In order to correct, with a wide bandwidth, wavefront distorsions due to various origins, such as thermic deformations of mirrors or atmospheric turbulences, we use a dither mirror, a correction mirror and powerful computers controlling a complete electronics system (detection, dithering and control). Wavefront compensation is achieved by a modal dithering and a modal control. Therefore we can control the main optical modes present in atmospheric turbulences and improve their compensation. Concerning to modal dithering, we send to each actuator of the dither mirror a linear combination of the dither frequencies which is a beating produced by a digital synthetizer which has a limited dynamic. Therefore the dither beating set must be normalized, which may reduce the control performances. So we propose a method to optimize the dynamic of the beatings by adjusting the phase of individual dither frequencies at the output of the synthetizers.

An adaptive optics system is considered as a multi loops servo system. The main components of the ioops are: - the wavefront sensor, - the real time digital processor, - the adaptive mirror and the high voltage control amplifiers. These loops are closed via the wavefront itself. Generally the detector of the wavefront sensor is a CCD camera. At first, images of the CCD matrix are digitized and the processor computes the local slopes of the wavefront ; secondly, the processor performs the calculation needed to output the values of the controls. Finally data are sended to the adaptive mirror through a digital/analog converter and the power amplifiers. A scheme of the loops is given where each major constituent is replaced by its transfer function. The time lags due to the CCD matrix and calculations are expressed. Using the Nichols criterion which defines the stability of the loops, the effects of the sampling rate and time lags on gain and bandwidth are studied. The results of a numerical model are given and comparisons are made with the COME-ON experiments. Same paper has been presented during the SPIE's 1990 Symposium on Astronomical Telescopes and Instrumentation for the 21St Century in February 1990.

The ultimate performance of an adaptive optics (AO) system can be sensitive to specific design parameters of individual components. The type and configuration of a wavefront sensor or the shape of individual deformable mirror actuator influence functions can have a profound effect on the correctability of the AO system. This paper will discuss the results of a theoretical study which employed both closed form analytic solutions and computer models. A parametric analysis of wavefront sensor characteristics, noise, and subaperture geometry are independently evaluated against system response to an aberrated wave characteristic of atmospheric turbulence. Similarly, the shape and extent of the deformable mirror influence function and the placement and number of actuators is evaluated to characterize the effects of fitting error and coupling.

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 (E.S.O.) 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.

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.

The wavefront retrieval method using a programmable optical signal processor with a spatial light modulator , lens and point- photodetector , which is performing the general Fourier series expansion is described.

Phasing an array with steep slopes and large surface errors, such as in NASA's Precision Segmented Reflector (PSR) Program for submillimeter astrophysics, is difficult using conventional optical techniques because the surface errors on the panels are large as compared with optical wavelengths. This paper extrapolates a procedure proposed by the Keck Observatory whereby physical optical techniques phase the edges of the panels, but with this technique they are phased with respect to a reference surface obtained from a high-quality central panel. A full-surface height-map is thereby obtained by integrating slope measurements, and full-surface phasing can be achieved using the modified and conventional Shack-Hartmann camera.

A deterministic algorithm is outlined which controls resonator loss by means of profiling active resonator mirrors to control the laser output power from an active medium. The control algorithm is designed to meet two basic requirements: that it provide a range of resonator transparency which permits laser output power ranging from zero to the maximum output power level for initial unsaturated gains or a wide range of pumping rates; and that the reference resonator output beam's quality and transverse dimensions be preserved during output power control. A stationary phase approximation is used to study the active resonator, and numerical calculations are performed for an active resonator with controlled losses. The curvatures of the axial areas of active mirrors are found to contribute most to resonator loss. Two channels of control over the curvature of the axial elements of active resonator mirrors are found to be adequate in the algorithm for loss control. Unstable control regions are identified in the numerical modeling, and the algorithm permits the control of laser output power over a range of pumping rates.

An absolute instability achieved at a zero-frequency detuning and at an input-signal energy of 10 to the -10th J is described, which has a high weak-signal reflection coefficient of 5 x 10 to the 6th. Absolute instability of the interacting waves is demonstrated both with and without having an external signal applied. The wave detuning can be varied smoothly by means of angular detuning of the oppositely directed pump reference waves, which also permits angular selectivity in the reflection of an external signal.

