Optical interferometry will open new vistas for astronomy over the next decade. The Space Interferometry Mission (SIM-PlanetQuest), operating unfettered by the Earth's atmosphere, will offer unprecedented astrometric precision that promises the discovery of Earth-class extra-solar planets as well as a wealth of important astrophysics. Optical interferometers also present severe technological challenges: laser metrology systems must perform with sub-nanometer precision; mechanical vibrations must be controlled to nanometers requiring orders of magnitude disturbance rejection; a multitude of actuators and sensors must operate flawlessly and in concert. The Jet Propulsion Laboratory, with the support of Lockheed Martin Advanced Technology Center (LM ATC) and Northrop Grumman Space Technology (NGST), has addressed these challenges with a technology development program that is now complete. Technology transfer to the SIM flight team is now well along and the project is proceeding toward Preliminary Design Review (PDR) with a quickening pace.

Successful Completion of SIM-PlanetQuest Technology

Optical and IR interferometry wil open new vistas for astronomy over the next decade. Space based interferometers, operating unfettered by the Earth's atmosphere, will offer the greatest scientific payoff. They also present the greatest technological challenge: laser metrology systems must perform with sub-nanometer precision; mechanical vibrations must be controlled to nanometers requiring orders of magnitude disturbance rejection; a multitude of actuators and sensors must operate flawlessly in concert. The Jet Propulsion Laboratory along with its industry partners, Lockheed Martin and TRW, are addressing these challenges with a development program that plans to establish technology readiness for the SIM by the end of 2004.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

SIM technology development overview: light at the end of the tunnel

The Integrated Modeling of Optical Systems (IMOS) Integration Workbench at JPL has been used to model the effects of structural perturbations on the optics in the proposed Orbiting Stellar Interferometer (OSI). OSI consists of 3 pairs of interferometers and delay lines attached to a 7.5 meter truss. They are interferometrically monitored from a separate boom by a laser metrology system. The spatially distributed nature of the science instrument calls for a high level of integration between the optics and support structure. Because OSI is designed to achieve micro-arcsecond astrometry, many of its alignment, stability, and knowledge tolerances are in the submicron regime. The spacecraft will be subject to vibrations caused by reaction wheels and on-board equipment, as well as thermal strain due to solar and terrestrial heating. These perturbations affect optical parameters such as optical path differences and beam co-parallelism which are critical to instrument performance. IMOS provides an environment that allows one to design and perturb the structure, attach optics to structural or non-structural nodes, trace rays, and analyze the impact of mechanical perturbations on optical performance. This tool makes it simple to change the structure and immediately see performance enhancement/degradation. We have employed IMOS to analyze the effect of reaction wheel disturbances on the optical path difference in both the science and metrology interferometers.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Integrated Structural and Optical Modeling of the Orbiting Stellar Interferometer

The U.S. Air Force Research Laboratory developed the deployable optical telescope testbed as part of the large deployable optics research program. The goal of this programwas to investigate the feasibility of a deployable space telescope concept and to mature critical enabling technologies to provide risk reduction for the general mission concept. This paper discusses many of the challenges encountered in laboratory testing of complex, sensitive, deployable systems. Implementation of a white-light interferometer for initialization and calibration, a pencil beam or single-pixel heterodyne interferometer for measuring primary mirror motion, and a Twyman–Green interferometer system for wave-front monitoring are discussed. Development of integrated system models and system identification methods for controller design are discussed. Active structural control was demonstrated to maintain optical alignment of the telescope while subjected to simulated reaction wheel disturbances, ambient vibration and atmospheric beam steering. Wave-front sensing and interferogram analysis were used to quantitatively assess optical performance, but wave-front error was not fed back to the controller.

Active Vibration Control of a Deployable Optical Telescope

Optical interferometry will open new vistas for astronomy over the next decade. The Space Interferometry Mission, operating unfettered by the Earth's atmosphere, will offer unprecedented astrometric precision that promises the discovery of Earth-class extra-solar planets as well as a wealth of important astrophysics. Optical interometers also present severe technological challenges: laser metrology systems must perform with sub-nanometer precision; mechanical vibrations must be controlled to nanometers requiring orders of magnitude distrubance rejection; a multitude of actuators and sensors must operate flawlessly and in concert. The Jet Propulsion Laboratory along with its industry partners, Northrop Grumman Space Technology, and Lockheed Martin, are addressing these challenges with a technology development program that is nearing completion. Emphasis is shifting from technology demonstration to technology transfer to the flight team that wil build and launch the space system.

