Showing papers on "Adaptive optics published in 1986"

Aberrations of optical systems

Abstract: Optical systems and ideal optical images. Geometrical optics. Gaussian optics. Finite raytracing. Finite raytracing through non-symmetrical systems. Optical invariants. Monochromatic aberrations. Calculation of the Seidel aberrations. Finite aberration formulae. Chromatic aberration. Primary aberrations of unsymmetrical systems and of holographic optical elements. Thin lens aberrations. Optical tolerances.

Configuring an electrostatic membrane mirror by least-squares fitting with analytically derived influence functions

Abstract: Electrostatic membrane mirrors are potentially useful as aberration generators. To realize this capability, it is necessary to know the influence functions (each giving mirror response to a single electrode) and then to use a fitting procedure to determine the optimal voltage settings for reproducing a desired surface shape. We show that an approximate analytical solution of Poisson’s equation exists that can generate the influence functions for a circular electrostatic mirror. We also show, by demonstration, that it is computationally feasible to calculate these influence functions and to use them with a fitting procedure to fit the surface of a 109-electrode mirror to a desired shape. Our methods allow one to test the theoretical performance of the 109-electrode mirror; we find that good fits are obtainable for Zernike polynomials of up to degree 6.

Wavefront Reconstruction from Noisy Slope or Difference Data Using the Discrete Fourier Transform

Abstract: A general algorithm is presented for reconstructing a two-dimensional wavefront optical path difference (OPD) map from noisy slope or difference measurements by means of a least squares fit using complex exponentials. This form of modal estimation can be described as a filtering operation in the spatial frequency domain. Thus fast Fourier transform (FFT) algorithms can be used for rapid reconstruction. The reconstruction is unbiased also in the case of finite data arrays. The error propagation from the noisy measurement data to the integrated wavefront is minimal in a least squares sense. It is believed that this reconstruction algorithm can be implemented in an adaptive optical system by using commercially available array processor hardware, thus reducing the total system cost and the need for specialized hardware.

National Optical Astronomy Observatories (NOAO) Infrared Adaptive Optics Program II: Modeling Atmospheric Effects In Adaptive Optics Systems For Astronomical Telescopes

Abstract: However pertect an adaptive optical system can he, it win never tuiiy correct tne image We discuss here the effects of amplitude errors due to stellar scintillation, the effects of chromatic errors due to both refraction and diffraction in a two-wavelength system, and the effects of non-isoplanicity All these errors are directly related to the height of turbulence layers An expression is derived for the scale height of turbulence which should be considered as an important parameter in selecting new sites for astronomica observations The performances of adaptive optical systems are usually described in terms of Strehl ratios We present here the results of computations of the whole transfer function for lon exposure compensated images It is shown that the point-spread function for a partially compensated image generally consists of an Airy disk surrounded with a halo The ratio of the energy in the Airy disk over the energy in the halo is independent of the telescope aperture and provides a better measure of the quality of the compensation

Method for improvidng the spatial resolution in an integrated adaptive optics apparatus

Abstract: Apparatus is disclosed for correcting wavefront errors in adaptive optics systems. A modified liquid crystal light valve is used as an integrated wavefront sensing and wavefront control system. A remote reference aberrated wavefront 10 first reflects off the liquid crystal substrate 20 and is then transferred by beam-splitters 14 and 15a, b and reflector 19 to the rear surface ofo the liquid crystal light valve 12. A one-to-one imaging system composed of lenses 70a, b and spatial filter 72 provide a new diffraction-limited optical transfer system which allows full use of the extremely high resolution capabilities of the liquid crystal light valve 12. The wavefront 10, after being imaged by the optical transfer system, is combined with a local reference plane 16, the interference pattern from which strikes the rear surface photoconductor 26. The photoconductor electrons liberated serve to alter the voltage across the liquid crystal at those points where the interference pattern has right maxima. Using a tunable birefringent liquid crystal layer 20, a voltage change causes a commensurate refractive index change in the liquid crystal substrate. These refractive index changes alter the optical path length of particular portions of the aberrated wavefront and the device's inherent negative feedback drives these phase errors to zero, at which point the wavefront 18 is completely corrected. A second laser beam 11 may be sent out of the device, reflecting off the corrective liquid crystal layer and predistorting it in order to correct for atmospheric path disturbances. The present invention provides an adaptive optics correction system possessing extraordinarily high spatial resolution.

