The finite element method is today strongly identified with structural mechanics and it was, in fact, developed in the middle
third of this century by the structures community in order to achieve useful solutions in the field of the general theory of
elasticity. It is often overlooked however that in the preceding centuries that were required for the development of the
general theory of elasticity, the effort was driven by and often spearheaded by the very physicists and mathematicians who
were hypothesizing about molecules, gravity, electro-magnetics and all the other disciplines that today make up the over-all
field of "mechanics." It is not possible to appreciate the present status of the finite element method without understanding
The finite element analyses of two large lenses for the Keck Telescope High Resolution Echelle Spectrograph are described. The two lenses, one simple lens, and one meniscus, are of fused silica and are approximately 800 mm (30 in.) in diameter. The purpose of the analyses is to determine the deformations of each optic under its own weight, and to identify the simplest, most cost effective mounting cell that will satisfy the optical requirements. Two common radial supports are analyzed, including varieties of hard point and band type mountings. Several types of axial supports are examined including simple three-point mounts, ring mounts, and static deformation mounts. A parametric finite element input routine is described, whereby a solid model and finite element mesh are automatically generated, given the lens diameter, central thickness, and surface radii of curvature. Deformation predictions from the models are compared with theoretical calculations, interferometric testing, and precision profilometry.
A method has been developed for mounting charge-coupled device (CCD) arrays in an optical telescope so as to minimize thermal defocusing errors. The mounting arrangement was developed for a six-inch aperture, visible band, off-axis reimaging telescope attached to an experimental satellite. The mounting arrangement consists of two pieces: a fiberglass frame which holds the actively cooled CCD package and provides thermal isolation from the telescope body; and a titanium flexure, which acts to minimize structural distortions caused by the difference in thermal expansion properties of the CCD array and the telescope body. This paper describes the design, analysis, and testing of this CCD array mounting arrangement. A detailed finite-element model of the CCD array and the mount was developed and used to predict thermally-induced defocus and gravity sag deformations, as well as natural frequencies. Experimental tests to verify the computer model results were performed using holographic interferometry. Vibration tests were also performed to verify the natural frequencies as well as structural integrity during launch. A comparison of the computer model predictions and the holographic interferometric measurements of thermally-induced defocussing indicates agreement to within 15 to 20%. Both the experimental and computer results indicate that the mounting structure provides focus stability over the operational temperature range of the telescope with sufficient structural integrity to survive the anticipated spacecraft launch loads.
Finite element analysis (FEA) of large space optics enhances cryogenic testing by providing an analytical method by which to ensure that a test article survives proposed testing. The analyses presented in this paper were concerned with determining the reliability of a half meter mirror in an environment where the exact environmental profile was unknown. FEA allows the interaction between the test object and the environment to be simulated to detect potential problems prior to actual testing. These analyses examined worse case scenerios related to cooling the mirror, its structural integrity for the proposed test environment, and deformation of the reflective surface. The FEA was conducted in-house on the System's Reliability Division's VAX 11-750 and Decstation 3100 using Engineering Mechanics Research Corporation's numerically integrated elements for systems analysis finite element software. The results of the analyses showed that it would take at least 48 hours to cool the mirror to its desired testing temperature. It was also determined that the proposed mirror mount would not cause critical concentrated thermal stresses that would fracture the mirror. FEA and actual measurements of the front reflective face were compared and good agreement between computer simulation and physical tests were seen. Space deployment of large optics requires lightweight mirrors which can perform under the harsh conditions of space. The physical characteristics of these mirrors must be well understood in order that their deployment and operation are successful. Evaluating design approaches by analytical simulation, like FEA, verifies the reliability and structural integrity of a space optic during design prior to prototyping and testing. Eliminating an optic's poor design early in its life saves money, materials, and human resources while ensuring performance.
This paper describes the structural design, analysis, and load/deflection tests of a generic all- beryllium telescope truss (metering structure). The assembly serves to verify the viability of building lightweight precision mirrors and structures for high performance space-based optics and to measure the integrated performance of state-of-the-art CCD focal planes. It demonstrates the feasibility of manufacturing lightweight and stiff structures that can support high performance optical systems and meet their alignment requirements. The goal of building a telescope worthy of space flight was met.
Optical mirrors have historically been constructed of ceramic materials (e.g., glasses) or high modulus metals (e.g., beryllium). The employment of these materials for space-based optical applications is undesirable due to their mass and magnitude of thermal expansion. Thermal gradients can produce stresses on the mirror which would influence the mirror form unfavorably. Composite materials have been developed to exhibit a near-zero coefficient of thermal expansion in conjunction with a reasonably high modulus and good thermal conductivity. There is considerable interest in utilizing typical metals, ceramic, and composite materials in optical mirror applications.
