The digitized archive of the Arcetri spectroheliograms. Preliminary results from the analysis of Ca II K images

Context. The increasing interest in the recovery of historic data and the availability of new hardware resources is leading to projects to digitize photographic archives of astronomical observations. In addition to preservation, solar archives are digitized because the observations stored in such archives have the potential of providing unique information about solar magnetism, which can improve knowledge about long-term solar changes.