Polarization calibration of the Tandem Etalon Magnetograph of the Solar Magnetic Activity Research Telescope at Hida Observatory

The Tandem Etalon Magnetograph (TEM) is one of the instruments of the Solar Magnetic Activity Research Telescope of Hida Observatory. The TEM is a partial disk (320″ × 240″) filter magnetograph which scans the wavelength around a Fe I line at 6303 Å and achieves polarimetric sensitivity of <5 × 10^-4 for each wavelength. To obtain the polarimeter response matrix of the instrument, we have carried out end-to-end polarization calibrations of the instrument. We have also measured the polarization characteristics of the polarization beam splitter (PBS), which is a crucial component of the instrument. As a result of end-to-end calibration, we found significant spatial variation in the response matrix across the field of view. From a laboratory test, we found that $1\%$ of the magnitude of a circular diattenuation of the PBS was due to the retardation caused by the stress in the cube and the linear diattenuation of the film. Although the spatial variation across the field of view is more than 10 times larger, to achieve the polarimetric sensitivity of <5 × 10^-4, this can be well explained by the polarization characteristic of the PBS and corrected by using the response matrix obtained in the end-to-end calibration. In addition, we also obtained the daily variation of the polarization property of the TEM. We found that the crosstalk from Stokes Q to V changes by an amount comparable to the tolerance through a day. In the present configuration, we require a pixel-by-pixel calibration every 100 min to meet the accuracy requirement.