This PDF file contains the editorial “Editorial: Is Photonics the Answer?” for OE Vol. 29 Issue 10

This PDF file contains the editorial “Book Rvw: Optical Processing and Computing," edited by H.H Arsenault, T.Szoplik and B. Macukow for OE Vol. 29 Issue 10

This PDF file contains the editorial “Book Rvw: Introduction to the Optical Transfer Function," by Charles S. Williams and Orville A. Becklund for OE Vol. 29 Issue 10

Adaptive optics has recently been thrust onto the international scene by the successes reported by the European Southern Observatory and by the problems associated with the Hubble Space Telescope.

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

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

The analysis of a plane symmetric, four-mirror optical system that has a real pupil to accomplish passive or active wavefront correction is presented. Several system solutions are possible, but the emphasis of the analysis is given to those that might simplify the fabrication of a large telescope.

An optical wavefront propagating through the atmosphere will be perturbed by local variations in the refractive index ofthe atmospheric gases. When accumulated over long optical path distances they will impart a spatial and temporally random distortion to the wavefront. These distortions have a characteristic spatial coherence length r0 and an atmospheric decorrelation time ?0. In directed energy applications, atmospheric distortions can reduce the peak target energy densities of larger diameter laser beams by orders of magnitude. The problem is not solved through the use of larger apertures; once the aperture size increases beyond one or two r0, the far-field spot remains constant in size. Hence, for large aperture systems, the overall performance is set by the spatial coherence of the atmosphere and not by the system's exit pupil.

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

A deformable mirror has been designed and built to correct thermally induced aberrations in a medium average power solid-state laser (MPSSL) system. The mirror is capable of correcting typically induced errors of up to two waves in the narrow dimension and up to six waves in the long dimension. It has a clear aperture of 12 mm x 130 mm and serves as one element in the laser cavity. Piezoelectric translators are attached to the mirror through a unique flexure mechanism. This simple mechanical design has proven adequate for controlling low-spatial-frequency aberrations. Although designed specifically for the MPSSL, modifications of this design have application to other optical systems. The mirror design and performance as well as the methodology and techniques used in this development are discussed.

A common method for correcting wavefront aberrations in an optical system involves using an array of actuators to deform the optical surface of a mirror until the aberrations are reduced or eliminated. Recent interest in metallic sandwich mirrors, where the optical surface is separated from the back surface by a lightweighted core, has created a need for a method to correct wavefront aberrations other than by applying actuator forces. One possible way to deform the optical surface of a sandwich mirror is by independently pressurizing cells within the core of the mirror. A method of active correction utilizing independent pressurized "macrocells" in the core of an 18 in. diameter perforated aluminum honeycomb sandwich mirror has been attempted. This paper describes the experiments and the finite element analyses conducted on 6 in. mirror blanks containing progressively more complex single macrocells and the experiments conducted on two 18 in. mirror blanks, one with a cellular honeycomb core and the other with a machined aluminum core, each containing 19 individual macrocells. Results from the multiple macrocell mirror blanks show that establishing a spatially variable pressure distribution in the core of a sandwich mirror can be an effective active method for deforming the mirror's optical surface.

In some applications, moment actuators provide a competitive alternative approach to the conventional piston type of actuators. A piezoelectric or similar device placed between two posts on the back surface of a deformable mirror constitutes one type of moment actuator. A closed form solution of the biharmonic equation of Kirchhoff's thin plate theory is obtained by the classical method of boundary value problems of linear elastomechanics and presented in this paper for the influence function of an arbitrarily located and oriented moment actuator of the above type on the back of a flat circular deformable mirror. The solution covers both radial and tangential moments. The results clearly show their global character in contrast to those for the piston type actuators with high stiffness. An application of these functions to the design of a 30-actuator circular deformable mirror with moment actuators, related hardware, and test results has been reported elsewhere.

Real-time wavefront sensors can be viewed as processing channels that convert a continuous, two-dimensional wavefront phase variation into a parallel array of sampled wavefront values. In general, two pieces of data are required for each wavefront sample: the x gradient and the y gradient. We achieve efficient use of the electro-optical portion of this channel through optical multiplexing techniques that combine the x and y data into one signal. Time multiplexing, temporal frequency multiplexing, and spatial frequency multiplexing have each been implemented successfully in high performance, grating-based, shearing interferometers. The two frequency multiplexing techniques are examined in this paper. A 1000 point wavefront sensor is described as an example of spatial frequency multiplexing, and a typical design is used as the example of temporal frequency multiplexing. Performance of multiplexed sensors is not sacrificed, as demonstrated by the nearly 1000:1 dynamic range (total tilt:rms error) of the latter type of sensor.

