Zero-level problem of solar magnetographs and observations of large-scale magnetic fields

Abstract

This paper discusses some aspects of the zero-level problem of solar magnetographs which is particularly important for observations of large-scale magnetic fields on the Sun. Experiments at the STOP telescope of the Sayan Solar Observatory (SSO) showed that in addition to the adjustment errors of the polarization analyzer, the focusing errors of the spectrograph, and the linear polarization of the light (these mechanisms were known previously [5]), “spurious” signals of the magnetograph are brought about by polarization effects in the optical details preceding the polarization analyzer (coelostat mirrors and the objective) and aberration errors of the spectrograph. Disadvantages of the method of monitoring the zero level from the nonmagnetic line λ 512.37 nm FeI are pointed out. A correlation was made between the observations of the solar mean magnetic field in the SSO and WSO (Wilcox Solar Observatory, Stanford, USA) — the observatories which use the different methods of zero-level monitoring.