The birefringent KCaII filter is ready for new observations

Spectral and filter observations in KCaII line play a crucial role in studying the structure and dynamics of chromospheric formations and in analyzing solar magnetism. However, most monochromatic filters for KCaII observations have a full width at half maximum (FWHM) 0.3–3 Å, which means that chromospheric structures are obscured by more intense photospheric radiation. In order to minimize the influence of the photosphere, increase the information content of filter observations, and correctly interpret the data, an FWHM of less than 0.2 Å is required. To narrow the passband of the well-known B. Halle birefringent filter with 0.3 Å FWHM, new high-order interference polarization elements with 0.15 Å FWHM have been manufactured. Thin film polarizers have been installed to preserve the optical stack dimensions, and a new preliminary interference filter has been made. We performed combined spectral alignment of the old and new BF elements and studied optical properties of the filter. The updated filter is installed on the KCaII chromospheric telescope of the Baikal Astrophysical Observatory (BAO). The filtergrams show characteristic chromospheric formations — solar filaments. This indicates that the birefringent filter has a high spectral resolution such that the contribution of the photosphere to image acquisition is insignificant.