Polarization analysis of the VLTI and GRAVITY^★

¹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstr. 1, 85748 Garching bei Muenchen, Germany
e-mail: fwidmann@mpe.mpg.de
² LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
³ Max-Planck-Institute for Astronomy, Königsstuhl 17, 69117, Heidelberg, Germany
⁴ Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937, Köln, Germany
⁵ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶ Universidade de Lisboa – Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
⁷ Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
⁸ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748, Garching, Germany
⁹ European Southern Observatory, Casilla 19001, Santiago 19, Chile
¹⁰ Max-Planck-Institute for Radio Astronomy, Auf dem Hügel 69, 53121, Bonn, Germany
¹¹ Departments of Physics and Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
¹² CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
¹³ Department of Astrophysical & Planetary Sciences, JILA, Duane Physics Bldg., 2000 Colorado Ave, University of Colorado, Boulder, CO 80309, USA
¹⁴ Department of Physics, University of Illinois, 1110 West Green Street, Urbana, IL 61801, USA
¹⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁶ Department of Astrophysics, IMAPP, Radboud University, 6500 GL Nijmegen, The Netherlands
¹⁷ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
¹⁸ Advanced Concepts Team, European Space Agency, TEC-SF, ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
¹⁹ ORIGINS Excellence Cluster, Boltzmannstraße 2, 85748 Garching bei Muenchen, Germany
²⁰ Leiden University, 2311 EZ Leiden, The Netherlands

Received: 20 June 2023
Accepted: 3 November 2023

Abstract

Aims. The goal of this work is to characterize the polarization effects of the beam path of the Very Large Telescope Interferometer (VLTI) and the GRAVITY beam combiner instrument. This is useful for two reasons: to calibrate polarimetric observations with GRAVITY for instrumental effects and to understand the systematic error introduced to the astrometry due to birefringence when observing targets with a significant intrinsic polarization.

Methods. By combining a model of the VLTI light path and its mirrors and dedicated experimental data, we constructed a full polarization model of the VLTI Unit Telescopes (UTs) and the GRAVITY instrument. We first characterized all telescopes together to construct a universal UT calibration model for polarized targets with the VLTI. We then expanded the model to include the differential birefringence between the UTs. With this, we were able to constrain the systematic errors and the contrast loss for highly polarized targets.

Results. Along with this paper, we have published a standalone Python package that can be used to calibrate the instrumental effects on polarimetric observations. This enables the community to use GRAVITY with the UTs to observe targets in a polarimetric observing mode. We demonstrate the calibration model with the Galactic Center star IRS 16C. For this source, we were able to constrain the polarization degree to within 0.4% and the polarization angle to within 5° while being consistent with the literature values. Furthermore, we show that there is no significant contrast loss, even if the science and fringe-tracker targets have significantly different polarization, and we determine that the phase error in such an observation is smaller than 1^°, corresponding to an astrometric error of 10 µas.

Conclusions. With this work, we enable the use by the community of the polarimetric mode with GRAVITY/UTs and outline the steps necessary to observe and calibrate polarized targets with GRAVITY. We demonstrate that it is possible to measure the intrinsic polarization of astrophysical sources with high precision and that polarization effects do not limit astrometric observations of polarized targets.