NIRPS (Near Infra Red Planet Searcher) is a near-infrared, fiber-fed, high-resolution, high precision radial velocity (pRV) spectrograph to be installed at ESO 3.6m telescope in La Silla Observatory in Chile. High precision radial velocity measurements require to have a very stable optical assembly. The gluing of optical elements in their mounts with A12 epoxy was selected as bonding process to minimize kinematic motion and optimize stability. However, coefficient of thermal expansion (CTE) mismatch between optical elements, their mountings and the glue may produce large local mechanical stress. Finite element analysis (FEA) was performed to estimate the thermal stress at room temperature and cryogenic temperature (80K). The selection of suitable bonding parameters (gluing setup, glue thickness, etc.) was a challenge given the CTE difference of optical elements (ZnSe and ZerodurTM) and holding flexures (SS304 and InvarTM). Extensive tests were performed to find a suitable bonding strategy. Gluing samples were tested under cryogenic temperature during several weeks. Mechanical shear stress tests were also performed to show that glued assembly could survive a 12g vertical load.
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