25 July 2008 NIRSpec OA development process of SiC components
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Abstract
The NIRSpec OA (optical assembly) design largely relies on SiC components. The properties of the SiC material and very tight stability budgets required a dedicated development process. Starting from validation of design principles by breadboard testing, this paper describes the development process up to the SM test of the NIRSpec optical assembly. From breadboard testing the design of the mounting interface was established. The test programme also included gluing processes, torque free mounting of mirrors and verification of stability of friction joints. The basic design rules for the mirrors to cope with distortion of mirror surfaces due to bi-metallic bending effects and flatness deficiencies were derived. A modular design using 3 TMAs (Three Mirror Anastigmats) was followed for the OA. From the overall design, budget allocations and design loads for the TMAs were determined. The detailed design process was then driven by distortion budget allocations derived from optical analysis. Due to stringent stability requirements and high mechanical loads, most elements needed several design iterations to meet the budget allocations. Finally, distortions and displacements of the optical elements under the predictable in-orbit conditions were calculated and used in the optical model. The effects can be partially compensated by adjustment. The budget allocation was then revised to account for non-predictable effects only. An extensive test programme on all level is applied. Proof testing is done on parts level; only for some structural parts the load introduction was too complex to allow full coverage. For those, proof test coverage was completed on TMA level. Structural qualification is done on TMA and OA level. Material properties of the SiC and associated design implications led to an extremely low structural damping. The strong responses found in sinusoidal vibration testing required high effort for test item monitoring, data evaluation and shaker control.
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Karl Honnen, Andreas Kommer, Boris Messerschmidt, Thorsten Wiehe, "NIRSpec OA development process of SiC components", Proc. SPIE 7018, Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, 70180R (25 July 2008); doi: 10.1117/12.788898; https://doi.org/10.1117/12.788898
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