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The Near Infrared Spectrometer and Photometer (NISP) of the EUCLID satellite project encompasses high precision large lens mounts of 168 mm diameter that are operated at cryogenic temperatures down to 135K. The four lenses of the optical system are made of different materials: SUPRASIL 30001, CaF2, and S-FTM16, which are mounted in a separate lens mount design using glue connections. Each lens assembly has its individual mechanical interface to the structure, the so called lens barrel. Exhaustive structural and thermal investigations have determined lens surface deformations and lens position changes that are introduced by various environmental loads, such as thermal-, mechanical-, interface-, and gravity loads, as well as mechanical stress of the lenses due to glue shrinkage during curing. All these impacts change the lens optical behaviours under real operational conditions of the optical assembly, which are thoroughly investigated in the optical performance assessment activity. Especially, great effort has been made for the simulation of interface tolerances. Due to the complexity of all mechanical interfaces (baffle, lens mounts, housing, telescope structure, etc.) statistical simulation is conducted applying Monte Carlo method. From the result of the statistical simulation 3 representative cases are selected for the optical performance assessment, which have 95% confidence level of the lens surface deformation. In the context of the evaluation procedure the surface form error of all EUCLID lenses as well as the RMS WFE at the focal plane is assessed, and results are compared with the nominal performance of the system, as well as with interferometrically measured results achieved during the interface– and gravity release test campaign. The performance of the lens holder design in terms of glue shrinkage effects, gravity release and interface tolerances is verified by an adapted test facility including an interferometer based optical metrology system. Finally, the measured values are compared with the analytical results, which show great confidence and hence proves validation of the analytical model. The paper presents the optical performance analysis results and the measured performance of the EUCLID high precision large cryogenic lens mounts. The achievements are presented on behalf of the EUCLID consortium.
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