A compact XeCl laser system made up of an oscillator and an amplifier is described. By applying a stimulated Brillouin scattering mirror (SBSM) to the amplifier, an output laser beam of optical and spectral characteristics very close to those of the oscillator has been obtained. By applying to the amplifier a phase conjugate cavity, 1.7m long, formed by an SBSM and a quartz flat window, a pulse train made of pulses separated by the cavity round trip time has been obtained. The first pulse was 8 nsec long while the last pulse was shorter than 2 nsec.

This contribution describes the results of an investigative study of the use of a commercial narrow band excimer laser in photolithography. The technique for image projection involves the use of phase conjugate mirrors which we produced by stimulated Brillouin scattering . Feature sizes down to 80 m could be projected. Attempts to transfer smaller structures failed since diffraction effects led to an inflation of the focal volume, which eventually reduced the power density below the threshold for stimulated Brillouin scattering. As will be shown the properties of the commercial light source represent the major obstacle towards a substantial improvement in the imaging performance.

A real-time phase visualization technique is proposed which utilizes a phase conjugating mirror that reflects an image of the phase derivative formed within an optical differentiation system. The technique compensates for the phase variation, and the amplitude variation is handled by a coherent optical integration system, allowing real-time quantitative visualization of the actual object phase. Computer simulation and experimental results demonstrate the implementation of the phase contour visualization.

A bimorph mirror seems to be a low voltage, large strokes device which can be used as a correction mirror in an adaptive optics system for infrared applications. A few theoretical results are recalled and have been used to develop a numerical method to solve the displacements of a bimorph mirror supplied by a distribution of voltages. An example is given which involves seven electrodes ; comparisons with theoretical and other numerical results are achieved. Same paper has been presented during the SPIE's 90 Symposium on Astronomical Telescopes and Instrumentation for the 21st Century in February 1990.

Adaptive optical system structures for deformable mirrors are compared with special attention given to analyzing the effects of the structures on the optical systems. A mathematical model considers both aberration criteria and RMS deformable mirror surface deviation from ideal shape to describe laser optical activity in terms of an automatic control system. A significant relationship is found between automatic control system structures and the chosen optical quality parameter, which defines the quantity of control loops and interconnections.

Several levels of wavefront control are necessary for the optimum performance of very large telescopes, especially segmented ones like the Large Deployable Reflector. In general, the major contributors to wavefront error are the segments of the large primary mirror. Wavefront control at the largest optical surface may not be the optimum choice because of the mass and inaccessibility of the elements of this surface that require upgrading. The concept of two-stage optics was developed to permit a poor wavefront from the large optics to be upgraded by means of a wavefront corrector at a small exit pupil of the system.

The Precision Segmented Reflectors (PSR) project represents a first step toward developing the technology base needed to support future advanced astrophysics missions, especially those that operate at submillimeter wavelengths. The focus of the effort is to develop a lightweight, low cost option for building large aperture, segmented reflecting space-based telescopes. The principal driver for PSR technology is the Submillimeter Telescope program including the proposed 20 meter Large Deployable Reflector (LDR) telescope, and its precursor-survey mission which is currently in early development and definition. Four major technical areas, reflector panels and materials, structures, and figure control are under development by PSR. These technical areas are, however, generic in nature and can be applied to other future missions such as optical communications, optical interferometers, and missions requiring large diameter, segmented reflectors. In this paper, specific project objectives, approaches and significant challenges will be described. Technology development issues encountered will also be discussed. Finally, project status will be reported.

An analytical model of an active flexible mirror is described in which a thin plate with discrete piezoactuators comprises the proposed mechanism. The model of the adaptive mirror (AM) surface formation is given in terms of the thin-plate theory of the substrate's and actuator's elastic interaction. AM parameters are determined by means of the model as is a numerical simulation of the AM operation in an adaptive optical system (AOS). The AOS control algorithm utilizes orthogonalization of the wavefront sensor to overcome phase distortion in phase conjugation. A Hartmann-type wavefront sensor and a mirrorlike phase corrector with discrete actuators are considered in the numerical simulation of the phase-conjugate AOS. Compensation error in the proposed 19-channel hexagonal-array AM algorithm is found to be between 1.5 and 4 times higher than that required for optimal control in the case of Seidel aberrations.

