In some applications, moment actuators provide a competitive altemate approach to the conventional piston type of actuators. A piezo-electrlc or a simIlar device placed between two posts on the back surface of a deformable mirror constitutes one type of moment actuators. A dosed form solution of the blharmonlc equation of Kirchoffs 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 this function to the design of a 30-actuator circular deformable mirror with moment actuators, related hardware, and test results has been reported elsewhere1.

The paper describes a computer program, called AXAFIT, which carries out optical performance analyses of deformed Wolter Type-I X-ray optics. The program was developed to assist the NASA AXAF program. NASTRAN finite element analysis is employed to determine rigid body deflections and elastic deformations of optical elements. NASTRAN data are then matched with a geometric ray tracing routine by means of an interpolation technique. Encircled energy, rms spot diameter, geometric point spread function, and effective collecting area are predicted. The performance analysis of the AXAF Verification Engineering Test Article-I employs the results of this analytical technique. Verification test case results for simple rigid body misalignments and closed-form deformations are demonstrated to be satisfactory. AXAFIT is shown to be useful in conducting image quality analyses for these optical systems by means of NASTRAN-generated deflections.

The geometric design of 60 laser beempeths was optimized to minimize pathength variation subjected to several design contraints. Nonlinear programming techniques were utilized to solve the 120 design variable problem. Realistic packaging constraints allowed for a "clean" final design configuration.

PR 1578 adhesive is often used for bonding a glass mirror to its mounting flexures (Invar or Aluminum) when the mirror assembly is designed to operate in a low temperature environment PR 1578 is used because it has high strength and remains elastic at low temperature. Several tests conducted at Itek, however, show that the thermal stress induced in the bond may cause fracture of the glass near the bond at cryogenic temperawre. This paper examines the reason of fracture and the effects of flexure material, adhesive thickness, and size of bonding. A finite element model is developed for simulating the bonding of flexure, adhesive, and glass. Temperature dependent material properties are used in the analysis. Good correlation between the analysis and the test is obtained.

One of the challenges in using either ground- or space--based high power lasers is keeping the beam on target in spite of various disturbances. Active control is one of the methods for maintaining laser beam direction. It requires (1) a Sensor which monitors the beam path and sends an error signal to the Controller when the beam is misaligned; (2) the Controller, after processing the error signal, sends a correction signal to (3) Actuators which control the position of (4) a Beam Steering Mirror (BSM), also in the optical path of the laser beam. The correction signal is designed to change the position of the BSM to that the laser beam is brought back on target. Since it is not obvious how a system of this sort will respond to various disturbances, it is helpful to be able to simulate such a system, during the design stages, so that one might have confidence that the system will work if it gets to the hardware stage. Before applying MSC/NASTRAN to the real systern,it seemed advisable to check out the implementation of MSC/NASTRAN on a small structures-optics-controls system that had the flavor of the real system but far less complexity. In that way it was hoped that procedural errors would be minimized when MSC/NASTRAN was used to analyze the real system. This paper explains the implementation of MSC/NASTRAN as applied to the analysis of a Small Structures-Optics-Controls System (SSOCS), a sketch of which is shown in Figure 1. The SSOCS comprises a Beam Steering Mirror (BSM) supported by two Voice-Coil Actuators (VCA's) (on structural support springs "8" and "9") which change the position of the BSM, a Disturbed Mirror (DM) (on structural support springs "6","7", and "57"), a Sensor which monitors translational and angular misalignrnents in the beam path, and a Controller which receives the error signals from the Sensor and sends correction signals to the VCA's. The response of the SSOCS to a step function disturbance applied to the structure of the DM via p7(t) through M7 (between springs "7" and "57") was calculated by MSC/NASTRAN; and, the results were compared against those computed for the SSOCS by a general purpose dynamics analysis program called TIMRSP. (See Block Diagram of SSOCS equations used in TIMRSP program.) The agreement between the results of MSC/NASTRAN and TIMRSP was considered excellent and increased confidence that there should be rio procedural errors.

For very sensitive applications, it may be necessary to adjust the spatial power spectral density (PSD), obtained by measuring any line figure on an optical surface, to eliminate the effect due to ground vibration. This is possible only if we know how to relate the standard random vibration output-numerous pointwise temporal PSDs-to spatial PSDs measured by interferometers. This paper serves as such a link. Two methods are presented here. The first uses direct integration starting from the mathematical definition of the spatial PSD of a function. The second borrows the relationship between two-dimensional spatial PSD and temporal PSD from atmospheric disturbance. As expected, both methods give nearly identical results.

