Visible-light channel of the Metis/Solar Orbiter coronagraph: On-ground polarimetric calibration

¹ INAF, Astrophysical Observatory of Torino, Via Osservatorio 20, Pino Torinese (TO) I-10025, Italy
² INAF, Arcetri Astrophysical Observatory, Largo Enrico Fermi 5, I-50125 Florence, Italy
³ Department of Physics and Astronomy, University of Florence, Via Giovanni Sansone 1, Sesto Fiorentino (FI) I-50019, Italy
⁴ Instituto Nacional de Técnica Aeroespacial, Torrejón de Ardoz, Madrid S-28850, Spain
⁵ INAF, Astronomical Observatory of Capodimonte, Salita Moiariello 16, Napoli I-80131, Italy
⁶ CNR – Institute for Photonics and Nanotechnologies, Via Trasea 7, I-35131 Padua, Italy
⁷ Dip. di Scienze Pure e Applicate, Università di Urbino, Via Santa Chiara, 27, 61029 Urbino, Italy
⁸ INFN Sezione di Firenze, Via B. Rossi 1, 50019 Sesto Fiorentino, Florence, Italy
⁹ Academy of Science of the Czech Republic Fričova 298, 251 65 Ondřejov, Czech Republic
¹⁰ University of Wroclaw, Center of Scientific Excellence – Solar and Stellar Activity, Kopernika 11, 51-622 Wroclaw, Poland
¹¹ NASA Headquarters, Washington, DC 20546-0001, USA
¹² Department of Physics and Astronomy, University of Padua, Via Francesco Marzolo 8, Padova I-35131, Italy
¹³ INAF, Astrophysical Observatory of Catania, Via Santa Sofia 78, Catania I-95123, Italy
¹⁴ ASI, Via del Politecnico, 00133 Roma, Italy
¹⁵ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany
¹⁶ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Alfonso Corti 12, I-20133 Milan, Italy

^⋆ Corresponding author: marta.casti@inaf.it

Received: 11 November 2024
Accepted: 7 July 2025

Abstract

Context. Metis is the solar coronagraph on board the ESA/NASA mission Solar Orbiter (SolO), launched in 2020. It is designed to acquire simultaneous images of the solar corona in visible polarized light (580−640 nm), and in the narrowband H I Lyman-α line (i.e., 121.6 nm). The instrument visible-light channel includes a polarimeter composed of two liquid crystal variable retarder (LCVR) cells, a quarter-wave plate, and a linear polarizer, with the LCVR cells acting as the modulating element. By applying a specific voltage to the cells, it is possible to modify the incoming polarized light by changing the direction of the associated vector by a known angle, i.e., the angle of retardance.

Aims. The polarimetric characterization of Metis is needed to correctly derive the properties of the plasma in the observed solar corona. This work describes the steps we took to fully characterize the visible channel of the Metis coronagraph, i.e., by deriving the modulation and demodulation matrices for each pixel, the latter being the key element for deriving the Stokes vector of the observed K corona. We completed the characterization by deriving the error associated with the derived values.

Methods. The first step is to fully characterize the Metis polarimeter. This is needed to derive the relation between the voltage applied to the liquid crystal cells and the angle of retardance imposed on the incoming polarized light. This step represents the starting point for the calibration of the full instrument.

Results. We derived (i) the voltage versus retardance curve associated with the polarimeter and (ii) the demodulation tensor of the Metis coronagraph, which is now used to retrieve the Stokes vector that describes the coronal light polarization state.

Conclusions. This calibration is fundamental to disentangle the coronal light from the instrumental effects, such as disuniformity and instrumental polarization.