Glue test results for high-precision large cryogenic lens holder

Abstract

The Near Infrared Spectrometer and Photometer (NISP) of EUCLID requires high precision large lens holders (O170 mm) at cryogenic temperatures (150 K). The lenses of the optical system are glued into separate lens holders, the so called adaption rings. For the selection and verification of a suitable adhesive extensive glue selection tests are performed and results presented in this paper. With potential glue candidates, handling, single lap shear, connection tension and shear tests are carried out at room temperature (RT) and 150 K (OPS). For the NISP optical system DP490 is selected as the most suitable adhesive. The test results have shown that an even distribution of the glue in the glue gap is of crucial importance for the functioning and performance of the bonded lens system. The different coefficients of thermal expansion (CTE) between lens and lens holder produce large local mechanical stress and might cause lens breakage or failure of bonding. The design of the injection channel and the gluing procedure are developed to meet the lens performance requirements. An example is shown that after thermal cycling the remaining 0.5 mm – 1 mm thick adhesive in the injection channel results in large local mechanical stresses, and hence, damage of the lens. For a successful performance of the glue interface not only an optimum glue gap of 80 – 150 μm is important, also micro-cracks of the glass at the gluing area have to be avoided. The performed glue tests with DP490 for 3 different lens/ring material combinations show sufficient mechanical tension and shear strength for bonding of the lens system. Titanium/LF5G15 and Invar/Fused Silica combinations have reached the strength of 30 MPa at RT and 50 GPa at 150 K. These results are presented on behalf of the EUCLID consortium.