for conventional imaging through turbulence is on the order of 5{endash}20{percent} of the diffraction-limited resolution at the best observatories in the world. The origin of these performance degradations is random turbulence-induced fluctuations in the index of refraction of the atmosphere. Random index-of-refraction fluctuations producing the optical path length of the atmosphere to be random in both space and time, producing random aberrations in the telescope pupil that degrade imaging performance. Over the past several years significant advances have been made in developing both hardware and image-processing-based techniques for improving the resolution of astronomical telescopes. Hardware-oriented correction techniques are based on wave-front sensing and adaptive optics. Image-processing-based methods include speckle-imaging techniques and hybrid imaging techniques that use elements of adaptive-optics systems and image reconstruction. Analysis techniques for predicting the performance of these imaging methods have been developed, and the comparative performance of these imaging techniques has been examined. This paper discusses turbulence and image-detection statistics, describes the fundamentals of methods for overcoming turbulence effects, and provides representative performance results. {copyright} {ital 1997} {ital The American Physical Society}

67 citations


Proceedings ArticleDOI
21 Mar 1997
TL;DR: In this article, the integral field unit (IFU) was proposed for two-dimensional area spectroscopy, which acts as a coupler between the telescope and spectrograph to reformat a square or rectangular field into a long slit.
Abstract: I propose a new design of integral field unit (IFU) for two- dimensional area spectroscopy. The unit acts as a coupler between the telescope and spectrograph to reformat a square or rectangular field into a long slit. In addition to providing good transmission, it avoids the geometrical losses inherent in previous designs based on fiber optics, lens array re- imaging and image slicing. The proposed design is a new type of image slicer in which the original two-dimensional image is sliced into narrow sub-images that are re-imaged side by side to form a long one-dimensional image at the spectrograph input. The new design is much more compact than previous designs making it easier to insert in the front of a spectrograph without any modification to the spectrograph support system. The design uses much smaller optics than previous designs. The small number of reflections -- 4 to 6 depending on the telescope focal ratio -- and the smaller instrument size, which simplify the cooling of the instrument, makes the design well suited for infrared spectroscopy. The application of this design to 8-m telescopes and its use with adaptive optics in the optical and infrared is discussed, particularly with respect to the multi-object spectrographs of the Gemini telescopes project. In this case, the proposed design gives a field of 8.3' by 11.4' along with a background field of 2.6' by 3.6' with a spatial resolution of 0.09' by 0.16' and roughly 1200 spectral resolution elements in each spectrum. The spatial resolution is well suited to images produced by the Gemini telescopes in tip/tilt mode (typically FWHM approximately 0.3' in 10th percentile seeing) and with higher order adaptive optics. This design is also applicable to 4-m telescopes with -- or without-- adaptive optics. The possibility of inserting such an instrument in front of existing spectrographs is discussed.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Journal ArticleDOI
TL;DR: Multiframe blind deconvolution - the process of restoring resolution to blurred imagery when the precise form of the blurs is unknown - is discussed as an estimation-theoretic method for improving the resolving power of ground-based telescopes used for space surveillance.
Abstract: Multiframe blind deconvolution - the process of restoring resolution to blurred imagery when the precise form of the blurs is unknown - is discussed as an estimation-theoretic method for improving the resolving power of ground-based telescopes used for space surveillance. The imaging problem is posed in an estimation-theoretic frame- work whereby the object's incoherent scattering function is estimated through the simultaneous identication and correction of the distorting effects of atmospheric turbulence. An iterative method derived via the expectation-maximization (EM) procedure is reviewed, and results obtained from telescope imagery of the Hubble Space Telescope are presented.

