Near-infrared HeI 1083nm Stokes Polarimeter Based on Liquid Crystal Variable Retarders

The spectral line of HeI 1083nm is important and potential to measure the magnetic field of the solar upper chromosphere. In this paper, we present a newly developed Stokes polarimeter for measuring the polarized signals at this wavelength. In this device, two Liquid Crystal Variable Retarders (LCVRs) were employed as electro-optical modulators and a Wollaston prism as analyzer and polarized beam splitter. Compared to the commonly used linear-polarized analyzer, the Wollaston prism analyzer has main advantage to minimize the seeing-induced contamination of earth's atmosphere, as it produces simultaneous images by the two perpendicular polarization states. A novel optical design which focuses the two beams on different detector areas is described. And the accurate calibration methods are introduced too.