Zero-polarization candidate regions for the calibration of wide-field optical polarimeters^★

¹ Department of Physics, University of Crete, Vasilika Bouton, 70013 Heraklion, Greece
e-mail: nmandarakas@physics.uoc.gr
² Institute of Astrophysics, Foundation for Research and Technology – Hellas, 100 Nikolaou Plastira str. Vassilika Vouton, 70013 Heraklion, Crete, Greece
³ Department of Space, Earth & Environment, Chalmers University of Technology, 412 93 Gothenburg, Sweden
⁴ Université Libre de Bruxelles, Science Faculty CP230, 1050 Brussels, Belgium
⁵ South African Astronomical Observatory, PO Box 9, Observatory, 7935 Cape Town, South Africa
⁶ Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa
⁷ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, 1216 E California Blvd, Pasadena, CA 91125, USA
⁸ Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007, India
⁹ Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA
¹⁰ Department of Physics, Stanford University, Stanford, CA 94305, USA
¹¹ Kavli Institute for Particle Astrophysics & Cosmology, PO Box 2450, Stanford University, Stanford, CA 94305, USA

Received: 28 September 2023
Accepted: 3 January 2024

Abstract

Context. The calibration of optical polarimeters relies on the use of stars with negligible polarization (i.e., unpolarized standard stars) for determining the instrumental polarization zero point. For wide-field polarimeters, calibration is often done by imaging the same star over multiple positions in the field of view (FoV), which is a time-consuming process. A more effective technique is to target fields containing multiple standard stars. While this method has been used for fields with highly polarized stars, there are no such sky regions with well measured unpolarized standard stars.

Aims. We aim to identify sky regions with tens of stars exhibiting negligible polarization that are suitable for a zero-point calibration of wide-field polarimeters.

Methods. We selected stars in regions with extremely low reddening, located at high Galactic latitudes. We targeted four ~40′ × 40′ fields in the northern and eight in the southern equatorial hemispheres. Observations were carried out at the Skinakas Observatory and the South African Astronomical Observatory.

Results. We found two fields in the north and seven in the south characterized by a mean polarization lower than p < 0.1%.

Conclusions. At least 9 out of the 12 fields can be used for a zero-point calibration of wide-field polarimeters.