Proceedings ArticleDOI
07 Jul 1997
TL;DR: In this paper, the authors show how aberrations can affect the choice of optimum NA and partial coherence for a given reticle object when considering critical dimension uniformity and depth of focus.
Abstract: This paper shows, that as resolution is pushed into regions below 0.6 (lambda) /NA, understanding the effects of wavefront aberrations is crucial to producing stepper systems that can meet end-user requirements. We show how aberrations can affect the choice of optimum NA and partial coherence for a given reticle object when considering critical dimension uniformity and depth of focus. The ability to measure the complete wavefront and extract meaningful full-field aberration data is shown using an advanced through-the-lens interferometer that operates at the wavelength and bandwidth of the lithographic radiation. The impact of aberrations an image quality criteria is shown through a sensitivity analysis using an imaging approximation model that represents various image criteria as a weighted sum of aberration coefficients. The validity and use of such a model is shown by correlation to full- field experimental measurements.

Journal ArticleDOI
TL;DR: In this article, a silicon-based, surface micromachined, deformable mirror device for optical applications requiring phase modulation, including adaptive optics and pattern recognition systems is described.
Abstract: A silicon-based, surface micromachined, deformable mirror device for optical applications requiring phase modulation, including adaptive optics and pattern recognition systems is described. The mirror will be supported on a massively parallel system of electrostatically con- trolled, interconnected microactuators that can be coordinated to achieve precise actuation and control at a macroscopic level. Several genera- tions of individual actuators as well as parallel arrays of actuators with segmented/continuous mirrors have been designed, fabricated, and tested. Deflection characteristics and pull-in behavior of the actuators have been closely studied. Devices have been characterized with regard to yield, repeatability, and frequency response. An electromechanical model of the system has been simulated numerically using the shooting method, and good correlation with experimental results has been ob- tained. A twenty-channel parallel control scheme has been developed

Journal ArticleDOI
TL;DR: Numerical simulations show that the very stringent constraints on the optical quality of a space IR interferometer aimed at detecting extrasolar planets can be relaxed to values achievable with current technology.
Abstract: We propose applying the techniques of spatial filtering to the concept of interferometric coronography. In such a system, provided that the object being studied is not resolved by the individual apertures of the interferometric array, the beams can be considered as coherent or, more exactly, single mode. Hence spatial filtering allows one to cleanse the beams of imperfections generated by defects on the optical components of the interferometer and thus to obtain very high rejection rates in the destructive output of the interferometer (coronographic output) for an on-axis star. Numerical simulations show that the very stringent constraints on the optical quality of a space IR interferometer aimed at detecting extrasolar planets can be relaxed to values achievable with current technology. In particular, we show that the difficulties induced by dust scattering, small micrometeorite impacts on the primary mirror, and high-frequency ripples of polishing residuals can be eliminated by simple pinhole spatial filtering. The effects, however, will be less dramatic on large-scale defects such as coating defects and pointing errors in the telescopes.

Journal ArticleDOI
TL;DR: The results of independent numerical simulations of adaptive optics systems for 8-m astronomical telescopes that use both Shack-Hartmann and wave-front curvature sensors are presented and the system performance in term of Strehl ratio and full width at half-maximum versus the magnitude of the (on-axis) guide star is reported.
Abstract: We present the results of independent numerical simulations of adaptive optics systems for 8-m astronomical telescopes that use both Shack-Hartmann and wave-front curvature sensors. Four differents codes provided consistency checks and redundancy. All four simulate a complete system and model noise and servo-lag effects. A common atmospheric turbulence generator was used for consistency. We present the main characteristics of the codes, and we report the system performance in term of Strehl ratio and full width at half-maximum versus the magnitude of the (on-axis) guide star. We show that a Shack-Hartmann plus stacked actuator mirror system with 10 x 10 subapertures or a curvature plus bimorph mirror system with 56 subapertures yields a 50% Strehl ratio at 1.6 mum for a m(R) = 14.7 magnitude star, with almost equivalent performance at both brighter and dimmer light levels.