Ultra High-precision operation is very essential in many optical instruments and modern manufacturing systems. However, mechanical vibrations introduced by various sources can easily disrupt this delicate operation. Thus, active vibration isolation and control becomes very important in many circimstance. This paper presents a "smart" active micro-position feedback control technique using a piezoelectric actuator. A general theory for the piezoelectric actuator subjected to mechanical excitations and feedback voltages is developed. This theory is applied to an active micro-position control by injecting a processed feedback voltage at variable feedback gains into the piezoelectric actuator. Effectiveness of the piezoelectric micro-position attenuation is evaluated analytically and experimentally. Theoretical solutions are compared favorably with the laboratory experimental results.

The Multiple Mirror Telescope (MMD on Mt. Hopkins AZ, which uses an array of six telescopes on a single mount to achieve an effective aperture of 4.5-rn, is to be converted to a 6.5-rn telescope with a single prirnary. This paper describes the objectives, constraints, and solutions for a conceptual design of the Optics Support Structure (OSS) for the 6.5-rn telescope.

The large, thin-walled grazing incidence X-ray optics for NASA's Advanced X-ray Astrophysics Facility (AXAF) present a significant challenge to maintain figure during fabrication, metrology, and final assembly. The largest of the X-ray optics is 1 m long, 1.2 m in diameter, only 23 mm thick and weighs 225 kg. Conical shells of this size and stiffness present particularly difficult mounting problems because of their local flexibility and extreme sensitivity to small loads caused by differences in thermal expansion and one-g effects. Support methods that minimize the optics distortion during fabrication, metrology and launch configuration is presented. We describe the important role that finite element modeling and breadboard testing play in determining the performance of the support structures and that of material effects such as epoxy shrinkage and moisture desorption.

The paper presents a mathematical model, based on Kirchoff's thin flat plate theory, developed to determine polishing pressure distribution for a flexible polishing tool. A two-layered tool in which bending and compressive stiffnesses are equal is developed, which is formulated as a plate on a linearly elastic foundation. An equivalent eigenvalue problem and solution for a free-free plate are created from the plate formulation. For aspheric, anamorphic optical surfaces, the tool misfit is derived; it is defined as the result of movement from the initial perfect fit on the optic to any other position. The Polisher Design (POD) software for circular tools on aspheric optics is introduced. NASTRAN-based finite element analysis results are compared with the POD software, showing high correlation. By employing existing free-free eigenvalues and eigenfunctions, the work may be extended to rectangular polishing tools as well.

A previously developed eigenvalue model is extended to determine polishing pressure distribution by rectangular tools with unequal stiffness in two directions on cylindrical optics. Tool misfit is divided into two simplified one-dimensional problems and one simplified two-dimensional problem. Tools with nonuniform cross-sections are treated with a new one-dimensional eigenvalue algorithm, permitting evaluation of tool designs where the edge is more flexible than the interior. This maintains edge pressure variations within acceptable parameters. Finite element modeling is employed to resolve upper bounds, which handle pressure changes in the two-dimensional misfit element. Paraboloids and hyperboloids from the NASA AXAF system are treated with the AXAFPOD software for this method, and are verified with NASTRAN finite element analyses. The maximum deviation from the one-dimensional azimuthal pressure variation is predicted to be 10 percent and 20 percent for paraboloids and hyperboloids, respectively.

The elaboration of an optical system which measures the displacement and velocity of large flexible space structures is reviewed. The motion-measuring system, comprised of a laser, optics, motorized mirror, two photodiodes and electronics, is designed to allow feedback for configuration control of flexible structures. A motor or scanner is employed to sweep the light sheet. The range of motion was shown to be 2 inches, and information was received at a rate of 30 Hz. The uncertainty in displacement measurement was better than +0.01 inch from a distance of 16 feet. The results are considered to be very good for two scanning light sheet systems. Higher sample rates and a more constant speed would improve the rotating mirror option, and the accuracy and range of the scanning mirror option could be upgraded as well. Both setups are shown to be viable noncontact measuring methods for the displacement and velocity of large space structures.

