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Various observatories around the globe started regular full-disk imaging ofthe solar atmosphere in the Ca II K line since the early decades of the 20thcentury. The archives made by these observations have the potential ofproviding far more detailed information on solar magnetism than just thesunspot number and area records to which most studies of solar activity andirradiance changes are restricted. We evaluate the image contents of three CaII K spectroheliogram time-series, specifically those obtained by thedigitization of the Arcetri, Kodaikanal, and Mt Wilson photographic archives.We describe the main problems afflicting these data and analyze their qualityby expressing the image contents through several quantities. We compare theresults obtained with those for similar present-day observations taken with theMeudon spectroheliograph and with the Rome-PSPT. We show that historic datasuffer from stronger geometrical distortions and photometric uncertainties thansimilar present-day observations. The latter uncertainties mostly originatefrom the photographic calibration of the original data and from stray-lighteffects. We also show that the image contents of the three analyzed series varyin time. These variations are probably due to instrument changes and aging ofthe spectrographs used, as well as changes of the observing programs. Ourresults imply that the main challenge for the analysis of historic data istheir accurate photometric calibration. This problem must be solved before theycan provide reliable information about solar magnetism and activity over thelast century. Moreover, inter-calibration of results obtained from independenttime-series is required to reliably trace changes of solar properties with timefrom the analysis of such data.

On the potential value of Ca II K spectroheliogram time-series for solar activity and irradiance studies