A Hartmann wavefront sensor measures optical wavefront aberrations by using a lens array to break the input sample into subapertures. The lens in each subaperture produces an image whose centroid position depends on the average phase front tilt in the subaperture. The simplest measurement implementation, a quad cell detector, is limited in tilt dynamic range to ± 1 wave because the (diffraction-limited) image has a radius equivalent to 1 wave of tilt. For greater tilts, the image is off-null and no positional information is available. Dynamic range can be increased by defocusing the spot but only at the expense of increased centroid variance. However, if the spot image is optically intensified before being detected, the intensified image can be defocused without increasing the centroid variance. A theoretical derivation of sensor performance with and without intensification is given. Results of simulations and laboratory experiment are presented showing good agreement with theory. The use of this technique may greatly extend the capabilities of wavefront sensor systems at modest increase in cost or complexity.

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

The first demonstration of acousto-optical excision using a GaAs photodetector array is reported. A maximum isolation of 40 dB was obtained using a six element GaAs interdigitated finger double heterojunction photo conductor array. The center frequency was 45 MHz, and the swept bandwidth was 6 MHz. The response of discrete GaAs heterojunction photo conductors was linear with bias voltage, potentially making them useful for performing multiplication functions in optical analog filters. New concepts of using these photo conductors for optical adaptive filters are presented.

An imaging method using the Doppler effect is proposed in this paper. An object is illuminated by a sinusoidal wave from a linearly moving transducer. The frequency components of the reflected wave can be considered to be projections along lines at specified corresponding angles from the transducer. One can reconstruct images by applying conventional CT (computerized tomography) algorithms to the projections thus obtained. The theory is derived for coherent and incoherent processing. Theoretical results are confirmed by numerical simulations. Experimental results show that coherent Doppler tomography is much higher in resolution but more noisy than incoherent Doppler tomography.

The vessel geometry and densitometry in single-view coronary angiograms are analyzed with an emphasis on the local features and how they vary along a vessel segment. With the aid of a tracking algorithm the two-dimensional contour of a vessel segment is represented by one-dimensional functions, in terms of lumen width, direction, and intensity along the vessel centerline. A quantitative validation of this contour representation approach is established by using the computer-generated imagery. The spatial frequency responses of the vessel contour functions are characterized for human coronary cineangiograms and for digital subtraction angiograms. The lumen width functions have 90% of the signal energy concentrating below a spatial cutoff frequency of 2.5 to 3.7 per average lumen width. The application of the contour representation to vessel image compression is demonstrated. The result of this study provides a guideline for spatial filtering of the coronary arteriograms. The methodology should also be useful in analyzing coronary arterial abnormalities and tortuosity.

The design concept of the mirrors for the European Space Agency's high throughput x-ray spectroscopy mission XMM is characterized by thin (?1 mm) graphite epoxy shells as the mirror structure and a replication technique to produce the mirror surface. The mirror microroughness is specified to ? ?0.5 nm (rms); profile errors should not exceed ±2.5 arcsec. An ESA research study has been performed by Carl Zeiss and subcontractors (Dornier GmbH, Max Planck Institut für Extraterrestrische Physik/Garching, University of Leicester, X-ray Astronomical Group, Dept. of Physics) to clarify whether graphite epoxy mirror structures are able to meet the tight XMM mirror figuring and surface requirements and to determine what measures have to be taken to overcome the typical problems associated with composite materials used as mirror substrates, such as print-through of fibers and inner structure, short term/long term stability, and water absorption and outgassing. The starting conditions and progress during development are described. The present status of optical and x-ray performance of replicated flat graphite epoxy mirrors is reported, and aspects for further improvement of graphite epoxy mirrors is discussed.

A large number of mirror shells are required for the European Space Agency's XMM telescope three mirror modules, each of which consists of 58 thin walled highly nested Wolter I mirrors. The specified performance of the XMM mirror modules requires new design concepts and application of new technologies for the production of the lightweight mirror shells. Under an ESA contract, Carl Zeiss is leading a European industrial/scientific consortium to develop technologies that allow production of real size XMM mirror shells by a dedicated metal/epoxy resin replication technique. In late 1988 the first full size XMM mirror shell was successfully replicated, having the following characteristics: total length 600 mm, maximum shell diameter 400 mm, carrier material/thickness graphite-epoxy/0.8 mm, replication layers gold/epoxy, and total replicated mirror surface 7500 cm2. The development, preparation, and mirror shell replication are described, and the first results of the performance of the mirror shell after measurements and x-ray testing are presented.

