Solar activity and irradiance studies with Ca II spectroheliogram time series: Potential, problems and possible solutions

Various observatories around the globe started to record synoptic observations of the solar full-disk atmosphere since the beginning of the 20th century . The archives of Ca II spectroheliograms made by these observations have the potential of providing far more detailed information on solar activity than just the sunspot number and area records to which most studies of activity variations, solar cycle and irradiance changes have restricted themselves. However, these data suffer significantly from various problems including numerous defects in the photographic plates, missing or inaccurate calibration of the blackening curve, changes in the positioning of the exit slit with respect to the spectral line, variable seeing and many more. These make any attempts to directly obtain results from the original plates more than dubious.
Here we describe the approach developed to calibrate and process the images obtained by the digitization of Arcetri and Kodaikanal Observatories. We have carried out a number of tests of the reliability of the correction or compensation for the shortcomings described above. Besides briefly mentioning these, we estimate the remaining uncertainties in the data.