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12 December 2003 Study of small variations of coefficient of thermal expansion in Corning ULE glass
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Thermal requirements for the Jet Propulsion Laboratory (JPL) Technology Demonstration Mirror (TDM) to support the Terrestrial Planet Finder (TPF) Mission push the state-of-the-art for lightweight mirror design. A detailed evaluation by Brasher LP during the TPF TDM Mirror Study shows that Corning ULE glass is the best choice to meet these requirements. Even using ULE, careful selection of the grade and individual boule stacking are required to optimize the design and achieve the desired operational performance. The grade of ULE varies from premium to tooling and is determined by the distribution of Coefficient of Thermal Expansion (CTE) variation about a mean value, both radially and axially. The total range of CTE throughout the mirror must be carefully evaluated to determine operational performance. To support this evaluation, a specialized Finite Element Method (FEM) model generator was modified to assign CTE values to each element based upon its position within the boule and the specified mean CTE, radial and axial gradients for each run. The user can specify different CTE mean and radial gradients for front and back face sheets and for two levels of core to match the expected two boule stack for the TDM mirror blank. The program (model generator) can select random values for each of these inputs within the range of a given grade and generate a new model within minutes. This permits statistical studies of ranges as well as worst-case evaluations with ease. This paper describes the evolution of this type of modeler and some of the results obtained from recent studies of small variations in the CTE affecting mirror performance.
© (2003) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
William R. Arnold "Study of small variations of coefficient of thermal expansion in Corning ULE glass", Proc. SPIE 5179, Optical Materials and Structures Technologies, (12 December 2003);

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