Chromospheric K CaII telescope of Baikal astrophysical observatory. New light

In recent years, most results of CaII studies of the solar chromosphere in H and K lines have been based on observations with narrow passband filters. The Baikal Observatory’s full-disk chromospheric telescope for the K CaII line with a birefringent filter (BF) has again been included in observation programs of the Sun Service Station and scientific research. We analyze the methods of telescope and modernized birefringent filter adjustment. After many years of continuous operation, a pressing need arose to replace the damaged parts of the chromospheric telescope. Optics and mechanics of the entire telescope have been cleaned and readjusted. Wavefront interferograms of the teleobjective lens and of the whole telescope show that wavefront distortions of the entire optical path are within 0.25 λ. We present an optical scheme and discuss optical-physical characteristics of BF elements. The interference prefilter and UV polarizers have been replaced, and passbands of BF tunable stages have been adjusted. The BF spectral characteristics provide a fairly high contrast of monochromatic images of large-scale phenomena. Unfortunately, with a collimator and Sony Cyber-Shot DSC-S85 camera, mounted on the telescope, the theoretical resolution of the 180-mm telescope is not realized. We have therefore calculated two optical schemes of telescope changes for possible improvement in spatial resolution.