Several years ago the Jet Propulsion Laboratory embarked upon a program to develop advanced polymer composite mirror elements as enabling technology for orbiting far IR/submillimeter telescopes. Structural composite mirrors have the advantages of high specific stiffness, good thermal stability and low cost that beneficially affect the entire telescope design. The goal of this panel development program is to design and fabricate prototype mirror panels: up to one meter in size with a surface precision and orbital thermal performance of a few microns, to achieve real densities close to 5 kg/sq m and to demonstrate the thermally stable performance of these panels experimentally. Studies leading to current mirror design are summarized. The precision and thermal performance of the mirror panels were tested in a dedicated test facility. Data showing panel performance are presented. Finally, the test results are compared to the analytically predicted panel performance.
The primary objective of this paper is to evaluate the optical pointing performance of the PSR Moderate Focus-Mission Structure when subjected to both mechanical and thermal disturbances. The mechanical disturbances are based on secondary mirror chopping. Results indicate that dynamic responses of the primary reflector and the secondary reflector subjected to chopping disturbances of the secondary reflector about its center of mass are within the figure maintenance control capabilities. The effects of modal damping, truss-type secondary support, interface boundary constraints, and alternate configurations, are also evaluated in the analysis. Thermal distortions of the structure were also evaluated based on the on-orbit temperature profiles derived from the submillimeter telescope missions. Results from thermal deformation analysis indicate that figure initialization control is needed for the PSR Moderate Focus-Mission. However, a figure maintenance system may not be required if adequate thermal isolation is incorporated into the support truss design for the PSR Moderate Focus-Mission Structure.
The rotational stiffness of hinge joints, and the gap of the joints applied in large deployable trusses, have been experimentally shown to have a significant role in determining such structures' dynamic behavior; an analytical validation of these results is presented for the case where linear rotation springs are used to model the hinge joints employed in a simple beam in trusses. The results obtained indicate that the natural frequencies of these structures depend not only on joint stiffness but also on joint location. Such gap parameters as gap size, stiffness, position, and excitation-force levels, are discussed with a view to a deeper understanding of their effects on a space interferometry system's simulated dynamic responses.
The Starlab telescope's design approach used extensive nonlinear FEM analyses of the suspension systems and primary mirror/bonded assemblies in order to achieve the requisite optical sensitivity to attachment system-induced forces, in combination with the distribution of launch loads through thin sections from concentrated attachment points. A test program verified all aspects of the design. Attention is presently given to the design principle used, involving the splitting of the suspension system's functionality.
Large glass mirrors made by fusing smaller pieces together are susceptible to distortions during manufacture and temperature changes due to differences in thermal expansion within the mirror. A method of predicting the size and shape of this distortion using the finite element method of computer simulation is described. The method is verified by comparison to a set of measurements on glass samples.
Finite element analyses were performed to predict the optical surface distortions for a 3.5-meter borosilicate glass structured mirror due to the effects of thermal variations. In order to evaluate the performance of the mathematical mirror model, a parallel experimental study was conducted at the National Optical Astronomy Observatories (NOAO). A total of 666 thermal sensors was bonded to the mirror for the experiment. Temperature distributions measured by the thermal sensors were directly translated by an interface program into a set of the nodal temperatures for input for the numerical model, and the optical surface distortions were calculated. Excellent agreement between the experimental and numerical results were found. Additionally, an analytical approach for a linear thermal gradient along the optical axis was made, and the result agreed closely with that from the finite element analysis.
Dome-induced thermal disturbances that degrade seeing can originate when temperature differences exist between the interior and exterior of a telescope enclosure. It is important to design enclosures which minimize the effect. One design aid is to model the enclosure and study the flow patterns in and around the model at various angles to the flow direction. We have used a water tunnel and models of spherical, octagonal, and rectangular enclosures to investigate the flow characteristics as a function of angle and venting configuration. In addition to a large video data-base, numerical results yield flushing times for all models and all venting arrangements. We have also investigated the comparative merits of passive venting as opposed to active forced flow circulation for the 4m telescope enclosure at the NOAO Cerro Tololo Interamerican Observatory at La Serena, Chile. Finally, the flow characteristics of a tracking half-shroud were studied as a possible shield for the enclosureless case.
The WIYN telescope primary mirror is the 3.5-m, second mirror cast at the University of Arizona. The borosilicate glass temperature of the mirror is regulated during use to limit the distortions caused from thermal expansion, and to limit mirror seeing caused by natural convection. The thermal regulation system includes a closed air cooling plenum within the mirror cell structure with blowers and heat exchangers. Heat generated by this system and that from the glass is carried away by another liquid cooling loop to a remotely positioned liquid chilling unit. The system can regulate the mirror temperature to +/- 0.2 C from a temperature set point near ambient air for typical static and dynamic environments. Evaluation of the system includes a full laboratory optical test of the mirror, support, cell, and temperature control system. Data on the performance of the thermal control is summarized in terms of the quantities of temperature deltas across the system components and flows of air, water, and heat.
This article describes several methods for rapidly and continuously measuring the rigidity of both dynamic and static with large high-precision instruments. It also provides some examples of measuring methods with large M.M. wavelength radio and large optical telescope.