The Control, Astrophysics and Structures Experiment in Space (CASES) will investigate critical control technology applicable to sthbilizing and pointing large flexible structures in space. This shuttle-based experiment will provide active control ofa 32 m extendibleboom structure usinggas thrusters for pointing and angular momentum exchange devices (AMED) for active damping to suppress vibrations. In conjunction with this controls-structures interaction experiment, an x-ray diffraction instrument will investigate high energy sources at the center of the galaxy and on the surface ofthe Sun. An occulter plate on the tip ofthe boom and proportional counters at the base of the boom comprise an x-ray telescope with a 32 m length. High spatial resolution depends on accurate knowledge and, to alesser extent, control ofthe position ofthe boom tip assembly relative to the proportional counters located at the base ofthe boom. The Remote Attitude Measurement Sensor (RAMS) was designed to provide high accuracy and update rates while measuring numerous reflective targets on a flexible structure. RAMS is presently baselined to fill two important sensor needs for CASES. First, as a tip displacement sensor, RAMS willprovide accurate knowledge ofthe position and orientation of the boom tip assembly. Second, as a boom motion tracker, RAMS will monitor 43 reflective targets that are distributed along the length ofthe boom and provide displacement (eigenvector) information for post-facto processing. This paper describes the design and operation of RAMS as both a tip displacement sensor and a boom motion tracker. It explains how RAMS interfaces with the CASES closed-loop control system and how systems identification is accommodated.

This paper contains a discussion of the functions of fast steering mirrors (FSMs) in optical systems which include target tracking, attenuation of disturbances including jitter, alignment beam and image stabilization, and extending optical sensor linear range. Dynamic and control modeling of the FSM mechanism, actuators, position sensors, and controllers are described in terms of mass and inertia properties, dynamic range, and bandwidth. Illustrative examples of the use of FSMs in optical systems and a simulation of a representative optical system using an FSM and corresponding results are shown. The use of software tools in performing control systems simulations is discussed.

This paper proposes a new approach of obtaining adaptive state estimation of a system in the presence of unknown system disturbances and measurement noise. In the beginning, a non-optimal Kalman filter with arbitrary initial guess for the process and measurement noises is implemented. At the same time, an adaptive transversal predictor (ATP) based on the recursive least-squares (ilLS) algorithm is used to yield optimal one- to p- stepahead output predictions using the previous input/output data. Referring to these optimal predictions the Kalman filter gain is updated and the performance of the state estimation is thus improved. If forgetting factor is implemented in the recursive least-squares algorithm, this method is also capable of dealing with the situation when the noise statistics are slowly time-varying. This feature makes this new approach especially suitable for the control of flexible structures. A numerical example demonstrates the feasibility of this real time adaptive state estimation method.

This paper describes simulation of a space-platform line-of-sight (LOS) stabilization system on a ground-based hardware simulator. A definition of the two-body space platform system is given followed by an explanation of how that system is simulated on the hardware. Sensor synthesis, optical magnification simulation, and structural mode simulation are discussed. Two control systems are described: 1) Alignment Inertial Reference (AIR) platform, and 2) aligtiment system. The platform control system is a digital variable structure system known as Proximate Time-Optimal Servomechanism (PTOS). The alignment control system uses an analog linear control scheme to control two steering mirrors.

It is well known that constrained viscoelastic layer damping treatments provide an effective means of passive control for structural vibration. These treatments dissipate vibrational energy by inducing shear strain in a thin layer of viscoelastic material. Our interest is in adding passive damping to a structure as an augmentation to active control. For such applications it is desirable to achieve high damping performance in a given frequency range with a minimum of added weight. Constrained layer damping treatments most commonly employ spatially continuous constraining layers over the entire viscoelastic layer. Plunkett and Lee have shown that the effectiveness of the damping treatment can be significantly increased by sectioning the constraining layer into segments of optimal length for the target frequency range. The authors have observed that few have recognized the degree of improvement achievable through this method. The purpose of this paper is to illustrate the effectiveness of the method, through examples. It is demonstrated that, for a particular laboratory structure, the damping of the modes of interest can easily be increased by a factor of 10 or more by properly sectioning the constraining layer. The structure considered is a simple aluminum flat bar which is the arm of a single link flexible robot experiment. The damping material is 3M ISD-11O, and the constraining layer is 10 mu steel shim. Data is presented to compare experimental results with theoretical and finite element predictions. Plunkett and Lee's theory is used for theoretical analysis. The finite element model was developed based on MSC/NASTRAN code.

Techniques are demonstrated which, at high temporal bandwidths, can control a segmented reflector with many segments. Phased Array Mirror, Extendible Large Aperture (PAMELA) technology is applied, where small hexagonal mirror segments are about equal to the atmospheric coherence length, to allow diffraction-limited visible imaging. Attention is first given to an adaptive optics technique which corrects the wavefront for atmospheric distortion in the isoplanatic patch of the observed object. Wavefront sensing control methods are presented for adjusting the mirror segments. The second technique considered is active optics, whereby a control system employing local figure sensing allows the utilization of the telescope as a traditional telescope. The latter technique is shown to work for objects that are not adequately bright. The incompatibilities of local and global iterative control methods are analyzed, as are hierarchical techniques which cluster the segments. The methods, in conjunction with the reduced stiffness and therefore low mass of a large segmented mirror, present high bandwidth wavefront correction capability in a system which can be used on earth and in space.

