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During qualification thermal testing of the Transiting Exoplanet Survey Satellite (TESS) lens assemblies, an anomalous focus change was observed during thermal-vacuum testing at its cold operating temperature (-75°C to - 85°C). Optical testing of the lens assemblies performed in the thermal-vacuum chamber indicated the magnitude and direction of the focus change, but did not identify the specific changes in the lens elements that were causing the focus shift. Individual lens motions measured using an interferometer indicated that lens vertices were moving relative to one another in a way that was inconsistent with predictions from detailed structural/thermal/optical (STOP) modeling. Further STOP analysis indicated the focus and vertex motion data were consistent with changes in lens curvature, suggesting that radial forces were deforming the lens. Finite element modeling showed that material property changes in the silicone adhesive material (room temperature vulcanizing, RTV 566) used to bond the lenses to the aluminum bezels could produce the necessary forces. The root cause of the focus shift was suspected to be unanticipated crystallization of the RTV 566 which has not previously been documented. Despite its widespread use, very little information has been published about the mechanical properties of RTV 566, and typical thermomechanical testing of its behavior has always utilized temperature sweeps. For this investigation, extensive testing was performed to characterize material property changes of RTV 566 samples under isothermal conditions at low temperatures (-75 to -85°C), for extended periods and at different levels of applied strain. The data presented here provide conclusive evidence that RTV 566 experiences time-dependent changes in mechanical properties that are consistent with crystallization phenomena.
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