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.
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