National Optical Astronomy Observatories (NOAO) Infrared Adaptive Optics Program I: General Description

Abstract: We describe the general principles behind a polychromatic adaptive optics prograff astronomy which was started at NOAO recently. In this program the atmospheric wavef distortions are measured at visible wavelengths (700nm) using an astronomical object vicinity of the infrared object of interest. The resulting wavefront corrections ar applied to an infrared imaging system which utilizes a two-dimensional detector arra

Segment Alignment For The Keck Telescope Primary Mirror

Abstract: We propose to use a Shack-Hartmann camera to align and phase the thirty-six segments of the ten meter primary mirror of the Keck Telescope. Tilt errors are determined by employing the Shack-Hartmann device in a manner exactly analogous to that used in mirror testing and in adaptive optics. Piston errors are determined (with the same device) by exploiting diffraction effects near the intersegment edges. These two tests are shown to have sensitivities that are comparable to each other and that are sufficient to achieve the design goals of the telescope. We describe how the tilt test can be extended to give information both about the individual segment figures and about the global mirror parameters. Some of the complications and potential systematic effects associated with these tests are discussed.

Cascaded adaptive optics system

Abstract: A cascaded adaptive optics system for correction of aberrations in a light beam. The system includes a plurality of spatial light modulators (SLMs) in a cascaded arrangement to provide an adaptive optics system characterized by wide range, rapid response, and high resolution. The spatial light modulators may be identical or have characteristics that are complementary in providing a combined output that advantageously corrects for aberrations. A conventional adaptive optics system such as a deformable mirror may be used as one or more of the SLMKs. Liquid crystal light valves (LCLVs) may be advantageously used as one or more of the SLMs in the invention. A single LCLV may be cascaded with itself in a multiple-pass configuration by means of suitable reflectors.

High bandwidth triple product processor using a shearing interferometer

Abstract: An apparatus is described which performs optical processing on electrical signals to calculate the triple product integral in a manner which accommodates high bandwidth signals, which processes were formerly possible only in mechanically unstable designs.

An Optical Phase Reconstructor Based On Using A Multiplier-Accumulator Approach

Abstract: Phase sensors that are most commonly used in the adaptive-optics area typically measure the gradient of the phase. A phase reconstructor is necessary to obtain the phase at the actuator positions of the deformable mirror. In the past reconstructors to obtain the optical phase from gradient measurements have been built using resistive nets. These nets simulate a least-squares reconstruction algorithm. There are other algorithms which can be used to mate wavefront sensors and deformable mirrors with different geometries or which can improve the noise performance by using the spatial correlation of the phase. These types of algorithms are difficult to implement and change using analog techniques. In addition, since the movement of an actuator can influence the position of adjacent actuators it is desirable to include this effect in the reconstructor. One may also want to remove the piston and the tip and tilt from the signal applied to the deformable mirror, and determine the values of the focus and tip and tilt terms in order to provide signals to auxiliary mirrors. A digital reconstructor can provide this capability. An approach to a digital reconstructor which can calculate an optical phase which is any linear function of the gradient measurements is described. This reconstructor is based on using a multiplier-accumulator circuit in each channel. A single phase value is calculated in each channel by summing the result of multiplying each gradient measurement by a stored matrix coefficient. Several sets of matrix coefficients are stored in memory to allow one to change the reconstruction algorithm quickly. The circuitry used and the time taken to perform the reconstruction will be described.

National Optical Astronomy Observatories (NOAO) IR Adaptive Optics Program III: Criteria For The Wavefront Sensor Selection

Abstract: In the NOAO IR Adaptive Optics Program, we have elected to develop a system to use the visual light from an object to sense the wavefront errors and generate the corrections re-quired to give diffraction-limited imaging in the near-IR (2-10 microns). We have evaluated the performance of a variety of possible sensor systems, computing both their ideal performance and their expected performance with available detectors. A major consideration in this evaluation is the ability of the sensor to measure mean wavefront tilts of the visual wavefront over subapertures corresponding to the (larger) IR wavefront scale lengths. We have chosen to use a Hartmann-Shack sensor with red-sensitive image intensifiers and a Reticon detector.

Adaptive optics for IC-engine measurements

Abstract: The present paper is concerned with the development of miniaturized LDA-optical systems to produce small-size, optical velocity sensors that can be easily employed to measure flow velocities. Miniaturization is achieved by employment of graded index fibers to guide the incident laser beams to the optical head. The same sort of fibers are used to guide the collected scattered laser radiation back to the photodetector. In this way, the bulky parts of a conventional LDA-optical system can be placed and operated away from the test section. Small optical heads result that can be easily handled and traversed. Three different LDA-optical systems are described. Test measurements in pipe and channel flows are presented and first measurements in a fired internal combustion engine are provided to establish the correct functioning of the system.

