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

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.

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Journal
Astronomy and Astrophysics
Published
2025-03-28
DOI
https://doi.org/10.1051/0004-6361/202450759
Citations
4
Primary Topic
Calibration and Measurement Techniques
Type
article
Field-Weighted Citation Impact
19.97

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article

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

M. Stangalini, A. Burtovoi, M. Messerotti, Fabio Frassetto et al.
4 citations
Astronomy and Astrophysics
Calibration and Measurement Techniques
19.97
article

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

M. Stangalini, A. Burtovoi, M. Messerotti, Fabio Frassetto, Paolo Chioetto, Paola Zuppella, G. Nicolini, G. Russano, C. Sasso, M. Uslenghi, L. Abbo, D. Spadaro, S. L. Guglielmino, V. Da Deppo, Michele Fabi, Enrico Magli, Alessandro Liberatore, C. Casini, M. Pancrazzi, T. Straus, U. Schühle, Yara De Leo, Daniele Telloni, P. Romano, G. Capuano, F. Frassati, V. Andretta, C. Grimani, Alessandra Slemer, M. Romoli, Marta Casti, P. Heinzel, G. Jerse, L. Teriaca, Silvano Fineschi, Roberto Susino, S. Giordano, J. D. Moses, L. Zangrilli, M. Giarrusso, G. Naletto, Maria Guglielmina Pelizzo, Gerardo Capobianco, Alain Jody Corso, А. Бемпорад, A. Volpicelli, Federico Landini, G. Massone
article en
4 citations

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.

Astronomy and AstrophysicsVol. 697
Goddard Space Flight Center (US), Jet Propulsion Laboratory (US), University of Padua (IT), University of Graz (AT), Istituto Nazionale di Fisica Nucleare (IT), Politecnico di Torino (IT), University of Urbino (IT), University of Wrocław (PL), Arcetri Astrophysical Observatory (IT), Istituto Nazionale di Fisica Nucleare, Galileo Galilei Institute for Theoretical Physics (IT), Czech Academy of Sciences, Astronomical Institute (CZ), Agenzia Spaziale Italiana (IT), Trieste Astronomical Observatory (IT), Astronomical Observatory of Capodimonte (IT), University of Catania (IT), Osservatorio Astrofisico di Torino (IT), Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano (IT), Max Planck Institute for Solar System Research (DE), Osservatorio Astrofisico di Catania (IT), Istituto Nazionale di Fisica Nucleare, Sezione di Firenze (IT), Center for Biomolecular Nanotechnologies (IT), Ospedale San Giuseppe (IT), Osservatorio Astronomico di Padova (IT), Institute for Microelectronics and Microsystems (IT), University of Florence (IT)
National Aeronautics and Space Administration, Ministero dell’Istruzione, dell’Università e della Ricerca, Akademie Věd České Republiky, Università di Catania, Bundesministerium für Wirtschaft und Energie, Uniwersytet Wrocławski
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Calibration and Measurement Techniques
19.97
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