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.

For the first time in ground-based astronomy, diffraction-limited imaging through atmospheric turbulence has been achieved in real time by adaptive optics in the infrared wavelengths range. This paper presents the first results and a short analysis, which demonstrate the considerable gain in resolution and sensitivity by the application of this technique. Single stars, close binary stars, and a satellite have been resolved. In one cases an other star several arcseconds apart has been used as reference for the wavefront sensing.

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).

A scalable wavefront correction device with high spatial and temporal frequency is described in terms of the concept and technologies developed for its realization. Electronic microcircuit technology is combined with traditional wavefront corrector methods to define the proposed integrated wavefront corrector (IWC) concept. The development of unique point-of-departure actuator material allows large electrostrains to be generated at reduced applied voltages. The specifications of the device are given and the detailed structure is shown in drawings. The single-layer actuator technology can be fabricated at a density of about 500 actuators per square cm. The composite structure developed for this adaptive optics technology is shown to preserve the structural integrity required for phase compensation, and provide the packing density requirements of high spatial frequency. The device is shown to permit large-scale wavefront correction and is of interest to deformable mirror technology.

The Very Large Telescope (VLT) presently being developed at ESO is described in terms of technological advances which make its use both technically effective and feasible. The VLT capitalizes on advances in materials, polishing techniques, and mirror support systems. The VLT consists of four 8-m alt-az telescopes and a 2-m auxiliary telescope in a single-dish configuration with Zerodur meniscus mirrors passively supported on a lateral system. A discussion of the tradeoffs between glass and metal mirrors is presented, and computerized polishing is described in relation to optical specifications. The mirror is supported with 150 axial and 60 lateral supports with electromechanical actuators to modulate applied force. The active optics concept is employed via the flexibility of the primary mirror, which generates elastomechanical deformations and the position and orientation of the secondary mirror.

The glass ceramic ZEROLXJR is excellently suited for the manufacture of mirror substrates hith is demonstrated by various astronomical telescope proj ects .The new spin casting tecbnique has been developed by ScFKYIT for the manufacture of thin monolithic mirror blanks exceeding 8 ra in diameter. This development resultel in sccirr obtaining the order for the manufacture of 4 meniscus shaped shells for the Very Large Telescope of the European Southern Observatoiy. The corresponding production facilities are currently being erected with the first casting Scheduled for the end of 1990 . Handling and suppcxtt of the huge blanks during the different process steps are the greatest challenges besides the spin-casting of about 50 t of glass . sciicrr has also performed considerable developmental work in the area of lightweighted ZEROLIJR mirror substrates which can be fabricated by applying various techniques.

The ArbeitsGemeinschaft Deutsches GrossTeleskop (AG-DG1) is concerned with the realization of a telescope in the 12 m class and has charged Carl Zeiss with a feasibility study concerning the optical figuring of large mirror blanks. The state of the art technology of spin-cast ZERODIJR mirror substrates at Schott Glaswerke allows fabrication of thin meniscus blanks up to 8.2 m in diameter. Therefore the primary is to be assembled from several single segments and should be optimized for the segmentation and the polishing process. Two methods of segmenting the primary mirror are considered: Combining either four 90-degree segments or a central 8 m monolith surrounded with 12 segments of 3Odegree each. Several methods of figuring and testing of out-of-axis segments are discussed. Techniques for a stiff connection of single segments are examined. Basic considerations of the axial support system of nonrotational segments are presented. The printthrough effect on the mirror's surface is calculated with varying pad distances and with varying geometry of the support points. The Membran Tool Processing (MTP) developed by Carl Zeiss will be used for polishing the mirror blank. With the active controlled strip tool large areas can be asphencal shaped and polished in a much shorter time than with the conventional technology.

