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

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.

Polarimetric data are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (ftp://138.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/684/A132