Time-slit intensity maps and the optically thin criteria for different spectral lines. Top: in grey, time-slit maps of the maximum intensity in the three observed spectral lines for the scan position number 3, Hα (left), He I D₃ (centre), and Ca II IR (right). In colour, the optically thin pixels with regular symmetric profiles used for the analysis. Bottom: same time-slit maps with the criterion by Zapiór et al. (2022) to define points that are optically thin (green) or thick (red). The black region represents the very small amplitudes rejected for the analysis.