  Various observatories around the globe started regular full-disk imaging of
the solar atmosphere in the Ca II K line since the early decades of the 20th
century. The archives made by these observations have the potential of
providing far more detailed information on solar magnetism than just the
sunspot number and area records to which most studies of solar activity and
irradiance changes are restricted. We evaluate the image contents of three Ca
II K spectroheliogram time-series, specifically those obtained by the
digitization of the Arcetri, Kodaikanal, and Mt Wilson photographic archives.
We describe the main problems afflicting these data and analyze their quality
by expressing the image contents through several quantities. We compare the
results obtained with those for similar present-day observations taken with the
Meudon spectroheliograph and with the Rome-PSPT. We show that historic data
suffer from stronger geometrical distortions and photometric uncertainties than
similar present-day observations. The latter uncertainties mostly originate
from the photographic calibration of the original data and from stray-light
effects. We also show that the image contents of the three analyzed series vary
in time. These variations are probably due to instrument changes and aging of
the spectrographs used, as well as changes of the observing programs. Our
results imply that the main challenge for the analysis of historic data is
their accurate photometric calibration. This problem must be solved before they
can provide reliable information about solar magnetism and activity over the
last century. Moreover, inter-calibration of results obtained from independent
time-series is required to reliably trace changes of solar properties with time
from the analysis of such data.