Journal ArticleDOI
TL;DR: A new modal linear predictive controller whose parameters are optimized by means of a recursive least-squares algorithm to minimize the residual optical phase variance and it is demonstrated that the optimal rejection transfer function for adaptive optics is proportional to the frequency signal-to-noise ratio.
Abstract: The correction efficiency of adaptive optics is limited mainly by measurement noise and time delay. To overcome this problem, we describe a new modal linear predictive controller whose parameters are optimized by means of a recursive least-squares algorithm to minimize the residual optical phase variance. The method copes with the recursivity of the closed-loop operation. We demonstrate that the optimal rejection transfer function for adaptive optics is proportional to the frequency signal-to-noise ratio. Finally, we present what we believe to be the first experimental results obtained with a predictor used to control the tip-tilt mirror of an adaptive-optics system.

Journal ArticleDOI
TL;DR: In this paper, the theoretical limits of performance of MEM-DMs for controlling fixed aberrations in optical systems were explored, and an alternative design that uses a lenslet array in conjunction with the MEM-DM was shown through theoretical studies to provide superior aberration correction with lower residual effects due to diffraction.
Abstract: Micro-electro-mechanical deformable mirrors (MEM-DMs) are solid-state electronic devices with small, movable reflective surface elements that can be used to manipulate the phase of optical wavefronts. MEM-DMs differ from more conventional continuous-facesheet deformable mirrors in that the movable surface of a MEM-DM consists of a set of segmented moving surfaces. The segmented, reflective surfaces of a MEM-DM give rise to larger diffraction effects than those provided by continuous-facesheet deformable mirrors. However, MEM-DMs are still attractive due to their low cost and the low drive voltages. We explore the theoretical limits of performance of MEM-DMs for controlling fixed aberrations in optical systems, and we present laboratory results demonstrating reduction of a fixed aberration using a MEM-DM device. Results presented here show that while a MEM-DM does provide some degree of aberration control, diffraction effects arising from the static support structures of the MEM-DM surface are significant. An alternative design that uses a lenslet array in conjunction with the MEM-DM is shown through theoretical studies to provide superior aberration correction with lower residual effects due to diffraction.

Journal ArticleDOI
TL;DR: In this paper, the temporal performance of a nematic liquid crystal phase modulator with 69 phase retardance cells was studied and the change of Strehl ratio for defocus under static and dynamic conditions was discussed.

Journal ArticleDOI
TL;DR: In this article, the authors obtained near-infrared, multispectral imagery of Jupiter and Saturn using the second generation, laser-beacon adaptive optics 1.5m telescope facility at the USAF Phillips Laboratory Starfire Optical Range (S.O.R.).
Abstract: We obtained near-infrared, multispectral imagery of Jupiter and Saturn during July, 1994 using the second generation, laser-beacon adaptive optics 1.5m telescope facility at the USAF Phillips Laboratory Starfire Optical Range (S.O.R.). The telescope optics were equipped with a fast, germanium array and image correlation processor for tip-tilt correction on extended, i.e., planetary objects, and a broadband CCD "scoring" camera with 0.089 arcsec/pixel sampling, for evaluating the overall system performance. Wavelength tunable imaging was accomplished by sharing the focal plane with a Goddard-built, acousto-optic tunable filter (AOTF) camera operating from 700 to 950 nanometers . Spectral images of the Jupiter-SL/9 collision sites were photometrically calibrated and reduced to absolute reflectivity (I/F) at each wavelength. Images of the sites show spectally dependent details which provide the starting point for one-dimensional plume models. Higher quality images of Saturn were acquired, with a spatial resolution of 0.29 arcseconds. We attribute this to robust tip-tilt correction made possible by the presence of Saturn's rings, which break the symmetry of an otherwise nearly circular image. The compensated images of Saturn indicate early evidence of a northern hemisphere atmospheric disturbance.

