A New Method of Calibration of Photographic Plates from Three Historic Data Sets

We analyze the synoptic data taken in the Ca ii K spectral line with spectroheliographs at Kodaikanal Observatory from 1907 to 1999, at Mount Wilson Observatory from 1915 to 1985, and at the National Solar Observatory at Sacramento Peak from 1963 to 2002. Photographic data were digitized and calibrated following the same set of procedures developed by the authors of this paper. Using calibrated data, we have outlined bright plages and have calculated a plage index defined as the fraction of solar hemisphere occupied by the chromospheric plages and enhanced network. We present a detailed description of our method and provide a brief comparison of Ca ii K plage indices derived using data from these three historic data sets.