Comparison of MD Hα maps for Carrington rotations 1427 (7 May to 2 June 1960, left, see also the middle column of Fig. 5) and 1497 (29 July to 25 August 1965, right, see also the top row of Figs. 1, 3 and 4 and right column of Fig. 6) produced manually by observations at Meudon (top row) and processed in this study (middle row). The bottom row shows the map obtained by overlapping the two corresponding maps shown in the upper two rows. To help visibility, this map is shown in greyscale, and the filaments from the manual processing (top panel) are shown in light grey instead of black.