In-flight radiometric calibration of the Metis UV H I Ly-α channel and comparison with UVCS data

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
² INAF – Catania Astrophysical Observatory, Via Santa Sofia 78, 95123, Catania, Italy
³ Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria
⁴ University of Florence – Physics and Astronomy Department, Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy
⁵ INAF – Turin Astrophysical Observatory, Via Osservatorio 20, 10025 Pino Torinese (TO), Italy
⁶ INAF – Arcetri Astrophysical Observatory, Largo Enrico Fermi 5, 50125 Florence, Italy
⁷ INAF – Capodimonte Astronomical Observatory, Salita Moiariello 16, 80131 Naples, Italy
⁸ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Alfonso Corti 12, 20133 Milan, Italy
⁹ CNR – Institute for Photonics and Nanotechnologies, Via Trasea 7, 35131 Padua, Italy
¹⁰ University of Catania – Physics and Astronomy Department “Ettore Majorana”, Via Santa Sofia 64, 95123 Catania, Italy
¹¹ Institute for Microelectronics and Microsystems (CNR-IMM), Catania, Italy
¹² CISAS – Center of Studies and Activities for Space “Giuseppe Colombo”, Via Venezia 15, 35131 Padua, Italy
¹³ The Catholic University of America at NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁴ INAF – Padua Astrophysical Observatory, Vicolo Osservatorio 5, 35122 Padua, Italy
¹⁵ University of Urbino – Dipartimento di Scienze Pure e Applicate, Via Santa Chiara 27, 61029 Urbino, Italy
¹⁶ INFN – Istituto Nazionale di Fisica Nucleare, Section in Florence, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
¹⁷ Astronomical Institute of the Czech Academy of Sciences, Fričova 28, 25165 Ondřejov, Czech Republic
¹⁸ University of Wrocław, Center of Scientific Excellence – Solar and Stellar Activity, Kopernika 11, 51-622 Wrocław, Poland
¹⁹ INAF – Trieste Astronomical Observatory, Via G.B. Tiepolo 11, 34143 Trieste, Italy
²⁰ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA-91109, USA
²¹ Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
²² NASA Headquarters, Washington, DC 20546-0001, USA
²³ University of Padova – Physics and Astronomy Department “Galileo Galilei”, Via F. Marzolo 8, 35131 Padua, Italy
²⁴ University of Padova – Department of Information Engineering, Via Gradenigo 6/B, 35131 Padova, Italy
²⁵ Italian Space Agency, Via del Politecnico, 00133 Rome, Italy

^★ Corresponding author.

Received: 17 May 2024
Accepted: 6 February 2025

Abstract

Context. We present the results of the in-flight radiometric calibration performed for the ultraviolet (UV) H I Ly-α channel of the Metis coronagraph on board Solar Orbiter.

Aims. The radiometric calibration is a fundamental procedure required to produce data in physical units. The quantity that allows us to pass from raw data into calibrated data is the radiometric calibration factor, ϵ_UV.

Methods. To obtain the ϵ_UV results, we used observations of stellar targets transiting the Metis field of view. We derived ϵ_UV by determining the signal of each calibration star by means of the aperture photometry and evaluating its expected flux in the Metis narrow bandpass (121.6 ± 10 nm). The analyzed data cover a time range from the beginning of the Cruise Phase in June 2020, up until August 2021.

Results. We find that the UV channel requires a significant additional correction of the response across the field of view, compared to that provided by the vignetting function measured on the ground and refined in flight, specifically tailored to the UV channel. This correction is provided by the ratio of images of the back-illumination of the closed door. Here, we use the stellar measurements to refine and improve such a correction map. After correcting for the spatial disuniformity, a radiometric calibration factor ϵ̅_UV = 0.20 ± 0.03 DN/photon was found. No significant changes in the UV channel throughput were observed during the period from June 2020 to August 2021. In addition, the analysis of a smaller number of stars observed in 2022 and 2023 enabled us to extend the validity of the radiometric calibration to that period, after considering a suitable scaling factor due to the change of operating voltages occurred in April 2022. From this second analysis, the value of the radiometric calibration factor is ϵ̅_UV = 0.11 ± 0.03 DN/photon. In order to support the radiometric calibration results, we performed a comparison between average radial profiles of the H I Ly-α intensity obtained from Metis UV images acquired in 2020–2021 and those measured with the Ultra-Violet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO) during the period of the activity minimum of solar cycle 22 in 1996. We found that intensity profiles of these instruments are consistent with each other.