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Chapter 1:
Introduction to Mechanical Analysis Using Finite Elements
Abstract
1.1 Integrated Optomechanical Analysis Issues 1.1.1 Integration issues The optical performance of telescopes, lens barrels, and other optical systems are heavily influenced by mechanical effects. Figure 1.1 depicts the interaction between thermal, structural, and optical analysis. Each analysis type has its own specialized software to solve its own field specific problems. To predict interdisciplinary behavior, data must be passed between analysis types. In this book, emphasis is placed on the interaction of the three analysis disciplines. 1.1.2 Example: orbiting telescope A simple finite element structural model of an orbiting telescope is shown in Fig. 1.2 and a corresponding optical model is shown in Fig. 1.3. Because of dynamic disturbances, the optics may move relative to each other and distort elastically. From the finite element model, the motions of each node point are predicted. To determine the effect on optical performance, it is necessary to pass the data to the optical analysis program in a form that is acceptable. This usually requires a special post-processing program as described in later chapters. Typically, the structural data must be converted to the optical coordinate system, optical units, and sign convention, then fit with Zernike polynomials or interpolated to interferogram arrays (Chapter 4). To create a valid and accurate structural model, the analyst must be aware of modeling techniques for mirrors, mounts, and adhesive bonds (Chapter 3). Incorporating image-motion equations inside the FE model (Chapter 4) allows for image-motion output directly from a vibration analysis. The vibrations may be due to harmonic or random loads. To determine if a mirror will fracture, the analyst must understand the type of failure analysis required and how to search over load envelopes. During the processing (grinding, polishing, and coating) of a mirror, it may be tested under various support conditions that require their own analysis (Chapter 3). Analysis of the assembly process (Chapter 3) will predict locked-in strains and create an optical back out that can be factored into the overall system performance. Performance of the flexible primary mirror can be improved by adding actuators and sensors to create an adaptive mirror (Chapter 6). Using optimum design techniques (Chapter 7), the design can be made more efficient and robust. The specific details of the analyses on this telescope are demonstrated in Chapter 8.
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