Journal ArticleDOI
TL;DR: An optical technique that allows us to estimate the inner scale by measuring the variance of angle of arrival fluctuations of collimated laser beams of different sections w (i) passing through a turbulent layer is proposed.
Abstract: We propose an optical technique that allows us to estimate the inner scale by measuring the variance of angle of arrival fluctuations of collimated laser beams of different sections w (i) passing through a turbulent layer. To test the potential efficiency of the system, we made measurements on a turbulent air flow generated in the laboratory, the statistical properties of which are known and controlled, unlike atmospheric turbulence. We deduced a Kolmogorov behavior with a 6-mm inner scale and a 90-mm outer scale in accordance with measurements by a more complicated technique using the same turbulent channel. Our proposed method is especially sensitive to inner-scale measurement and can be adapted easily to atmospheric turbulence analysis. We propose an outdoor experimental setup that should work in less controlled conditions that can affect astronomical observations. The inner-scale assessment might be important when phase retrieval with Laplacian methods is used for adaptive optics purposes.

Proceedings ArticleDOI
27 Mar 1997
TL;DR: In this article, a wavefront sensor was used for testing various optical elements including lenses, mirrors and laser rods. But the wavefront sensors are not suitable for in-situ measurements.
Abstract: Measurement is an integral part of optics manufacture, where grinding and polishing steps are linked iteratively with the testing steps. While numerous test methods have been developed, many of these are prone to vibration effects that limit their application to in-situ monitoring, or have other limitations. We have applied Shack-Hartmann wavefront sensors to the problems of optics measurement. We have developed an instrument that allows testing in common configurations, and also provides new ways to test optics. The current device is extremely sensitive. We have demonstrated this device for testing various optical elements including lenses, mirrors and laser rods.

Journal ArticleDOI
TL;DR: A precise measuring method for detecting the in situ distortion profile of a high-heat-load mirror for synchrotron radiation by use of a pentaprism long trace profiler (LTP) is presented and suggestions for improving the accuracy and stability are presented.
Abstract: A precise measuring method for detecting the in situ distortion profile of a high-heat-load mirror for synchrotron radiation by use of a pentaprism long trace profiler (LTP) is presented. A maximum distortion of 0.47 µm across a length of 180 mm was measured for an internally water-cooled mirror on an undulator beam line at ELETTRA while exposed to a total emitted power of 600 W. This first successfully tested in situ distortion profile points out the importance and need for this method. Two configurations for performing in situ LTP tests are discussed. For this measurement the configuration with all the equipment external to the vacuum chamber was used. The experiment has an accuracy and a repeatability of 0.04 µm. Suggestions for improving the accuracy and stability are presented.

Journal ArticleDOI
TL;DR: The results of simulation studies designed to explore how wave-front aberration information for near-diffraction-limited telescopes can be estimated through a regularized, low-pass filtered version of the Gonsalves (least-squares) phase-diversity technique are given.
Abstract: Because of mechanical aspects of fabrication, launch, and operational environment, space telescope optics can suffer from unforeseen aberrations, detracting from their intended diffraction-limited performance goals. We give the results of simulation studies designed to explore how wave-front aberration information for such near-diffraction-limited telescopes can be estimated through a regularized, low-pass filtered version of the Gonsalves (least-squares) phase-diversity technique. We numerically simulate models of both monolithic and segmented space telescope mirrors; the segmented case is a simplified model of the proposed next generation space telescope. The simulation results quantify the accuracy of phase diversity as a wave-front sensing (WFS) technique in estimating the pupil phase map. The pupil phase is estimated from pairs of conventional and out-of-focus photon-limited point-source images. Image photon statistics are simulated for three different average light levels. Simulation results give an indication of the minimum light level required for reliable estimation of a large number of aberration parameters under the least-squares paradigm. For weak aberrations that average a 0.10λ pupil rms, the average WFS estimation errors obtained here range from a worst case of 0.057λ pupil rms to a best case of only 0.002λ pupil rms, depending on the light level as well as on the types and degrees of freedom of the aberrations present.

