Correction to: Non-invasive assessment of plasma parameters inside an ion thruster combining optical emission spectroscopy and principal component analysisBenny T. Nauschütt, Limei Chen, Kristof Holste and Peter J. KlarEPJ Techn Instrum, 8 1 (2021) 18DOI: https://doi.org/10.1140/epjti/s40485-021-00075-6