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24 September 2011 Adhesive bond cryogenic lens cell margin of safety test
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Abstract
The Near Infrared Camera (NIRCam) instrument for NASA's James Webb Space Telescope (JWST) has an optical prescription which employs four triplet lens cells. The instrument will operate at 35K after experiencing launch loads at approximately 295K and the optic mounts must accommodate all associated thermal and mechanical stresses, plus maintain an exceptional wavefront during operation. Lockheed Martin Space Systems Company (LMSSC) was tasked to design and qualify the bonded cryogenic lens assemblies for room temperature launch, cryogenic operation, and thermal survival (25K) environments. The triplet lens cell designs incorporated coefficient of thermal expansion (CTE) matched bond pad-to-optic interfaces, in concert with flexures to minimize bond line stress and induced optical distortion. A companion finite element study determined the bonded system's sensitivity to bond line thickness, adhesive modulus, and adhesive CTE. The design team used those results to tailor the bond line parameters, minimizing stress transmitted into the optic. The challenge for the Margin of Safety (MOS) team was to design and execute a test that verified all bond pad/adhesive/ optic substrate combinations had the required safety factor to generate confidence in a very low probability optic bond failure during the warm launch and cryogenic survival conditions. Because the survival temperature was specified to be 25K, merely dropping the test temperature to verify margin was not possible. A shear/moment loading device was conceived that simultaneously loaded the test coupons at 25K to verify margin. This paper covers the design/fab/SEM measurement/thermal conditioning of the MOS test articles, the thermal/structural analysis, the test apparatus, and the test execution/results.
© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
David M. Stubbs, Craig L. Hom, Howard C. Holmes, Joseph C. Cannon-Morret, Obert F. Lindstrom, J. Wes Irwin, Leigh A. Ryder, Troy T. Hix, Jane A. Bonvallet, Hsin-Kuei S. Hu, Ira V. Chapman, Curtis Lomax, E. Todd Kvamme, Gregory S. Feller, and Mark M. Haynes "Adhesive bond cryogenic lens cell margin of safety test", Proc. SPIE 8125, Optomechanics 2011: Innovations and Solutions, 81250N (24 September 2011); https://doi.org/10.1117/12.891872
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