Characterizing instrumental effects on polarization at a Nasmyth focus using NaCo

Jozua de Boer; Julien H. Girard; Dimitri Mawet; Frans Snik; Christoph U. Keller; Julien Milli

doi:10.1117/12.2054970

28 July 2014 Characterizing instrumental effects on polarization at a Nasmyth focus using NaCo

Jozua de Boer, Julien H. Girard, Dimitri Mawet, Frans Snik, Christoph U. Keller, Julien Milli

Proceedings Volume 9147, Ground-based and Airborne Instrumentation for Astronomy V; 914787 (2014) https://doi.org/10.1117/12.2054970
Event: SPIE Astronomical Telescopes + Instrumentation, 2014, Montréal, Quebec, Canada

Abstract

We propose a new calibration scheme to determine the instrumental polarization (IP) and crosstalk induced by either the telescope or an instrument at Nasmyth focus. We measure the polarized blue sky at zenith with VLT/UT4/NaCo for different NaCo derotator and telescope azimuth angles. Taking multiple measurements after rotating both the instrument and the telescope with angles of 90° allows use to determine the IP and most crosstalk components separately for the telescope and the instrument. This separation of the Mueller matrices of UT4 and the NaCo is especially important for measurements taken in the conventional polarimetric mode (field stabilized), because the rotation of the instrument with respect to M3 causes a variation in the IP and crosstalk throughout the measurement. The technique allows us to determine the IP with an accuracy of 0.4%, and constrain or determine lower or upper limits for most crosstalk components. Most notably, the UT4 U → V crosstalk is substantially larger than theory predicts. An angular offset in NaCo's half wave plate orientation is a possible source of systematic errors. We measure this offset to be 1.8° ± 0.5°.

Citation Download Citation

Jozua de Boer, Julien H. Girard, Dimitri Mawet, Frans Snik, Christoph U. Keller, and Julien Milli "Characterizing instrumental effects on polarization at a Nasmyth focus using NaCo", Proc. SPIE 9147, Ground-based and Airborne Instrumentation for Astronomy V, 914787 (28 July 2014); https://doi.org/10.1117/12.2054970

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