Kepler primary mirror assembly: FEA surface figure analyses and comparison to metrology

John W. Zinn; George W. Jones

doi:10.1117/12.732667

14 September 2007 Kepler primary mirror assembly: FEA surface figure analyses and comparison to metrology

John W. Zinn, George W. Jones

Proceedings Volume 6671, Optical Manufacturing and Testing VII; 667105 (2007) https://doi.org/10.1117/12.732667
Event: Optical Engineering + Applications, 2007, San Diego, California, United States

Abstract

The Kepler primary mirror assembly (PMA), designed and fabricated by the Brashear Division of L-3 Communications (Brashear), consists of a 1.45 meter, lightweight, frit bonded, ULE mirror supported on a composite strut hexapod. During the early stages of fabrication, finite element analysis results are integrally combined with the in-process mirror metrology to determine the cryogenic, gravity free surface figure of the Kepler primary mirror. The successful operational design of the primary mirror assembly hinges upon the accurate estimation of the cryogenic figure. This paper describes a combined test and analysis approach developed by Brashear which yielded an analytical FEA cryogenic surface distortion within 8% of the actual measured rms surface. The primary mirror thermally induced surface distortion is strongly influenced by the nonlinear, strain rate dependent adhesive mechanical properties. Because of the difficulty in accurately characterizing and implementing the adhesive behavior, an optical test of a representative geometry is used to characterize the distortion caused by the adhesive and bond pad. By correlating FEA results to the optical results, an equivalent, linear elastic, adhesive stiffness is derived and used in the primary mirror assembly cryogenic FEA analysis. Additionally, during in-process metrology, the gravity induced FEA surface figure for each test configuration is numerically subtracted from the raw metrology data in order to estimate the gravity free figure. Thus, the accuracy of the final in-process mirror figure is dependent upon the accuracy of the FEA results which are subtracted. The in-process results are then compared to the final gravity free figure for the integrated primary mirror assembly which is obtained by averaging the metrology results for two diametrically opposed support orientations.

Citation Download Citation

John W. Zinn and George W. Jones "Kepler primary mirror assembly: FEA surface figure analyses and comparison to metrology", Proc. SPIE 6671, Optical Manufacturing and Testing VII, 667105 (14 September 2007); https://doi.org/10.1117/12.732667

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