18 December 2012 Ultra stable off-axis telescope: lessons learnt from the optical design to the correlation of the test results
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Abstract
RUAG Space developed, manufactured and demonstrated an afocal mirror telescope for space applications. The telescope is part of a Laser Communication Terminal (LCT) for GEO and LEO satellites. The design is off-axis and free of central obscuration. Optical interfaces are provided by pupils outside the telescope towards space (ø=135 mm) and towards the payload (ø=12.5 mm). The magnification is Γ=-10.8. The main characteristics are a WFE of ≤35nm, transmission <96%, low extinction ratio of linear and circular polarization, low stray light and low mass. The performance stability was demonstrated under various environments including vibrations, shock and thermal-vacuum up to 55°C. These properties enable a broad use, not limited to space. The layout is composed of four mirrors (Zerodur and Fused Silica) integrated in a nearly zero expansion Carbon Fibre (CFRP) structure. A detailed characterisation and advanced understanding of the CFRP represents a main achievement. The water absorption of CFRP in air causes elastic distortions of the structure until saturation. Certain optical performances are affected by this phenomenon which has to be considered when testing the system in thermal-vacuum environment. These effects were characterised and precompensated during integration in order to tailor the performance to the in-orbit conditions. The stability of the performances confirmed the selection of the CFRP as nearly-zero CTE material. Combined effects of moisture release and thermo-elastic distortions under thermal-vacuum loads were detected. The optical performances verification was then consequently and successfully tailored in order to distinguish these effects and prove the telescope stability under thermal-vacuum environment.
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Antonio Casciello, Thomas Weigel, Matthias Raunhardt, Andreas Herren, Steffen Blum, Nicolas de Roux, Thibault Seure, Ulrich E. Krähenbühl, Martin Herbert, "Ultra stable off-axis telescope: lessons learnt from the optical design to the correlation of the test results", Proc. SPIE 8550, Optical Systems Design 2012, 855017 (18 December 2012); doi: 10.1117/12.981201; https://doi.org/10.1117/12.981201
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