Bright-Moon sky as a wide-field linear Polarimetric flat source for calibration

¹ Institute of Astrophysics, Foundation for Research and Technology - Hellas, Vasilika Vouton, 70013 Heraklion, Greece
e-mail: sidh345@gmail.com
² Department of Physics, University of Crete, Voutes University Campus, 70013 Heraklion, Greece
³ Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune - 411 007, India
⁴ South African Astronomical Observatory, PO Box 9, Observatory, 7935, Cape Town, South Africa
e-mail: siddharth@saao.ac.za
⁵ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, 1216 E California Blvd, Pasadena, CA 91125, USA
⁶ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, NO-0315 Oslo, Norway
⁷ National Institute of Science Education and Research, An OCC of Homi Bhabha National Institute, Jatni, Bhubaneswar - 752050, India
⁸ Astrophysics Division, National Centre for Nuclear Research, Pasteura 7, Warsaw, 02093, Poland
⁹ Department of Space, Earth & Environment, Chalmers University of Technology, 412 93 Gothenburg, Sweden
¹⁰ Institute of Computer Science, Foundation for Research and Technology-Hellas, Vasilika Vouton, 70013 Heraklion, Greece
¹¹ Department of Computer Science, University of Crete, Voutes, 70013 Heraklion, Greece
¹² Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa
¹³ Owens Valley Radio Observatory, California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA
¹⁴ Steward Observatory, University of Arizona, 933 N Cherry Ave, Tucson, Arizona, 85721, USA
¹⁵ Université Libre de Bruxelles, Science Faculty CP230, Av. Franklin Roosevelt 50, 1050 Brussels, Belgium

Received: 7 May 2023
Accepted: 30 August 2023

Abstract

Context. Next-generation wide-field optical polarimeters such as Wide-Area Linear Optical Polarimeters (WALOPs) have a field of view (FoV) of tens of arcminutes. Wide-field polarimetric flat sources are essential to the efficient and accurate calibration of these instruments. However, no established wide-field polarimetric standard or flat sources exist at present.

Aims. This study tests the feasibility of using the polarized sky patches of the size of around 10 × 10 arcminutes², at a distance of up to 20° from the Moon, on bright-Moon nights as a wide-field linear polarimetric flat source.

Methods. We observed 19 patches of the sky adjacent to the bright-Moon with the RoboPol instrument in the SDSS-r broadband filter. These patches were observed on five nights within two days of the full-Moon across two RoboPol observing seasons.

Results. We find that for 18 of the 19 patches, the uniformity in the measured normalized Stokes parameters q and u is within 0.2%, with 12 patches exhibiting uniformity within 0.07% or better for both q and u simultaneously, making them reliable and stable wide-field linear polarization flats.

Conclusions. We demonstrate that the sky on bright-Moon nights is an excellent wide-field linear polarization flat source. Various combinations of the normalized Stokes parameters q and u can be obtained by choosing suitable locations of the sky patch with respect to the Moon.