Rapid maneuvers of large flexible structures are proposed by integrating the singular perturbation method and the Lyapunov- based nonlinear control design. The singular perturbation technique is applied for dividing the flexural vibration of large flexible structures into a slow-model and a fast-model subsystems in two separate time scales. The fast-model states characterized by the high frequency modes are significantly active only during a short initial transient period such that they decay in the fast time scale. After that period, behaviors of the system are dominated by its slow-model states associated with the low frequency modes. Based on the linear slow-model subsystem, a linear output feedback gain can be achieved by using the linear control theory such as the optimal control law or the pole-placement method. The control input of the nonlinear fast-model subsystem can be derived through a Lyapunov-based nonlinear control design. Namely, a feedback control of the fast-model subsystem is designed to guarantee its stability by having a positive-definite Lyapunov function which is a decreasing function of time. Combination of these two parts thus ensures a stable feedback control design for nonlinear two-time-scale flexible structures in a Lyapunov sense. This approach is also effective due to its reduced-order of two-time-scale models while dealing with the nonlinear control design. A numerical example is given to demonstrate control designs for rapid slewing maneuvers of large flexible structures with coupled rotational and translational motions while simultaneously suppressing vibrational motion during the control process.

An experimental implementation of a new distributed parameter shape control design methodology is presented. The technique is based upon the input/output representation of distributed parameter systems in a spatially- and temporallytransformed frequency space. The analysis is specialized to shape control through the introduction of generalized spatial transforms of the plant response which explicitly parameterize the shape control task. Experiments are summarized wherein a 4Oin pinned-pinned steel beams shape is controlled, in the presence of quasi-static and resonant disturbances, over a bandlimited set of four sinusoidal shape basis functions at a closed-loop temporal bandwidth of 2Hz, using distributed piezoelectric actuators. Temporal compensation is provided by digitally-implemented LQG/LTR compensators. A novel inner-loop damping formulation, based upon the second method of Lyapunov, is developed and implemented to damp the beam resonant response beyond the LQG/LTR control bandwidth.

Important considerations are given in connection with achieving submicroradian jitter for body-fixed payloads employing inertial reference sensors. The derivation of the requirements, key design aspects, and specifications for important components are treated. The submicroradian-level pointing system for use on the body-fixed telescope (BFT) is described, emphasizing line-of-sight (LOS) jitter application. The pointing subsystem design is described. The BFT is tested on a motion table and shaker table, and compared with a simulation of LOS jitter due to base motion. Performance expectations are verified by the testbed, and are shown to vary from predicted results by less than 10 dB at all frequencies. A submicroradian-jitter stabilization subsystem may be built with qualified components that are presently available.

A description of the requirement definition process is given for a new wideband attitude determination subsystem (ADS) for image motion compensation (IMC) systems. The subsystem consists of either lateral accelerometers functioning in differential pairs or gas-bearing gyros for high-frequency sensors using CCD-based star trackers for low-frequency sensors. To minimize error the sensor signals are combined so that the mixing filter does not allow phase distortion. The two ADS models are introduced in an IMC simulation to predict measurement error, correction capability, and residual image jitter for a variety of system parameters. The IMC three-axis testbed is utilized to simulate an incoming beam in inertial space. Results demonstrate that both mechanical and electronic IMC meet the requirements of image stabilization for space-based observation at submicroradian-jitter levels. Currently available technology may be employed to implement IMC systems.

The optical line-of-sight (LOS) system on a two body spe platform must be stabilized to sub-microradian levels in the presence of base disturbances to obtain blur free images. An example of a two body spacecraft has a telescope (cw beam expander) as the fore body and a sensor suite which can include an active tracking system on the aft body. An isolation system between the two bodies will keep the diswrbances generated in each from effecting the other. Such a system could be used for satellite communications, weather tracking systems, etc. The need for accurate control of angular motion is accomplished with stabilization and isolation piatforms and active control of structures and optics. This paper will discuss a method to isolate the LOS from the base motion of the space platform over a wide frequency range. This method includes a concept to blend inertial measuring sensors (IMS) that erate in different frequency ranges to produce an IMS sensor system that can measure base disturbances from DC to over 1000 Hz with minimum distortion. Experimental data from this blending technique will also be included in this report.

Precision Aerospace structures must be actively controlled to achieve stringent operating specifications; however, persistent disturbances will drastically reduce the benefits of active structure control unless the controller can be designed to counteract such disturbances. We present a scheme for reduced-order model based disturbance accommodating controllers for precision structures using the idea of a disturbance generator from Johnson. We show that closed-loop stability can be achieved using a compensation technique known as the residual mode filter from Balas3 for spillover-induced instabilities due to model reduction.