Space Interferometry Mission (SIM): technology completion and transition to flight

An optomechanical model of a segmented reflector telescope is developed. An image quality metric is used to optimize the placement of damping elements in the telescope backup truss. Comparisons with an alternative placement criteria are made.

Modeling and optimization of a segmented reflector telescope

SRT disturbances dominated by tip/tilt corrected through stiffening secondary support structure...wavefront compensation.

Limits on adaptive optics systems for lightweight space telescopes

The present multidisciplinary telescope-analysis approach, which encompasses thermal, structural, control and optical considerations, is illustrated for the case of an IR telescope in LEO; attention is given to end-to-end evaluations of the effects of mechanical disturbances and thermal gradients in measures of optical performance. Both geometric ray-tracing and surface-to-surface diffraction approximations are used in the telescope's optical model. Also noted is the role played by NASA-JPL's Integrated Modeling of Advanced Optical Systems computation tool, in view of numerical samples.

Integrated multidisciplinary analysis of segmented reflector telescopes

This poster session paper describes an integrated modeling and analysis capability being developed at JPL under funding provided by the JPL Director's Discretionary Fund and the JPL Control/Structure Interaction Program (CSI). The posters briefly summarize the program capabilities and illustrate them with an example problem. The computer programs developed under this effort will provide an unprecedented capability for integrated modeling and design of high performance optical spacecraft. The engineering disciplines supported include structural dynamics, controls, optics and thermodynamics. Such tools are needed in order to evaluate the end-to-end system performance of spacecraft such as OSI, POINTS, and SMMM. This paper illustrates the proof-of-concept tools that have been developed to establish the technology requirements and demonstrate the new features of integrated modeling and design. The current program also includes implementation of a prototype tool based upon the CAESY environment being developed under the NASA Guidance and Control Research and Technology Computational Controls Program. This prototype will be available late in FY-92. The development plan proposes a major software production effort to fabricate, deliver, support and maintain a national-class tool from FY-93 through FY-95.

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Integrated modeling of advanced optical systems

Integrated modeling of spacecraft systems is a rapidly evolving area in which multidisciplinary models are developed to design and analyze spacecraft configurations. These models are especially important in the early design stages where rapid trades between subsystems can substantially impact design decisions. Integrated modeling is one of the cornerstones of two of NASA's planned missions in the Origins Program -- the Next Generation Space Telescope (NGST) and the Space Interferometry Mission (SIM). Common modeling tools for control design and opto-mechanical analysis have recently emerged and are becoming increasingly widely used. A discipline that has been somewhat less integrated, but is nevertheless of critical concern for high precision optical instruments, is thermal analysis and design. A major factor contributing to this mild estrangement is that the modeling philosophies and objectives for structural and thermal systems typically do not coincide. Consequently the tools that are used in these discplines suffer a degree of incompatibility, each having developed along their own evolutionary path. Although standard thermal tools have worked relatively well in the past. integration with other disciplines requires revisiting modeling assumptions and solution methods. Over the past several years we have been developing a MATLAB based integrated modeling tool called IMOS (Integrated Modeling of Optical Systems) which integrates many aspects of structural, optical, control and dynamical analysis disciplines. Recent efforts have included developing a thermal modeling and analysis capability, which is the subject of this article. Currently, the IMOS thermal suite contains steady state and transient heat equation solvers, and the ability to set up the linear conduction network from an IMOS finite element model. The IMOS code generates linear conduction elements associated with plates and beams/rods of the thermal network directly from the finite element structural model. Conductances for temperature varying materials are accommodated. This capability both streamlines the process of developing the thermal model from the finite element model, and also makes the structural and thermal models compatible in the sense that each structural node is associated with a thermal node. This is particularly useful when the purpose of the analysis is to predict structural deformations due to thermal loads. The steady state solver uses a restricted step size Newton method, and the transient solver is an adaptive step size implicit method applicable to general differential algebraic systems. Temperature dependent conductances and capacitances are accommodated by the solvers. In addition to discussing the modeling and solution methods. applications where the thermal modeling is "in the loop" with sensitivity analysis, optimization and optical performance drawn from our experiences with the Space Interferometry Mission (SIM), and the Next Generation Space Telescope (NGST) are presented.

Laura Needels

Papers

Modeling and optimization of a segmented reflector telescope

Limits on adaptive optics systems for lightweight space telescopes

Integrated multidisciplinary analysis of segmented reflector telescopes

Integrated modeling of advanced optical systems

Integrated Modeling Tools for Thermal Analysis and Applications