In order to address the variance in the focal lengths of the segments for the Deutsches Grossteleskop (DGT), a method is delineated to calculate permissible focal tolerances. Sets of arbitrarily distributed delta-f (differences in focal lengths), where for each the average is zero and the standard deviation is normalized to 1 mm, are used to calculate the permissible tolerance. The DGT's correlation function provides the criteria for the analysis, and the results demonstrate that the two types of segmentation considered should have a delta-f of 0.072 for the four-quadrant configuration and of 0.360 for the 13-piece configuration.

The ten meter diameter Reck telescope, currently the largest of the telescopes under construction in the world today, consists of thirty-six 1.8 meter diameter off-axis hyperbolic hexagonal mirror segments. These are comprised of near zero coefficient of expansion glass ceramic substrates manufactured by Schott Glaswerke of Germany under the trade name of Zerodur. The blanks are approximately 1.9 meters in diameter and 7.5 centimeters thick. To produce the aspheric segments (consisting of six different configurations) in a timely fashion, scientists at the University of California have developed the technique of stressed mirror polishing. This method employs the introduction of shears and moments about the segment periphery, in its circular shape, to bend the mirror into the reverse of the desired shape . A true sphere is then ground and subsequently polished into the segment, after which the loads are removed and the desired optical prescription obtained. Next, the segment is cut to the hexagonal shape and a central hole, 0.25 meters in diameter, is core drilled partially through its hack. The segment is then mounted to its final support structure, Mirrors constructed in this fashion have many advantages, which include avoidance of the large capital facilities needed to cast large monolithic blanks, and logistical problems of transportation or risk in fabrication or repair. Conventional polishing techniques for segments, however, which have no symmetrical axis, are very expensive and time-consuming, unlike the stressed mirror process which involves the fabrication of spheres using a single, large tool. The process involves theoretical prediction of the loads required to bend the surface, and iterative solutions based on test measurements to fine tune the desired shape. The iteration requires the use of the theoretical math model, which consists of a closed form solution to a flat plate, as well as a detailed finite element model of the segment, which includes the effects of shear and curvature. This paper discusses some of the theoretical and test correlations achieved to produce the required configurations. Very good agreement is found, such that the deformed surface can be achieved after as little as one iteration accurate to the tenth of a micron level. After the segments are cut, their shape may warp slightly due to residual stress levels in the blank itself. Using the detailed finite element model, a first order analysis of the springing effect, including the cutting of the central hole, is presented, and further correlation to test data made. Stress birefringence data has been successfully utilized in determining the magnitude and shape of the expected change. Again, results show a reasonable correlation can be made, allowing anticipation of the presumed spring prior to polish of the segment. Prediction allows for adequate range of adjustment and minimized force application for the corrective devices used to warp the segments to their final shape.

Progress on the German large telescope project (DGT) since 1988 is described, with emphasis given to advances in mounting design and important manufacturing aspects related to segmented optics. The two segmentation possibilities, various polishing methods, focal tolerance considerations, and control of the segmented primary for the DGT are set forth, and the Hexa-pod mounting design and dome structure are mentioned. Site selection, funding considerations, and the possibility of an international consortium to continue the project are listed as issues still to be addressed.

Extensive finite-element calculations have been done on a novel type of optical telescope mounting to estimate the dynamic performance as well as the dynamic sensitivity of specific components of the structure. The finite-element analysis led to improved dynamics by welldirected stiffening of the basic structure. At the end of the process a lowest structural eigenfrequency of 8 Hz could be achieved for a 12-m telescope. The finite-element model of this design was also used to estimate the tracking performance of the mount directly. Moreover, it was used to establish and test a control scheme for the telescope that is able to damp the structure within less than a second but residuals in tracking error remain on smaller time scales. Those can not be removed by driving the entire mount but are to be eliminated via fine correction in a second stage.

A practical method for reducing geometric aberrations resulting from the misalignment of an astronomical telescope is presented. The wavefront distortion is measured with a lateral shearing interferometer placed near the telescope focus. Atmospheric seeing conditions do not need to be particularly good. Ray tracing optimization is applied to the optical design and is combined with the observed aberrations to compute the corrections required for the telescope alignment. Applications to a 0.5m Ritchey-Chretien telescope are discussed.

