AIMS: We investigate whether restoration techniques, such as those developedfor application to current observations, can be used to remove stray-lightdegradation effects on archive CaII K full-disk observations. We analyze towhat extent these techniques can recover homogeneous time series of data.METHODS:We develop a restoration algorithm based on a method presented byWalton & Preminger (1999). We apply this algorithm to data for both present-dayand archive CaII K full-disk observations, which were acquired using the PSPTmounted at the Rome Observatory, or obtained by digitization of Mt Wilsonphotographic-archive spectroheliograms. RESULTS:We show that the restoringalgorithm improves both spatial resolution and photometric contrast of theanalyzed solar observations. We find that the improvement in spatial resolutionis similar for analyzed recent and archive data. On the other hand, theimprovement of photometric contrast is quite poor for the archive data, withrespect to the one obtained for the present-day images. We show that thequality of restored archive data depends on the photographic calibrationapplied to the original observations. In particular, photometry can berecovered with a restoring algorithm if the photographic-calibration preservesthe intensity information stored in the original data, principally outside thesolar-disk observations.
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