The photographic calibration constitutes a key step for the processing of the digital images considered in this study. We evaluated the calibration curve, which relates the photographic density (output), to the logarithm of the plate exposition (input), starting from the average value of transparency T and its standard deviation measured on the various steps of the wedge exposition. Following
Caccin et al. (1998), the curve was computed through a least squares third degree polynomial fitting performed on the measured values, after suitable data re-arrangement and interpolation. In particular, the measured values T for the various steps of the wedge exposition were sorted in order of increasing exposition. The obtained values, together with intensity values expected by the step-wedge exposition, were linearly interpolated to produce a 200×2 element matrix showing the correspondence between density and intensity for the analyzed image.
This matrix was used as input for the polynomial fitting, which is performed taking into account the standard measurement errors for the transmission measured in each step. The fitting returned the function coefficients used to get the photographic calibration of pixel values on the analyzed image. The curve computation was applied to each image including a calibration wedge. The whole sample of CaII K images can be distinct in four sub-samples, depending on both instrumental and observational changes occurred during the about fifty years of Arcetri observations.
However, there is a large scatter among the curves obtained for each sub-sample.The calibration of the plates stored without calibration exposures was performed by using a reference curve. This curve was singled out among all the computed ones, since it allows to calibrate the largest sample of images carrying no specific calibration information by obtaining images in which the intensity pattern on solar observations appears as it is seen on present-day observations. However, the selected curve exhibits large scatter with respect to some computed curves.