A New Product For Photon-Limited Imaging

Abstract: Adaptive Optics Associates, Inc. (AOA) has recently constructed a new commercial low-light imaging detector which we call the P4oton Digitizing Camera (PDC). Based on the PAPA detector developed at Harvard Universityl", the PDC generates (x,y,t) coordinate data of each detected photo-event. .Because the positional address computation is performed opti-cally, very high counting rates are achieved even at full spatial resolution. Careful optomechanical and electronic design results in a compact, rugged detector with superb per-formance. The PDC will be used for speckle imaging of astronomical sources and other astro-nomical and low-light applications.

Interferometric Optical Test And Diffraction Based Image Analysis

Abstract: Interferometric modification of the Hartmann test method has been used for observatory tests of the optics of the Danish 1.5 m telescope on La Silla. Some results of the tests and our analysing methods are presented. We use the measured wavefront aberrations to compute the point source diffraction tmage. The computing method can also be used for theoretical analysis, e. g. for investigating the effects of the mirror deflections due to the supporting system.

Dynamic characteristics of adaptive optical systems

Abstract: An analysis is made of the dependence of the Strehl ratio of an optical system with correction of distortions on the time delay. A comparison is made of the efficiency of two adaptive correction systems: one with a constant delay and the other when not only a function but its derivative are used in correction. The required frequency band of an adaptive optical system is determined.

Cramer-Rao Lower Bound On Wavefront Sensor Error

Abstract: Wavefront sensors that can operate at low light levels, be built from present technology components, and provide accurate wavefront phase estimates in real time are required for use with adaptive optics systems. The use of estimation theory makes possible the evaluation of wavefront sensors without specification of the wavefront phase estimation algorithms. The Cramer-Rao method was used to find a lower bound on integrated rms wavefront sensor estimation error. In addition to an analysis of the general case, the error lower bound was numerically evaluated for the shearing interferometer wavefront sensor. Computer simulations of the atmosphere and wavefront sensor measurements including noise were performed. Using an appropriate algorithm, the phase was estimated and the resulting phase error was compared with the lower bound. The results support the validity of using the Cramer-Rao lower bound to evaluate wavefront sensor performance.

ESO Active Optics System: Verification On A Lm Diameter Test Mirror

Abstract: Active optics, as envisaged for the ESO NTT, is based on four principles: 1. The linear relationship between the applied support force and the deformation of the mirror resting on a force based support system. 2. The orthogonality of the corrected aberrations. 3. The restriction to correct only those aberrations for which small forces are necessary, i.e. long wavelength aberrations as e.g. astigmatism and spherical aberration, which are also those modes induced by errors in the system. 4. The use of precalibrated forces for these corrections. Using the Shack-Hartmann test method for the analysis of the wavefront, we have confirmed these principles in an experiment with a lm diameter spherical mirror with a thickness of 18,9 mm and 78 supports.

Aberration Field Properties Of Simple Non-Axially Symmetric Optical Systems.

Abstract: A new approach describing the third-order aberration fields of simple tilted and decentered optical systems (plane-parallel plate, wedge, mirror, including aspherics, and thin lens) is presented.

Optical processing and Space Station automation

Abstract: Optical information processing research aimed at Space Station automation applications is reviewed. The emphasis of the NASA Ames Research Center program is on intelligent optical pattern recognition and optical control processing. Attention is given to the primary functions of an overall scene understanding system: distortion-invariant optical feature generation, dimensionality reduction, object classification, and contextual information processing. A method of using synthetic discriminant functions to facilitate learning in a high-speed optical correlator is described. A discussion is presented of candidate analog and digital architectures for the optical implementation of state-estimation algorithms needed for the control of high-dimension dynamic systems. The multivariate system chosen for the optical control technology demonstration - a segmented, adaptive mirror and interferometrically based wavefront sensor - is also described.

Self-referenced High Resolution Adaptive Wavefront Estimation And Compensation

Abstract: A relatively simple 'all-optical' system, called a self-referenced interference phase loop (IPL), is presented for measuring spatial and temporal phase fluctuations over an optical wavefront, and also adaptively correcting that wavefront and/or generating a conjugate wave. It is shown that this system can unambiguously (with no phase quadrant ambiguity) estimate and compensate phase in real time over multiple Tr radians of dynamic range/ and is essentially unaffected by simultaneous wave amplitude variations. Furthermore, this system requires no external reference wavefront and can operate on partially coherent and multispectral (i. e, "white light") wavefronts, in which case it estimates and compensates optical-path-distance errors. In addition, the IPL readily lends itself to high resolution "all-optical" implementations containing thousands to millions of spatial resolution elements. The IPL can also be operated as a bistable array in optical information processing and computing applications.

