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Proceedings Paper

Characterizing instrumental effects on polarization at a Nasmyth focus using NaCo
Author(s): Jozua de Boer; Julien H. Girard; Dimitri Mawet; Frans Snik; Christoph U. Keller; Julien Milli
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Paper 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 UV 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°.

Paper Details

Date Published: 28 July 2014
PDF: 15 pages
Proc. SPIE 9147, Ground-based and Airborne Instrumentation for Astronomy V, 914787 (28 July 2014); doi: 10.1117/12.2054970
Show Author Affiliations
Jozua de Boer, European Southern Observatory (Chile)
Leiden Observatory, Leiden Univ. (Netherlands)
Julien H. Girard, European Southern Observatory (Chile)
Dimitri Mawet, European Southern Observatory (Chile)
Frans Snik, Leiden Observatory, Leiden Univ. (Netherlands)
Christoph U. Keller, Leiden Observatory, Leiden Univ. (Netherlands)
Julien Milli, European Southern Observatory (Chile)
IPAG, Univ. Joseph Fourier, CNRS (France)

Published in SPIE Proceedings Vol. 9147:
Ground-based and Airborne Instrumentation for Astronomy V
Suzanne K. Ramsay; Ian S. McLean; Hideki Takami, Editor(s)

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