Initial radiometric calibration of the HRI_EUV telescope of the EUI instrument on board Solar Orbiter

¹ Solar-Terrestrial Centre of Excellence–SIDC, Royal Observatory of Belgium Ringlaan -3- Avenue Circulaire B-1180 Brussels, Belgium
² Institut d’Astrophysique Spatiale, CNRS, Univ. Paris-Sud, Université Paris-Saclay Bât. 121 91405 Orsay, France
³ Centre Spatial de Liège, Université de Liège Av. du Pré-Aily B29 4031 Angleur, Belgium
⁴ Max Planck Institute for Solar System Research Justus-von-Liebig-Weg 3 37077 Göttingen, Germany
⁵ European Commission, DG Energy, Directorate D: Nuclear Energy, Safety and ITER 24 rue Demot B-1049 Brussels, Belgium
⁶ UCL-Mullard Space Science Laboratory Holmbury St. Mary Dorking Surrey RH5 6NT, UK
⁷ Physikalisch-Technische Bundesanstalt (PTB) Abbestr. 2-12 10587 Berlin, Germany
⁸ Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center 7260 Davos Dorf, Switzerland
⁹ Laboratoire Charles Fabry, Institut d’Optique Graduate School, Université Paris-Saclay 91127 Palaiseau Cedex, France
¹⁰ Centre for Astrophysics & Relativity, School of Physical Sciences, Dublin City University Glasnevin Campus Dublin D09 V209, Ireland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 5 August 2024
Accepted: 6 October 2025

Abstract

Context. The HRI_EUV telescope was calibrated on ground at the Physikalisch-Technische Bundesanstalt (PTB), Germany’s national metrology institute, using the Metrology Light Source (MLS) synchrotron in April 2017 during the calibration campaign of the Extreme Ultraviolet Imager (EUI) instrument on board the Solar Orbiter mission.

Aims. We used the pre-flight end-to-end calibration and component-level (mirror multilayer coatings, filters, detector) characterization results to establish the beginning-of-life performance of the HRI_EUV telescope, which will serve as a reference for radiometric analysis and monitoring of the telescope in-flight degradation.

Methods. Calibration activities at component level and end-to-end calibration of the instrument were performed at PTB/MLS synchrotron light source (Berlin, Germany) and the SOLEIL synchrotron (Orsay, France). Each component optical property is measured and compared to its semi-empirical model. This pre-flight characterization is used to estimate the parameters of the semi-empirical models. The end-to-end response is measured and validated by comparison with calibration measurements, as well as with its main design performance requirements.

Results. The telescope spectral response semi-empirical model is validated by the pre-flight end-to-end ground calibration of the instrument. It is found that HRI_EUV is a highly efficient solar EUV telescope with a peak efficiency superior to 1 e⁻.ph⁻¹, low detector noise (≈2.8 e rms), low dark current at operating temperature, and a pixel saturation above 120 ke- in low-gain or combined image mode. The ground calibration also confirms a well-modelled spectral selectivity and rejection, and low stray light. The EUI instrument achieves a state-of-the-art performance in terms of signal-to-noise and spatial resolution.