Upper part of each panel: TSI reconstructions from photographic Ca II K observations. Daily values from the MW and MD1 datasets are shown as red dots and the corresponding 81-day running means as dashed blue lines. For comparison, the PMOD irradiance composite series is overplotted (black dots for daily values and the solid black line for the 81-day running means), which was used as the reference for the reconstructed series from MD1 Ca II K data. The running means have been computed considering only days on which both the reconstructions and the PMOD series were available. Lower left part of each panel: difference between the PMOD composite and the reconstructed TSI (red dots for daily values and solid light blue for 81-day running means). The dashed blue line is a linear fit to the residuals. The horizontal black dotted line marks the zero difference. Lower right part of each panel: distribution of the residuals in bins of 0.05 Wm⁻². The numbers in the bottom part of each panel indicate the solar cycle number, which are placed roughly at the maximum of each cycle.