Fibre Bragg gratings as an alignment aid in JWST MIRI

T. Stevenson; J. Sykes; P. Samara-Ratna; J. Pye

doi:10.1117/12.669756

7 July 2006 Fibre Bragg gratings as an alignment aid in JWST MIRI

T. Stevenson, J. Sykes, P. Samara-Ratna, J. Pye

Author Affiliations +

Proceedings Volume 6273, Optomechanical Technologies for Astronomy; 62733G (2006) https://doi.org/10.1117/12.669756
Event: SPIE Astronomical Telescopes + Instrumentation, 2006, Orlando, Florida , United States

Abstract

Largely thermal considerations have led the James Webb Space Telescope (JWST) Mid Infra Red Instrument (MIRI) European Consortium to specify a CFRP hexapod with rigidised Invar endfittings and brackets to form the Primary Structure of the instrument. Each bracket incorporates a pair of orthogonal flexures to provide kinematic mounting to JWST. The principal alignment of the instrument, namely the placing of the Pick-off Mirror (POM) in the telescope frame, must be known and be trackable by a combination of measurement and prediction. Contributors to the alignment are many and various, but potentially great uncertainty lies with the use of a hexapod with field separable joints. In order to provide continuous measurement of the response of the Primary Structure hexapod to integration, g release effects and thermoelastic effects, we have installed a strain gauge array in proximity to the flexures. In this way, asymmetrical strains, inadvertantly introduced during integration, may be detected. The technology employed is that of optical Fibre Bragg Gratings (FBGs), which allow us to measure strains continuously from room temperature down to cryogenic temperatures, with a modest investment in temperature calibration. The strain array has been used during the integration and testing of the Structural Thermal Model of the instrument, and some data have been obtained regarding the utility and effectiveness of this technique in diagnosing sources of alignment error buildup. This paper describes the technology employed, the logic behind these measurements and experience with integration and calibration. Analysis, and the results of some tests, both mechanical and thermal, are presented and discussed.

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T. Stevenson, J. Sykes, P. Samara-Ratna, and J. Pye "Fibre Bragg gratings as an alignment aid in JWST MIRI", Proc. SPIE 6273, Optomechanical Technologies for Astronomy, 62733G (7 July 2006); https://doi.org/10.1117/12.669756

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