This paper contains a presentation of the techniques needed to develop simulations for large scale systems which are composed of control systems, optics, and structures. A case study is presented which consists of a generic, controlled specraft whose purpose is to maintain a precise line-of-sight (LOS) in the presence of disturbances and structural deformations. The spacecraft model consists of a six degree-of-freedom (DOF) rigid body with superimposed small structural deformaüons obtained from high-order finite element model. The optical system consists of a telescope with a controlled Fast Steering Mirror (FSM) for maintaining an accurate line-of-sight. Reaction wheels and an Inertial Measurement Unit (IMU) are used for spacecraft attitude control. Disturbances consist of onboard payload motions, sensor noises and reation wheel imbalance. Results of the simulation are shown.

Designers of lightweight flexible space structures are faced with the significant challenge of achieving position and shape control while avoiding structural vibration. A potentially attractive approach to the vibration and shape control problems lies in the use of piezoelectric films as actuators and sensors. Piezoelectric materials deform when exposed to an electric field, or conversely, when deformed they produce an electric field. This, property considered along with the fact that they are lightweight, inexpensive and exhibit a very wide dynamic response bandwidth, make piezoelectric films attractive as actuators and sensors for certain applications. In this paper, some fundamental relationships for beams incorporating piezoelectric film actuators and sensors are examined. The differential equation of motion for a beam with piezoelectric film bonded to both sides is used to develop Laplace domain transfer function models of the system. These transfer functions are exact Laplace domain representations of the system equations of motion. The transfer functions are cast into a closed rational form using Maclaunn series expansions representing a specific number of modes. In this form, the transfer functions lend themselves to classical control analysis. It is shown that the transfer function relating a voltage applied to a full coverage actuating layer, to the voltage induced in a full coverage sensing layer on the opposite beam face, behaves like a classic colocated system with alternating poles and zeros and accordingly the system is easy to stabilize with low order compensation. In contrast to this result, it is shown that in spite of the effective colocation of actuator and sensor in the case of the transfer function from actuating voltage to tip position, the desirable alternating pole/zero pattern is not exhibited due to incompatibility of actuating and sensing signals. This result is verified experimentally.

Many future space missions involving flexible structures for large optics may require active vibration control to satisfy mission objectives. Thus, it is important for active control of flexible structures to be practically demonstrated in ground based experiments. These experiments can validate (or invalidate) existing theories and technology and provide directions for future research. This paper discusses three experiments conducted by Harris which successfully demonstrate control of flexible structures. The paper concludes with some remarks on the lessons learned from conducting these experiments.

This paper describes the use of composite materials in the Soft X-Ray Telescope (SXT). One of the primary structural members of the telescope is a graphite epoxy metering tube. The metering tube maintains the structural stability of the telescope during launch as well as the focal length through various environmental conditions. The graphite epoxy metering tube is designed to have a negative coefficient of thermal expansion to compensate for the positive expansion of titanium structural supports. The focus is maintained to + or - 0.001 inch by matching the CTE of the composite tube to the remaining structural elements.

The Mars Observer Camera (MOC) is one of several instruments aboard the Mars Observer Spacecraft, which is scheduled to launch in September 1992, and begin monitoring the Martian surface (from Martian orbit) in December 1993. The MOC is the only instrument, however, that will record visual images of the surfe of Mars. The MOC comprises three separate optical systems: the Narrow-Angle system, for relaying high-resolution images of the Martian surface to Earth; the Red Wide-Angle system, for relaying limb-to-limb images in the 575 to 625 nanometer spectral range; and the Blue Wide-Angle system, for relaying limb-to-limb images in the 400 to 450 nanometer spectral range. The Mars Observer Project is conducted by the Jet Propulsion Laboratory (JPL), while the MOC experiment is conducted by the Arizona State University (ASU). Responsibility for the MOC instrument design, fabrication, integration, and test lies with the California Institute of Technology (Caltech). Perkin-Elmerts Applied Optics Operations (now OCA Applied Optics) and Composite Optics' involvement in the MOC program began in Iate-1986 when Caltech awarded this team a contact to fabricate the MOC Engineering Model and, later, two Right Models (one being a spare) based on the novel approach of utilizing graphite/epoxy composites for a majority of the MOC structure to achieve minimum weight. Since a majority of the structure is made of graphite/epoxy, including the sensitive metering structure between the pnmary and secondary mirrors of the Narrow-Angle system, charterizing the MOC structure became mandatory. Major concerns during the design of the MOC were not only structural integrity (designing the MOC such that its lightweight strucwre would withstand the shock and vibration of launch) and thermal stability (maintaining focus during the extreme thermal environment in Martian orbit), but also hygroscopic issues (paphite/epoxy absorbs atmospheric moisture and expands, causing defocus). This paper also briefly addresses the methods employed to reduce stray light from the walls of the graphite/epoxy structure.

