Showing papers on "Adaptive optics published in 1987"

Active optics. I. A system for optimizing the optical quality and reducing the costs of large telescopes.

Abstract: A system of ‘active optics’ control for the optical imagery of astronomical telescopes has been under development in the European Southern Observatory for about ten years. Its first application will be in the 3·5 m New Technology Telescope (NTT) scheduled for operation in 1988. A model test with a thin 1 m mirror (aspect ratio 56) has given remarkably successful results which will be reported in Part II of this paper. Part I gives a complete presentation of the theoretical principles of this technique of active optics and its scope of application. The subject is treated from the viewpoint of the temporal band-pass of error sources, ‘active optics’ being concerned with the low-frequency band-pass. The high-frequency band-pass (‘adaptive optics’) is principally concerned with atmospheric correction and is only briefly referred to for comparison. ‘Active optics’ correction of the low-band-pass system errors should bring major improvements in image quality together with a large cost reduction. While ...

Partial atmospheric correction with adaptive optics

Abstract: Adaptive optics has seen only limited application in astronomical facilities, despite its significant potential for improving seeing conditions and increasing observing efficiency and productivity. Expense and technological difficulty appear to be the reasons that this is the case. Correction of large apertures requires hundreds of active elements in both the wave-front sensor and the adaptive mirror. We have performed some one-dimensional numerical simulations to test atmospheric wave-front correction when the active element is not matched to the correlation scale in the pupil. The results demonstrate that substantial seeing improvement can be obtained with an adaptive optical system having a limited number of active elements.

Dynamic characteristics of optical adaptive systems.

Abstract: The effect of time delay on characteristics of an optical adaptive system is considered. Classification of the optical adaptive systems as dynamic feedback systems has been made. Systems with constant delay, a highspeed adaptive system as well as a new class of systems, i.e., predicting adaptive systems, are investigated.

Adaptive optics for optimization of image resolution.

Abstract: Adaptive optics can be used to improve the quality of optical imaging instruments otherwise limited in resolution by atmospheric turbulence. To achieve a given level of resolution improvement at the lowest cost, design of adaptive mirrors must be carefully optimized. We have calculated the mean value of the optical transfer function (OTF) of an instrument corrected by adaptive optics using the OTF as a criterion for resolution optimization and describing perturbations through their phase structure function. A four-parameter numerical program has been written to compute the OTF. Assuming that the optics diameter and the perturbation Fried's parameter are known, it then becomes possible to optimize the system by a proper choice of the number of actuators and of the influence diameter, a parameter which characterizes the size of the required corrections.

Adaptive optical system with synchronous detection of wavefront phase

Abstract: An adaptive optical system with local wavefront sensing and control interferometrically determines wavefront phase aberration in an incoming electromagnetic beam and adjusts the reflective surface of a deformable mirror. The system is characterized by wavefront phase sensing local to the deformable mirror and detection apparatus which synchronously detects phase differences between an interferometrically modulated beam and a uniform modulator.

Hartmann Sensing At Adaptive Optics Associates

Abstract: A review of the current practice in wavefront sensor design at Adaptive Optics Associates (AOA) is presented. The principles of Hartmann sensing and matrix reconstruction are reviewed, and sensor performance required for compensation of atmospheric imaging is estimated. The structure of the digital processor based sensors built at AOA is discussed, with two recent systems as examples.

Adaptive optics for the European very large telescope

Abstract: Adaptive optics is one of the main features of the Very Large Telescope (VLT) of the European Southern Observatory (ESO) - an array of four 8 meter telescopes. These telescopes can be operated individually, in an incoherent and in a coherent interferometric beam combination mode. Each telescope will be equipped with adaptive optics systems for real-time correction of atmospheric turbulence effects. First results with a prototype system developed for the VLT demonstrated the feasibility and the significant gain of this technology for astronomical imaging. This paper describes the VLT adaptive optics system and its implementation program.