Proceedings ArticleDOI
17 Oct 1997
TL;DR: In this article, the authors present laboratory test results obtained during integration of the Palomar 200” Adaptive Optics System at the Jet Propulsion Laboratory (JPL) and present a wide-ranging summary of subsystem performance measurements.
Abstract: We present laboratory subsystem test results obtained during integration of the Palomar 200” Adaptive Optics System at Jet Propulsion Laboratory. These results pertain to the 241 actively controlled actuator, Shack-Hartmann sensed, initial delivery of a system optimized for near infrared observation with the 5 meter diameter telescope at Palomar Mountain. This system initially exploits natural guide stars. Our intention is to provide a wide-ranging summary of subsystem performance measurements and several lessons learned. Noteworthy among these results is our measurement of 6.3 ± 0.2 electron read noise performance of our initial WFS camera, based upon a 64 x 64 pixel MIT/LL CCD detector, running at 600 kilopixel per sec per output amplifier. This camera was constructed by SciMeasure Analytical Systems, Inc. of Decatur, GA.

Proceedings ArticleDOI
04 Apr 1997
TL;DR: In this article, the effect of nonlinear optical processes in the atmosphere as boundaries on propagation is discussed, and how to choose system parameters to guarantee optimum conversion of laser energy to target momentum.
Abstract: Nearly 200,000 pieces of debris in the 1 - 20 cm range in low- Earth orbit (LEO), a legacy of 35 years of spaceflight now threaten long-term space missions. An economical solution to the problem is to use a ground-based laser to create a photoablation jet on the objects and cause them to re-enter the atmosphere and burn up. A sensitive optical detector is required to locate objects as small as 1 cm at 1500 km range. Applied when the object is rising and between about 45 and 15- degree zenith angle, the necessary (Delta) v is of order 100 m/s. A laser of 30 kW average power at 5-ns pulsewidth and a 4 - 6 m mirror with adaptive optics can clear near-Earth space of the 1-20-cm debris in 2 years of operation. A high altitude site minimizes turbulence correction, interference from nonlinear optical effects, and absorption. We discuss the effect of nonlinear optical processes in the atmosphere as boundaries on propagation, and how to choose system parameters to guarantee optimum conversion of laser energy to target momentum. The laser might be Nd:glass (1.06 micrometer/530 nm), or iodine (1.3 micrometer).

Journal ArticleDOI
TL;DR: In this paper, a method is proposed for obtaining simple figures of merit (FoM) for rating the thermal lensing performance of mirror-substrate materials as well as window-material candidates.
Abstract: High-power, high-energy laser (HEL) systems include an op- tical train consisting of mirrors and windows, which must be capable of transporting and directing the beam without seriously degrading the nominal performance. Since catastrophic failure modes are not a major threat at beam-power levels of current interest, the system's perfor- mance as measured in terms of achievable target irradiances may de- grade as a result of thermal lensing, that is, the wavefront distortion caused by thermally induced phase aberrations. Analytical investigations that address the problem of evaluating the nature of laser-driven mirror/ window distortions are reviewed and updated. In this context, a method is proposed for obtaining simple figures of merit (FoM) for rating the thermal lensing performance of mirror-substrate materials as well as window-material candidates. The performance of cooled HEL mirrors re- flects their ability to minimize irradiance-mapping wavefront distortions, which leads to defining a thermal distortion coefficientj that controls the out-of-plane growth of the faceplate. It is then straightforward to derive equations for characterizing the root mean squared surface deformation and to assess the merit of mirror-faceplate materials in a pulsed or a cw environment. Since state-of-the-art heat exchangers exhibit relatively modest Biot numbers, the thermal conduction is not a critical parameter but the modulus of elasticity must be properly factored into the FoM for cw operation. Window-induced wavefront deformations require special attention because they involve not only position-dependent variations of the window thickness but also position- and polarization-dependent variations of the refractive index. This situation leads to introducing sym- metric and antisymmetric distortion coefficients, which can be combined into an effective optical distortion coefficientx that specifies the relative weight of birefringence compared to all other sources of distortion and shows that zero distortion can be achieved only with stress- birefringence-free material having a negative dn/dT. As in the case of mirror-faceplate materials, FoMs for the prediffusion and the steady- state regimes emerge in a direct manner and demonstrate that fluoro- zirco-aluminate glass by far outperforms other window-material candi- dates in implementing the zero-distortion goal. © 1997 Society of Photo- Optical Instrumentation Engineers. (S0091-3286(97)00106-2)

