Detailed evolution of the sunspot. The upper panel displays the continuum intensity image for reference, and the lower panel shows the vertical magnetic field in the local reference frame. To cover the most important times of the penumbra formation, we display one HMI frame each hour between 08:00 and 12:00, and additionally one frame at 05:00, roughly three hours before the SST observations, and one frame at 14:00, roughly two hours after our last observation. The small black box indicates FOV selection, which we inverted with STiC.