Materials For Large Time Apurture Bragg Cell

Abstract: A guide for the selection of materials for large time aperture Bragg cells is presented. Effects of acoustic attenuation and acoustic nonlinearities are analyzed and the results are applied to summarize the material and performance trade-off. Since optical quality and relatively large optical aperture size are the fundamental requirements for a large time aperture Bragg cell, this analysis is limited to high quality optical materials such as Tellurium dioxide, TeD2 .

Optical Design Of The Hipparcos Telescope

Abstract: The design and the development of the HIPPAR-COS payload all reflective Schmidt telescope have been driven by the highly demanding challenge of reaching an accuracy of 2 milli-arcseconds (1.E-8 radians) in star position measurements. This achievement is made possible by an original optical arrangement of the telescope, based on the capability to develop the so-called "beam combiner", which consists in a reflective Schmidt corrector cut in two halves and reassembled by bonding with an angle of 29 degrees, with a final wavefront distortion of lambda / 60 RMS. The final optical performances of the tele-scope can be summarized by a loss of MTF better than 7 % with respect to the diffrac-tion limit and a chromaticity lower than 3 milliarc-seconds. After a short description of the principle of Hipparcos measurement and of the telescope optical configuration and a recall of the chromaticity concept, this paper gives a presentation of the development of the optics especially of the Beam Combiner, and technics used for the alignment and test of the telescope.

Family Of Three Mirror Telescope

Abstract: In order to develope the three mirror telescope, a formula group of third order aberrations and a program had been developed. Two families of the aplanat with zero astigmatism or zero distortion were found. One of them is a new result. Based on the family 2, a 10 me-ter diameter telescope with a spherical primary mirror had been designed. The primary's F-number is 7/1.5 and the full field angle of view is 20 arc minutes. The RMS spot diameter (5 colours) is less than 0.313 arc second.

National Optical Astronomy Observatories (NOAO) Infrared Adaptive Optics Program Iv: Ir Background Speckle Noise Induced By Adaptive Optics In Astronomical Telescopes

Abstract: The infrared emission of astronomical sources is buried in a large background due to the thermal emission of the telescope. The real time deformations of an adaptive flexible mirror are likely to produce a spurious modulation of this background, adding noise to the signal. We estimate here the amount of noise introduced by such a mirror.

Reversal of CO2 laser radiation wavefront in a system of three interacting beams

Abstract: A study was made of reconstruction of the wavefront of CO2 laser radiation in the course of forward resonance parametric scattering by gaseous SF6 and a reverse passage after reflection from a mirror.

Spatial Interferometry With The European VLT

Abstract: The array concept for ESO's Very Large Telescope (VLT) project opens the possibility of observations with very high angular resolution. The long baseline provides a resolution span from approx. 0.5 marcsec in the blue to approx. 30 marcsec at 20 um wavelength. Realistic target for the interferometric operation is the IR range from 5 to 20 pm and later a gradual expansion to shorter wavelengths. In the interferometric mode the gain of the 8 meter single apertures of the VLT is only given if adaptive optics is applied for a real-time partial or full phase compensation of the degradations due to atmospheric turbulence,

Adaptive optical correction for extended source imaging.

Abstract: The problem of imaging the solar surface is considered. A theoretical study is presented that indicates that traditional adaptive optic techniques are of limited usefulness for solar imaging. A new method for adaptive optic image sharpening for extended sources is described. Results from numerical simulations show that this technique shows promise for image sharpening under conditions of moderately severe turbulence.

A Shearing Interferometer For Astronomical Wavefront Sensing

Abstract: A wavefront sensor has been designed to measure wavefront aberrations occurring during astronomical observations with high spatial and temporal resolution. Results of laboratory experimentation are presented. The sensor consists of a grating lateral shear heterodyne interferometer using a solid-state detector array. It has high light efficiency and is a self-referencing interferometer, thus allowing the use of white-light extended astronomical sources for the measurements. The wavefront sensor employs a heterodyne technique and is insensitive to intensity variations across the pupil and detector nonuniformities. To obtain a wavefront map, two sets of orthogonal wavefront difference data from the lateral shear interferometer are required. Both sets are measured simultaneously using a single laminar grating to produce the sheared diffraction orders and a single solid-state detector array placed in a plane conjugate to the telescope pupil plane. The signal from the detector array is digitized and fed into a computer where the wavefront differences are calculated. From these differences a wavefront map on a square array of 24 by 24 points is obtained by filtering the data in the spatial frequency domain. The time required for the acquisition of a complete set of data for one wavefront map is 13.3 ms. A wavefront reconstruction may be done within several milliseconds by using fast algorithms and floating point processor hardware.