Proceedings ArticleDOI
17 Oct 1997
TL;DR: In this article, the phase distortion is estimated from two or more well-sampled, full-aperture images of the NGS measured with known adjustments applied to the phase profile.
Abstract: The effect of focus anisoplanatism upon the performance of an astronomical laser guide star (LGS) adaptive optics (AO) system can in principle be reduced if the lowest order wavefront aberrations are sensed and corrected using a natural guide star (NGS). For this approach to be useful, the noise performance of the wavefront sensor (WFS) used for the NGS measurements must be optimized to enable operation with the dimmest possible source. Two candidate sensors for this application are the Shack-Hartmann sensor and “phase-diverse phase retrieval,” a comparatively novel approach in which the phase distortion is estimated from two or more well-sampled, full-aperture images of the NGS measured with known adjustments applied to the phase profile. We present analysis and simulation results on the noise-limited performance of these two methods for a sample LGS AO observing scenario. The common parameters for this comparison are the NGS signal level, the sensing wavelength, the second-order statistics of the phase distortion, and the RMS detector read noise. Free parameters for the two approaches are the Shack-Hartmann subaperture geometry, the focus biases used for the phase-diversity measurements, and the algorithms used to estimate the wavefront. We find that phase-diverse phase retrieval provides consistently superior wavefront estimation accuracy when the NGS signal level is high. For lower NGS signal levels on the order of 103 photodetection events, the Shack-Hartmann (phase diversity) approach is preferred at a RMS detector read noise level of 5 (0) electrons/pixel.

ReportDOI
01 Jun 1997
TL;DR: In this article, a sodium-layer laser guide star beacon with high-order adaptive optics at Lick Observatory produced a 2.4 intensity increase and 2.2 decrease in full width at half maximum for an astronomical point source, compared with image motion compensation alone.
Abstract: A sodium-layer laser guide star beacon with high-order adaptive optics at Lick Observatory produced a factor of 2.4 intensity increase and a factor of 2 decrease in full width at half maximum for an astronomical point source, compared with image motion compensation alone. Image full widths at half maximum were identical for laser and natural guide stars (0.3 arc seconds). The Strehl ratio with the laser guide star was 65% of that with a natural guide star. This technique should allow ground-based telescopes to attain the diffraction limit, by correcting for atmospheric distortions.

Patent
23 Dec 1997
TL;DR: In this article, a point source produced on the retina of a living eye by a laser beam is received at a lenslet array of a Hartmann-Shack wavefront sensor such that each of the lenslets forms an aerial image of the retinal point source on a CCD camera.
Abstract: An apparatus for improving vision and resolution of retinal images comprises a point source produced on the retina (101) of a living eye (100) by a laser beam (102). The laser beam reflected from the retina is received at a lenslet array of a Hartmann-Shack wavefront sensor (148) such that each of the lenslets forms an aerial image of the retinal point source on a CCD camera (146) located adjacent to the lenslet array. The output signal from the CCD camera is acquired by a computer (150) which processes the signal and produces a correction signal which may be used to control a compensating optical or wavefront compensation device such as a deformable mirror (118). It may also be used to fabricate a contact lens or intraocular lens, or to guide a surgical procedure to correct the aberrations of the eye. Any of these methods could correct aberrations beyond defocus and astigmatism, allowing improved vision and improved imaging of the inside of the eye.