Aspherical Surfaces Engraved On Photoresist Coatings : Manufacture Of A Zonal Corrector Plate For An Aberrating Cassegrainian Telescope

Abstract: We investigate how aspherical optical surfaces engraved in photoresist coatings avail against static wavefront aberrations up to 3 μ. The spherical aberration of a singlet lens is corrected for. The image given by a cassegrainian telescope of 143 mm diameter with a peak to valley 0,62 µ wavefront error and 7" slopes is improved by a 16 mm diameter intrafocal phase equalizer corrector plate. We then discuss extrapolations of our laboratory experiments.

Continuously Deformable Mirrors

Abstract: Low spatial and temporal frequency correction of optical wavefronts can be achieved with continuously deformable mirrors that are being developed by the Atomic Vapor Laser Isotope Separation (AVLIS) Program at Lawrence Livermore National Laboratory. These devices are simple in design, low in cost, require relatively few actuators, and are capable of submicron deformation control using conventional stepper motors. The technique involves bending a mirror substrate with actuators that apply variable bending moments typically around the perimeter of the mirror. The location and orientation of these actuators and the thickness variation of the substrate determine the particular static shape that will be generated and the optical distortion that it can correct. For constant-thickness substrates the deformation will generally follow a curve that can be described by a quadratic function. However, by contouring the back surface of the substrate, higher-order deformations can be generated. Among the optical aberrations that can be generated by this technique are focus, astigmatism, coma, and spherical aberration. More complex shapes and other applications are being investigated. This method may also be useful in the manufacturing of aspherical optics.

Construction And Performance Of Minibench Acousto-Optic Signal Processors

Abstract: Construction and performance of minibench acousto -optic signal processorsG. C. Messick, R. R. Ulrich, and J. M. PellegrinoHarry Diamond LaboratoriesAdelphi, Maryland 20783R. D. Felock*AAI CorporationHunt Valley, Maryland 21030AbstractThe requirements of fielding acousto -optic signal processors for electronic warfareapplications place stringent demands on mechanical design considerations. Theenvironments that optical processors are required to operate in can drastically limittheir functionality. This paper discusses the design constraints, construction, andperformance results of two operationally different acousto -optic signal processors.IntroductionAcousto -optic (AO) signal processors are extremely well suited to handle therequirements of high- density electronic warfare (EW) environments. Problems arise,however, when optical processors are required to perform these EW functions under actualfield conditions outside the laboratory. Airborne systems, for example, must withstandlarge variations in temperature and pressure as well as the shock and vibration of theflight itself. Moving ground -based systems face contamination by dust and otherparticles, in addition to more severe shock and vibration.Reducing the physical size and weight of the processors solves some stability andperformance problems with minimal engineering effort. However, to construct and field areliable signal -processing tool, additional investigation into material selections andstructural integrity of the design is required.Many of the problems associated with optical elements in adverse environments havealready been solved under certain operating conditions (e.g., satellite -based optics,telescopes, etc). Lightweight optical structures have been developed with excellentperformance parameters. In addition much work has been done on optical channelizerscentered at frequencies above 1 GHz.' These channelizers, however, tend to have only alimited number of output channels (between 10 and 50) and correspondingly very shortoptical moment arms for the Fourier transform lens. By contrast, the AO spectrum analyzerwork discussed herein relates to center frequencies of 45 and 100 MHz, with requiredresolution of 25 and 60 kHz, respectively. The required time apertures restrict theoptical aperture length to between 14 and 31 mm. Much longer optical moment arms areneeded than in the gigahertz regime. An 18 -mm deflection in the Fourier plane dictatesmoment arms of 230 to 600 mm (depending upon whether the desired channelizer bandwidth is30 or 60 MHz). Proposed new optical detector arrays have increased center -to- centerspacings by nearly a factor of two, allowing even longer moment arms in future designs.Integrated optics provides an alternative to conventional optics that promises muchgreater compactness. However, the optical apertures and deflections required here are toolarge to be feasible in integrated optical systems for the foreseeable future.This paper deals with some of these considerations and the way they have been appliedin the construction of two AO signal processors: a compact spectrum analyzer (CSA) and aminibench one -dimensional time -integrating correlator (mini 1 -D).Description of the two AO processorsThe CSA is an AO spectrum analyzer that provides real -time optical Fourier spectrumanalysis of a wide band of electrical signals. The CSA is packaged in a 15.2 x 12.7 x*Work performed while employed at Harry Diamond Laboratories.

