A universal polarimeter using liquid crystal variable retarders at the Norikura Solar Observatory

A universal Stokes polarimeter was developed for the 25-cm coronagraph of the Norikura Solar Observatory. The polarimeter utilizes two liquid crystal variable retarders (LCVR) as the polarization modulator and a Savart plate as the polarization analyser. By controlling the voltages applied to LCVRs, modulations of the polarization states are achieved more flexibly and efficiently in a wavelength range of 450-1,600 nm. The polarimeter is installed at the primary focus of the coronagraph to minimize the instrumental polarization caused by optics. Two orthogonal polarization states are measured simultaneously to minimize the effect of guiding errors and seeing effects. Combined with the coronagraph and the high resolution spectrograph at the coude focus, the polarimeter enables us not only observations of photospheric Zeeman effect but also various types of diagnostics of the solar atmosphere, including coronal Zeeman effect, Hanle effect, impact polarization, and Stark effect.