A brief development history and current development status evaluation are presented for active (as opposed to 'adaptive') optical techniques, giving attention to this technology's successful application to date in large telescopes using active primary mirror controls and prospective, more ambitious active optics systems. Reliable models and simulation tools are noted to already exist for establishing the flexural limits of active structures. Current examples include the ESO's 3.5-m New Technology Telescope and the Keck 10-m TMT; future applications are in Japan's NLT, the ESO VLT, and the SOFIA airborne IR telescope.
The application of monolithic piezoelectric bimorph deformable mirrors for optical wavefront correction is studied. These mirrors are fabricated from laminations of biaxial piezoelectric polymer sheets. The equations of motion governing flexural and thickness motions are derived from first principles, assuming linearly elastic biaxially-poled laminae. The bimorph is segmented into actuation 'zones' for structural control, providing actuation in terms of distributed moments. It is shown that this actuation strategy provides influence functions with a significantly more localized response (e.g., a higher spatial bandwidth) than point force/displacement actuation for structures having equivalent geometric and material properties. This is due to the bimorph actuation providing a spatial differentiation of the structural response as compared to point force actuation.
Active optical system approaches offer the possibility of designing large, spaceborne optical systems that weigh considerably less than conventional passive designs. They offer the further possibility of active compensation for on-orbit thermal and mechanical disturbances. However, a key enabling technology is the ability to accurately and rapidly sense optical system misalignments and figure errors. This paper describes the sample point interferometer (SPI), a measurement technique that can meet the sensing requirements of active optical systems. By monitoring the optical path lengths of an array of pencil beams projected to a corresponding array of retroreflectors affixed to the mirror, the SPI can provide simultaneous measurements of rigid body alignment and figure. The SPI uses semiconductor laser diodes as a light source for two-wavelength interferometry, and provides alignment and figure measurements that are repeatable, have a large dynamic range, and can be acquired at high bandwidths.
The Polarization Phase Sensor (PPS) is a white light polarization shearing interferometer that has successfully demonstrated absolute mirror segment alignment in the Rome Laboratory Optical Systems Engineering Laboratory. Operating at the center of curvature, the PPS demonstration has been configured to perform closed-loop phasing of a three segment spherical mirror. The PPS may be adapted for use with other optical systems, including aspheric surfaces. PPS device simulation and testing have verified this. This paper will address the optical layout of the device, experimental testing, and the results of subsequent analysis.
The optical characteristics of a 5-axis secondary mirror are discussed, stressing the infrared sky chopping and adaptive tip/tilt control capabilities. Sources of image degradation such as misalignments and manufacturing errors are considered. Different modes of operation for active, chopping, and adaptive control are described and examples of the required speed, range, and precision of these modes are presented.
Requirements were specified to design and build two 1000 Hz bandwidth steering mirrors to be used in a complex, high precision optical system. This paper describes the design, manufacture, tests, and performance of the Extremely Fast Steering Mirrors (EFSM) with an extensive use of a computer network. A mathematical model of a two-sided EFSM is derived. Tuning of the analog EFSM controllers is described. Experimental performance results for the closed loop EFSM system are presented. Also presented are results of the system's MATRIXx computer simulation. Performance predictions are compared to actual test results. A brief discussion of other components of the optical system and of the total system is also included.
The use of holographic optical elements (HOEs) to discriminate between coherent irradiation and
broadband, noncoherent light has been experimentally demonstrated under adverse scattering and
attenuating conditions. As a passive sensor component in a laser irradiation detection system, an HOE can
be used in several application areas, e.g., data transmission systems, aircraft warning system, underwater
communications, and alignment systems, where wavelength and direction of arrival information can be
The efficient concentration or focusing of laser light by an HOE onto a detector stage and, of equal
importance, the ability to form bright, unique geometric patterns are characteristics that establish the
HOE's use as a readily compatible irradiation sensor component. In addition, there is a considerable size
and weight advantage over other functionally comparable optical components. Finally, as a passive
element, an HOE can fmd use with CW or pulsed illumination.
The properties and advantages, pros and cons, of the use of HOEs as sensor elements are
discussed in the paper and illustrated in several laboratory experiments and a field test.
The on-orbit performance of the GOES satellite's scan mirror has been predicted by means of thermal, structural, and optical models. A simpler-than-conventional thermal model was used to reduce the time required to obtain orbital predictions, and the structural model was used to predict on-earth gravity sag and on-orbit distortions. The transfer of data from the thermal model to the structural model was automated for a given set of thermal nodes and structural grids.
During the past 15 years, Composite Optics, Incorporated (COI) has been very successful in the aerospace industry in the design and manufacture of thermally stable structures and high- precision, lightweight, solid reflector systems. COI's latest program, the Special Sensor Microwave Imager Sounder (SSMIS) Reflector, extends its capabilities into the area of ultra- lightweight, solid, deployable reflectors. This endeavor incorporates and improves the technologies developed on the stationary reflectors with the deployment mechanism technologies while maintaining the system's thermal stability.