In-flight radiometric calibration of the Metis Visible Light channel using stars and comparison with STEREO-A/COR2 data

Context. We present the results for the in-flight radiometric calibration performed for the Visible Light (VL) channel of the Metis coronagraph on board Solar Orbiter.Aims. The radiometric calibration is a fundamental step in building the official pipeline of the instrument, devoted to producing the calibrated data in physical units (L2 data).Methods. To obtain the radiometric calibration factor (& epsilon;(VL)), we used stellar targets transiting the Metis field of view. We derived & epsilon;(VL)by determining the signal of each calibration star by means of the aperture photometry and calculating its expected flux in the Metis band pass. The analyzed data set covers the time range from the beginning of the Cruise Phase of the mission (June 2020) until March 2021.Results. Considering the uncertainties, the estimated factor & epsilon;(VL)is in a good agreement with that obtained during the on-ground calibration campaign. This implies that up to March 2021 there was no measurable reduction in the VL channel throughput. Finally, we compared the total and polarized brightness visible light images of the solar corona acquired with Metis and STEREO-A/COR2 during the November 2020 superior conjunction of these instruments. A general good agreement was obtained between the images of these instruments for both the total and polarized brightness.

Automated summary

This study presents the results of in-flight radiometric calibration for the Visible Light channel of the Metis coronagraph on Solar Orbiter. The calibration was performed using stellar targets in the field of view, and the estimated calibration factor showed good agreement with the on-ground calibration. The comparison of images acquired by Metis and STEREO-A/COR2 also demonstrated a general good agreement.