This document discusses material selection, design, and analysis of a composite gimbal for use on a high precision inertial guidance test table with active magnetic bearing suspension. The test table's system performance goals of 0.1 arc second angular pointing accury and one part per million angular rate stability, can only be achieved by using a gimbal with high specific stiffness, highly symmetric elastic properties, and high dimensional stability. These characteristics are achieved by proper selection of the ginthal's construction material, configuration, and fabrication processes. Both traditional and advanced composite materials are considered and evaluated for specific stiffness, coefficient of thermal expansion, thermal conductivity, dimensional stability, fabrication problems, and cost. Using the candidate materials, several gimbal configurations are evaluated with respect to the test table's system performance goals for angular pointing accuracy and angular rate stability. Specific gimbal design parameters affecting the system performance goals for angular pointing accuracy and angular rate stability include: the angular payload deflections due to torsional wind-up and asymmetrical stiffness; the linear payload deflections that cause torque disturbances and shaft wobble; and the natural frequencies affecting the control system bandwidths. Detailed finite element models of each configuration are used to predict the performance charteristics and demonstrate the advantages of the graphite/epoxy composite design.

A T5OJERL1962 graphite epoxy laminate was selected as the primary structural material for an cptical test bed manufactured by Composite Optics, Incorporated (COT) for use by Eastman Kodak's Federal Systems Division to develop the design of lightweight optical systems with dynamic damping and active conirol of multi-segment primary mirrors. The composite structures for this project consist of a 100 inch diameter 12 inch deep Aft Structure Assembly and six 190 inch long round tubes double tapered from a 3.65 inch midspan I.D. to a 0.90 inch I.D. at the ends. A pseudoisotropic .09 inch thick layup was used for the aft structure to provide a low coefficient of thermal expansion and to meet requirements for stiffness and weight. The tubes were made with a ,06 inch thick (O0/±6O0)s laYup for low longitudinal coefficient of thermal expansion (CTh) and high axial stiffness. The aft structure will support a primary mirror consisting of a center segment and six outboard petal segments. The six composite tubes which attach to actuators at the outboard ends of the aft structure will support the secondary mirror for the Ritchey Chretien optical system.

The paper deals with the feasibility study and use of advanced composite materials in the Low Altitude Navigation and Targeting Infrared for Night (LANTIRN) program's field-test equipment. Emphasis is placed on thermal expansion in the optical test-equipment structure under elevated-temperature conditions. A design approach using a low-CTE graphite/epoxy composite for the optical bed is considered, and based on several constraints, four basic design approaches are evaluated: honeycomb, separated plate, machine solid, and thin solid. A prototype system is covered, and the test results of coupon samples of the composite baseplate material indicate that CTE is less than 1 ppm/deg C over a wide temperature range.

The stability and coregistration requirements for future radiometric instrument designs spawn the need for a totally integrated instrument structure and thermal control scheme. To meet the requirements of the future Geostationary meteorological missions an Ultra Stable Instrument Structure (USIS) will be needed. An instrument structure of lightweight construction is described that takes advantage of composite materials that combine high stiffness, low density along with low Coefficient of Thermal Expansion (CTE). In addition, this paper will outline the mission objectives, the operating environment and stability requirements needed for future spaceborne radiometer structures. A conceptual design of a composite instrument structure along with its thermal control system will be outlined, and various design trade-offs will be presented.

A conceptual design and manufacturing program for a lightweight graphite/epoxy steering mirror with a 0.5-2.0-m aperture for use at 10.6 micron is presented. An analysis is performed for the optimization of the mirror natural frequency as a function of component material, structural geometry, and physical dimensions. A 65-m-diameter eggcrate sandwich-core mirror constructed of graphite/epoxy composite material clad on all exposed surfaces with aluminum foil or tin/indium solder is manufactured, and it is indicated that this design can be scaled to larger diameters. Results of a trade study suggest that this type of mirror can be hardened against nuclear X-ray effects given the appropriate choice of cladding materials.

