Examples of Hα observations from the various archives analysed in this study. The images within each row correspond to the same day, with the exception of the Kh image (taken on 26 May 2000). In particular, the dates of the observations are 4 August 1965 for Ar, Ko, MD, MM, and Mi; 14 January 1980 for Bo, Ka, MD, and SP; 2 September 2011 for BB, Ka, La, Mi, and UP; and 13 September 2013 for BB, CT, Co, Ka, and PM. The images are shown after the pre-processing to identify the disc and to resample them to account for the disc ellipticity (when applicable) and convert the historical data to density values. The images have been aligned to show the solar north pole at the top.