Stray-light restoration of full-disk CaII K solar observations: a case study

TL;DR

This study evaluates the effectiveness of a restoration algorithm in removing stray-light effects from full-disk CaII K solar observations, improving spatial resolution and contrast, especially in recent data, with limitations in archival data.

Contribution

The paper demonstrates that a Walton & Preminger (1999) based restoration algorithm can enhance the quality of both current and archival CaII K solar observations, highlighting the importance of proper photographic calibration.

Findings

Restoration improves spatial resolution in both recent and archival data.

Photometric contrast improvement is limited in archival data.

Quality of archival data restoration depends on photographic calibration.

Abstract

AIMS: We investigate whether restoration techniques, such as those developed for application to current observations, can be used to remove stray-light degradation effects on archive CaII K full-disk observations. We analyze to what extent these techniques can recover homogeneous time series of data. METHODS:We develop a restoration algorithm based on a method presented by Walton & Preminger (1999). We apply this algorithm to data for both present-day and archive CaII K full-disk observations, which were acquired using the PSPT mounted at the Rome Observatory, or obtained by digitization of Mt Wilson photographic-archive spectroheliograms. RESULTS:We show that the restoring algorithm improves both spatial resolution and photometric contrast of the analyzed solar observations. We find that the improvement in spatial resolution is similar for analyzed recent and archive data. On the other hand, the improvement of photometric contrast is quite poor for the archive data, with respect to the one obtained for the present-day images. We show that the quality of restored archive data depends on the photographic calibration applied to the original observations. In particular, photometry can be recovered with a restoring algorithm if the photographic-calibration preserves the intensity information stored in the original data, principally outside the solar-disk observations.