The employment of composites in RF structures such as antennas, feedhorns, and waveguides is outlined, and focus is placed on the parameters of a composite mirror operating in the 3-5- and 8-12-micron areas. A large beam-steering composite mirror fabricated from ultrahigh-modulus graphite/epoxy is described, including its three subassemblies: the core subassembly and two facesheet subassemblies. Attention is given to an alternative approach in which a gel coat resin is applied to the glass surface and the mirror substrate is pressed to the tool to cover the mirror with the resin. Another method is to seal the composite from the effects of moisture expansion by applying a eutectic coating; voids and crystal-grain growth are the main sources of surface perturbation on such mirror surfaces.

Production tolerances must allow for deviations in ply angles in composite laminates. A Monte Carlo simulation of these deviations has been used to evaluate the distribution of properties for two symmetric laminates formed from P7SS/930 (graphite/epoxy) uniaxial composite plies. Although the nominal laminate mechanical properties are essentially identical for the two cases considered, the consistency of these properties varies with the layup. This analysis was used to select the laminate which provides the greatest consistency in Critical properties. Techniques used in this evaluation may be applied to any general laminate. This type of analysis is particularly useful in laminate evaluation whenever mechathcal properties must be tightly controlled, as typically required for optical support structures. For any given ply angle alignment tolerance and mechanical property variation, the reject rate can be estimated. Two types of ply misalignment are considered. In the first case, the alignment error of each layer is considered to be random. In the second case, systematic errors such as those which might be caused by imperfections in the cutting template that would affect all plies at a specified angle are considered. Graphs are used to present a statistical summary of the results. A program for IBM-PC1 computers using a commercially available matrix manipulation program has been developed to perform the analysis.

One of the critical technology needs for large precision reflectors required for future astrophysics and optical communications is in the area of structural materials. Therefore, a major area of the Precision Segmented Reflector Program at NASA is to develop light-weight composite reflector panels with durable, space environmentally stable materials which maintain both surface figure and required surface accuracy necessary for space telescope applications. Results from the materials research and development program at NASA Langley Research Center are discussed. Advanced materials that meet the reflector panel requirements are identified. Thermal, mechanical and durability properties of candidate materials after exposure to simulated space environments are compared to the baseline material.

In order to develop dimensionally space stable laminate composite structures a detailed understanding of moisture absorption of composite laminates and the effect of"dryout" on the shrinkage oflaminate structures in the space environment is required. Several new epoxy resins have recently been introduced which are reported to absorb less moisture than the baseline epoxy resins currently used for fabrication of space structures. These are 3M's SP-500, Hexcel F-584 and Toray 3631and are all modified structural epoxy resins. This study describes the mechanical properties and moisture absorption ofthese laminates as well as detailing the coefficient ofthermal expansion and moisture expansion for quasi-isotropic graphite laminates made from these resin systems.

Carbon fiber/resin matrix composites are uniquely suited for the construction of light-weight dimensionally-critical structures on account of their high specific stiffness, and low coefficient of thermal expansion (CTE). Many spacecraft components, such as optical support structures, exploit these properties. Newer applications are focusing on the use of composites for thermal management, where the high specific thermal conductivity of pitch-based carbon fibers is an advantage in lightweight radiators and passive cooling devices for electronics modules. In both mechanical and thermal management applications, resistance to microcracking under thermal cycling is desirable, in order to preserve the initial dimensions and Cm. This paper describes the use of ultra-thin prepregs, with thicknesses down to 0.001 inch, that can reduce or eliminate microcracking under thermal cycling.

Sandwich-construction mirrors with foam cores and solid face and back sheets can provide high stiffness and 1c weight if the materials of construction exhibit the correct properties. A number of metallic materials with proven performance as solid mirror substrates would be suitable for solid face sheets. However, structural foams having the desired canbination of properties (low density, high shear iodulus, and high microyield/microcreep strength) have not been available. A new aluminum-silicon carbide (SXA®) canposite foam with a combination of properties which makes it very attractive for mirror cores has ten developed. The structure and properties of this foam are described in this paper. The construction and performance of SXA aluminum-matrix canposite-face sheet/SXA® canposite-foam sandwich construction mirrors are also discussed.

Successful development of space-based surveillance and laser systems will require large precision mirrors which are dimensionally stable under thermal, static, and dynamic (i.e. structural vibrations and retargeting) loading conditions. Among the advanced composites under consideration for large spe mirrors, graphite fiber reinfced magnesium (Gr/Mg) is an ideal candidate material that can be tailored to obtain an optimum combination ofproperties, including a high modulus of elasticity zero coefficient of thermal expansion, low density, and high thermal condtxtivity In addition, an innovative technique, combining conventional filament winding and vuum casting has been developed to produce near-net shape Gr/Mg composites. This approach can significanfly reduce the cost of fabricating large mirrors by decreasing required machining. However, since Gr/Mg cannot be polished to a reflective surface, plating is required. This paper will review research at Martin Marietta Astronautics Group on GrIMg mirror blank fabrication and measured mechanical and thermal properties. Also, copper plating and polishing methods, and optical surface characteristics will be presented.

