Volume 633, January 2020
|Number of page(s)||26|
|Published online||14 January 2020|
Polarimetric imaging mode of VLT/SPHERE/IRDIS
Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
2 European Southern Observatory, Alonso de Córdova 3107, Casilla, 19001 Vitacura, Santiago, Chile
3 Space Telescope Science Institute, Baltimore 21218 MD, USA
4 Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
5 Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
6 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
7 ETH Zurich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
8 CRAL, UMR 5574, CNRS, Université de Lyon, Ecole Normale Supérieure de Lyon, 46 allée d’Italie, 69364 Lyon Cedex 07, France
9 Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
10 Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
11 Heidelberg University, Institute of Theoretical Astrophysics, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
12 Université Côte d’Azur, OCA, CNRS, Lagrange, France
13 INAF – Osservatorio Astronomico di Padova, Vicolo della Osservatorio 5, 35122 Padova, Italy
14 DOTA, ONERA, Université Paris Saclay, 91123 Palaiseau, France
15 LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC, Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
16 Geneva Observatory, University of Geneva, Chemin des Mailettes 51, 1290 Versoix, Switzerland
Accepted: 14 October 2019
Context. Circumstellar disks and self-luminous giant exoplanets or companion brown dwarfs can be characterized through direct-imaging polarimetry at near-infrared wavelengths. SPHERE/IRDIS at the Very Large Telescope has the capabilities to perform such measurements, but uncalibrated instrumental polarization effects limit the attainable polarimetric accuracy.
Aims. We aim to characterize and correct the instrumental polarization effects of the complete optical system, that is, the telescope and SPHERE/IRDIS.
Methods. We created a detailed Mueller matrix model in the broadband filters Y, J, H, and Ks and calibrated the model using measurements with SPHERE’s internal light source and observations of two unpolarized stars. We developed a data-reduction method that uses the model to correct for the instrumental polarization effects, and applied it to observations of the circumstellar disk of T Cha.
Results. The instrumental polarization is almost exclusively produced by the telescope and SPHERE’s first mirror and varies with telescope altitude angle. The crosstalk primarily originates from the image derotator (K-mirror). At some orientations, the derotator causes severe loss of signal (> 90% loss in the H- and Ks-band) and strongly offsets the angle of linear polarization. With our correction method we reach, in all filters, a total polarimetric accuracy of ≲0.1% in the degree of linear polarization and an accuracy of a few degrees in angle of linear polarization.
Conclusions. The correction method enables us to accurately measure the polarized intensity and angle of linear polarization of circumstellar disks, and is a vital tool for detecting spatially unresolved (inner) disks and measuring the polarization of substellar companions. We have incorporated the correction method in a highly-automated end-to-end data-reduction pipeline called IRDAP, which we made publicly available online.
Key words: polarization / techniques: polarimetric / techniques: high angular resolution / techniques: image processing / methods: observational / protoplanetary disks
Based on observations made with ESO telescopes at the La Silla Paranal Observatory under program ID 60.A-9800(S), 60.A-9801(S) and 096.C-0248(C).
The data-reduction pipeline IRDAP is available at https://irdap.readthedocs.io
© ESO 2020
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