Absolute velocity measurements in sunspot umbrae J. Löhner-Böttcher, W. Schmidt, R. Schlichenmaier, H.-P. Doerr, T. Steinmetz and R. Holzwarth A&A, 617 (2018) A19 Published online: 12 September 2018 DOI: 10.1051/0004-6361/201832886