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 the need for a method to correct wavefront aberrations other than by applying tuator forces. One possible way to deform the optical surface of a sandwich mirror is by ndependently pressurizing cells within the core of the mirror. A method of active correction utilizing independently ressithzed "mro-ce11s" in the core of an 18-inch di& aluminum honeycomb sandwich mirror has been attempted. This paper describes the experiments and the finite element analyses which were conducted on 6-inch mirror blanks containing progressively more complex single macro-cells; and the experiments condtxted on two 18-inch mirror blanks, one with a cellular honeycomb core and the other with a machined aluminum core, each containing 19 individual macro-cells. Results from the multiple macro-ceU 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.

A method is described to assess the figure change created by thermal distortion in beryllium sandwich mirrors during solar exposure in space. A NASTRAN finite element model of a mirror is used to evaluate the procedure, which uses embedded temperature sensors. The method is tested, with respect to six parameters, on an aluminum thermal surrogate mirror. Thermal gradients in most full-depth sandwich configurations are shown to be minimized, due to the high thermal conductivity of aluminum. Overall rms figure and defocus deformation are predicted to within 15 percent; the coma and astigmatism are predicted to within 20-25 percent; and the variability in predicting higher order distortions is wide. Defocus is shown to be the most likely dominant term in temperature-induced distortion. The notion is posited that provisions are necessary to counterbalance the high CTE of metal mirrors during solar exposure.

A method is developed to determine the weight, center of gravity, areal properties, and mass inertial properties for typical mirrors. A number of support conditions were considered to examine optical surface deflections, surface quality, and fundamental natural frequency for single- and double-arch mirror shapes. Structural performance estimates were made with the NASTRAN program, and optical performances were evaluated with the FRINGE program, using an SXA 40-in mirror. To show the behavior of element types from the NASTRAN program, finite element validity and sensitivity studies were performed in optical model applications. Material parameters, contoured back shapes, and support locations are shown to have significant effects on structural and optical performances. Optimal support locations and support points are given. Fundamental natural frequencies for some shapes are found with the closed-form solution. The plate models results may not be acceptable for determining real mirror optical performances.

In the late 1960's Malvick and Pearson' used dynamic relaxation techniques to solve for the deflections ofa four meter mirror in a variety of mounting configurations of interest to the astronomical community. The material has been widely used to design the mounting of large primary mirrors in a generation of optical instruments. In view of the advances in computational techniques and hardware in the intervening twenty years there has been interest in duplicating the analysis in a contemporary finite element analysis code. The writers have been investigating each of the fourteen mounting configurations used by the original authors by coding the problems for solution inMSC/NASTRAN, Version 66, and solving for the sameloads and boundary conditions. This paper is a discussion of the formulation of the problem in the fmite element code and a comparison of some of the initial results.

This paper investigates the radial growth and facet surface distortion of spinning polygon mirrors used in xerographic laser printers. The general purpose finite element code MSC/NASTRAN is first validated with a closed form analysis of spinning disks and then used to predict the radial growth of a polygon as a function of facet number. Successive finite element calculations of facet surface distortion are then completed on polygons made of 6061 aluminum, ranging from 5 to 18 facets, 1.0 to 3.0 inches in inscribed radii, and 5 to 30 krpm in rotational speed. Using the data from these calculations, a series of parametric curves are developed plotting maximum surface irregularity vs. facet number for various inscribed radii and rotational speeds. Using these curves, an example calculation is detailed so future determination of radial and facet deformations under rotational loads is more readily available to the engineer. The paper also includes a spatial representation of deformed facet shape as a function of facet number, for a polygon with an inscribed radius of 1.5 inches and an operating speed of 30 krpm. Also discussed is the effect of center hole size on surface deformation for the 12 facet case.

An analytical method to compute the optical errors due to three-dimensional photoelastic effect of isotropic transparent optical elements is presented. The finite element method is used to compute the stresses in optical elements of arbitrary shape due to general quasi-static mechanical/thermal loads. A parametric finite element model which enables accurate stress, thermal and optical analyses for design study is presented. Equations governing the stress birefringence of the optical element subject to general three-dimensional state of stress, material properties and non-normal beam incidence are derived. Each point in the aperture is characterized by a 2 x 2 complex matrix transformation relating the input beam to the output beam in terms of Jones optical vector. This method allows computation of output optical errors for the entire aperture, in terms of phase retardation and amplitudes of polarization, for an arbitrary input polarization.