Wavelength Effects On Images Formed Through Turbulence

Abstract: The characteristics of wavefronts propagated through turbulence and the resulting images are discussed. Hot wire anemometry and holographic interferometry techniques are used to measure wavefront distortion. A model is developed to describe the associated images. The effect of viewing wavelength is examined in detail. A short wavelength limit is found in which the image size is aerodynamically limited and independent of wavelength. The distortion weakens with increasing wavelength until long wavelengths are reached where the image size is diffraction limited. The behavior of imaging systems throughout this wavelength region and its dependence on aero-optic parameters is examined.

Acousto-Optic Binary Data Recorder

Abstract: The design, fabrication, and testing of an acousto-optic-based binary data recorder are described. The data to be recorded are applied to an acousto-optic (AO) Bragg cell illuminated by a pulsed laser diode, and the bit sequence present in the cell is imaged onto a linear detector array. Emphasis was placed on incorporating the optical system and the linear detector array with its control board into a rugged module of modest size capable of recording non-return-to-zero data at rates of 200 Mbits/s or greater. The design specifications of the AO cell and the pulsed laser were based on earlier analysis and proof-of-principle experiments. An in-house-built TI3AsS4 AO cell with a bandwidth of 260 MHz centered at 400 MHz gave 11% diffraction efficiency at the 810 nm wavelength used in the system. When used with a laser pulse of 4 ns full-width at half-maximum, this system had a maximum operating rate of 240 Mbits/s. The same optical system used with an AO cell with 500 MHz bandwidth and a 1.7 ns laser pulse had a maximum rate of 500 Mbits/s. The quality of the optical imaging did not appear to limit the performance, suggesting the possibility of either further increasing the data rate or reducing the size. Possible modifications to explore these options are discussed.

Active Forces For Figure Control On A 1.8-Meter Mirror

Abstract: The surface accuracy of the primary mirror of a large telescope can be compromised by polishing errors, thermal distortions, material inhomogeneities, stress relaxation and errors in support forces One method of correcting the mirror figure is to apply forces to the mirror to introduce deformations that cancel out these errors The image quality goals of the National New Technology Telescope (NNTT) will require highly accurate surfaces on 8-meter diameter primary mirrors, and it is anticipated that an active force system will be necessary to maintain the required accuracy An active support system has been built and tested on a 18-meter prototype of the primary mirrors for the NNTT The existing astatic counterweight support system was modified to allow computer-controlled variation of the forces exerted Tests of the system have been conducted in two different modes; it has been used to compensate measured optical surface distortions, as well as to correct the thermal distortion predicted by computer modeling based on measured mirror temperatures Test results show that distortions corresponding to astigmatism, coma, spherical aberration, trefoil and quatrefoil can be effectively countered

Molded Glass Collimator/Objective Lens Pair For Optical Pickup Applications

Abstract: The design and expected performance of an all-glass, high-NA, collimator/objective lens pair are described. The numerical apertures are: 0.55 (objective) and 0.40 (collimator). Both lenses are biaspheric and both are designed to perform over relatively broad wavelength and temperature ranges. The as-fabricated, on-axis wavefront error of both lenses is expected to be less than 0.04 waves (RMS, 765-845 nanometers). This is one of the first such glass matched pairs to be designed for use with laser diodes in compact-disc, video-disc and optical-memory applications.

Mirror Optics for High-Power Laser Beams

Abstract: Modern CO2-lasers for materials processing exceed an output power of 10 kW. Additionally, the beam quality, e.g. given by the focussability, is increased. The achieved high intensity requires mirror optics. The conditions for their use were examined and will be reported. Additionally, future aspects such as correctable telescopes or intelligent optics are discussed.

Focusing of optical beams by elastic deformable mirrors

Abstract: The finite-element method is used to model an elastic mirror which is deformed at predetermined points. We consider a phase corrector consisting of a square mirror with four actuators, simply supported at its center. The mirror size and actuator configuration are optimized. We investigate the suitability of this mirror for correcting distortions of an optical beam propagating in a randomly inhomogeneous or nonlinear medium.