Future synchrotron radiation facilities such as the Advanced Photon Source to be constructed at Argonne National Laboratory will produce dedicated and powerful beams ofx-ray radiation instrumental in advanced research in key areas of science, engineering, and medicine. The development of critical beamline optical components such as monochromators is pivotal in the successful utilization of the generated x-ray beam. The high heat load of the beam can severely distort, and thus adversely affect the performance of, such components. In this paper, a five-step numerical procedure to predict the performance of an x-ray monochromator subjected to high-power synchrotron radiation is described. The predicted performance of a model double-crystal monochromator system expressed in terms of its rocking curves satisfactorily presents the experimental results. This procedure can reliably be used in the design, evaluation, and optimization of monochromators and other optical components.

We have developed compact soft x-ray/EUV (XUV) and farultraviolet (FUV) multilayer coated telescopes for the study of the solar chromosphere corona, and corona/solar wind interface. Because they operate at short wavelengths (~40 < ? < 1550 Å), the modest apertures of 40 to 127 mm allow observations at very high angular resolution (0.1 to 0.7 arcsec). In addition to permitting traditional normal incidence optical configurations such as Cassegrain, Ritchey-Chrétien, and Herschelian to be used at XUV wavelengths, multilayer coatings also allow a narrow wavelength band (?/?? ~30 to 100) to be selected for imaging. The resuIting telescopes provide a very powerful and flexible diagnostic instrument for the study of both the fine scale structure of the chromosphere/ corona interface and the large scale structure of the corona and corona/ solar wind interface. In previous papers we described a new solar rocket payload, the Multi-Spectral Solar Telescope Array (MSSTA), composed of 17 of these compact telescopes. In this paper we report on the performance of the seven MSSTA Ritchey-Chrétien telescopes.

The background in x-ray telescopes in near-Earth orbits from low-energyelectrons (<100 keV) haslong been a sourceofconcern. Recent data on low-energy electron populations and improvements in the modeling of the propagation of low-energy electrons through Wolter I grazing- incidence optical systems now allow better quantitative estimates of the electron throughput of such systems. The modeling that has recently been done with respect to the U.K. soft x-ray telescope (Wide Field Camera) on ROSAT is described. The results from the modeling are then applied to several other missions using grazing-incidence optics, and the implications are discussed.

The rigid body motions and low frequency surface errors of grazing incidence Wolter telescopes are studied. The analysis is based on surface error descriptors proposed by Paul Glenn [Opt. Eng. 23(4), 384-390 (1984)1. In his analysis, the alignment and surface errors are expressed in terms of Legendre-Fourier polynomials. Individual terms in the expression correspond to rigid body motions (decenter and tilt) and low spatial frequency surface errors of mirrors. With the help of the Legendre-Fourier polynomials and the geometry of grazing incidence telescopes, exact and approximated first order equations are derived in this paper for the cornponents of the ray intercepts at the image plane. These equations are then used to calculate the sensitivities of Wolter type I and II telescopes for the rigid body motions and surface deformations. This theory also provides a tool to predict how rigid body motions and surface errors of the mirrors compensate each other.

We describe a conceptual plan for the development of a comprehensive long duration solar space observatory, the Advanced Solar Observatory (ASO). The ASO is intended to provide solar astronomers with the observational power (spectral, spatial, and temporal resolution, sensitivity, and breadth of wavelength coverage) necessary to address fundamental problems relating to the solar convection zone and activity cycle, the thermal and nonthermal processes that control the transport of energy, mass, and magnetic flux in the solar atmosphere, the generation of the solar wind, and the dynamics of the inner heliosphere. The ASO concept encompasses three proposed space station based instrument ensembles: (i) the High Resolution Telescope Cluster, which includes far ultraviolet, extreme ultraviolet, and x-ray telescopes, (ii) the Pinhole/Occulter Facility, which includes Fourier transform and coded aperture hard x-ray and gamma ray telescopes and occulted ultraviolet and visible light coronagraphs, and (iii) the High Energy Facility, which contains neutron, gamma ray, and low frequency radio spectrometers. Two other facilities, the Orbiting Solar Laboratory, which will contain high resolution visible and ultraviolet telescopes on a free-flying platform, and a package of Global Dynamics Instrumentation will, with the space station ensembles, form a comprehensive capability for solar physics. We describe the scientific program of the ASO, current instrument concepts for the space station based ASO instrument ensembles, and plans for their accommodation on the space station.