The Talbot effect is utilized to correct an alignment problem related to a neural network used for image recognition, which required the alignment of a spatial light modulator (SLM) with the input module. A mathematical model which employs the Fresnel diffraction theory is presented to describe the method. The calculation of the diffracted amplitude describes the wavefront sphericity and the original object transmittance function in order to qualify the lateral shift of the Talbot image. Another explanation is set forth in terms of plane-wave illumination in the neural network. Using a Fourier series and by describing planes where all the harmonics are in phase, the reconstruction of Talbot images is explained. The alignment is effective when the lenslet array is aligned on the even Talbot images of the SLM pixels and the incident wave is a plane wave. The alignment is evaluated in terms of source and periodicity errors, tilt of the incident plane waves, and finite object dimensions. The effects of the error sources are concluded to be negligible, the lenslet array is shown to be successfully aligned with the SLM, and other alignment applications are shown to be possible.

For the assembling of the optical systems with high performance and submicron tolerances it is necessary to optimize the system while it is being assembled. This means that parts of the system must be positioned and fixed with sufficient accuracy, and that the process must be checked by optical measurement. Sensor controlled optical assembly is the combination of these activities, integrated with optical and mechanical design. We expect that sensor controlled optical assembling will make possible the manufacturing of new optical systems with high performance. In this paper we discuss the methods and technologies necessary to implement sensor controlled optical assembling and we give examples of applications. The theoretical background of this work was given in a paper delivered at the SPIE Annual Meeting 1989 in San Diego.

Interferometrically measured system wavefronts combined with a differential ray-traced lens model are used to determine alignment adjustments to a lens system. This is achieved by minimizing the difference between measured and ray-traced wavefronts by means of a damped least-squares method, without information regarding individual surface shapes or misalignment sources in the system. The accurate quantitive adjustments provided by the technique reduce the time needed to align lens systems, and even complicated systems can be aligned.

The principles of the design of high-precision diffraction quality optical systems on the basis of the computer modeling of real optical systems have been considered.The modeling is performed with the use of the information about wave front deformations by optical surfaces, components and the system as a whole. The application of these methods at different stages of the manufacturing and an example of the alignment of a multi element lens are also given.

A new class of long focal apochromatic optical systems is described which permits the formation of images in different spectral ranges. The systems are composed of kinoform elements combined with conventional optical elements. The correction of monochromatic and chromatic aberrations is discussed theoretically with respect to their occurrence in long-focal, large-f-number astronomical lenses. Kinoform elements are compared to traditional optical elements, and the synthesis of long focal apochromats is shown to be possible with kinoform elements and typical optics. A discussion of diffraction efficiency and spectral selectivity in kinoform elements shows that acceptable characteristics for these optics can be achieved over a wide spectral range. Kinoform element objectives can be applied to high-resolution refractors, stellar sensors, large-telescope guiders, and collimators. The apochromatic lenses based on kinoform elements weigh 30-40 percent less than traditional lenses and offer high correction of chromatic aberrations, low thermal sensitivity, and a minor augmentation of stray light.

We have developed a software package for interactive design of optical systems. This software has extensive graphic visualization methods to interactively study the alignment characteristics of an optical system. The optical system layout, the multi-dimensional optical parameter spaces, and the image space can be displayed on a computer screen. The user can interactively vary one or more design and alignment parameters and analyze the corresponding behavior of the system. This results in a very powerful design tool. In this paper we present some details of the software package and preliminary results in optical system design and alignment.

Recent studies in the field of EM optics are presented in terms of the methods used and the potential and limits of the field of research. Problems are addressed which can be reduced to the scalar level, where Maxwell's equation (ME) is reduced to Helmholtz's equation (HE) and often requiring the use of ME to amplify problems encountered in HE. Consideration is given uniquely to time-harmonic fields represented systematically by complex functions of space and with attention given to time-dependence in exp(-i omega t). The closed-form solutions are given, and the integral, differential, and modal expansion methods are set forth. Possible applications for the methods are discussed, with emphasis given to the properties of gratings. Diffraction is described for rough surfaces, for a slit, and for integrated optics. The EM theory of anisotropic gratings, homogenization techniques, and asymptotic analysis are shown to be significant concepts that can have practical applications in optics.