Liquid crystal polarimeter: a solid state imager for solar vector magnetic fields

Abstract

The liquid crystal polarimeter (LCP) is a low-voltage complete Stokes polarimeter and spectral analyzer designed for measuring solar vector magnetic fields. The polarimeter consists of polarization and spectral analyzer sections, each a stack of commercially available nematic and ferroelectric liquid crystals. Used in conjunction with a Lyot birefringent filter and 2 CCDs, the system provides an essentially simultaneous polarization-spectral measurement by modulating input and output ferroelectric liquid crystals in phase at up to 31 .5 kHz. Each measurement determines a single magnetic-field physical parameter unaffected by atmospheric seeing. The sequence of four Stokes parameters spectrally sampled in even and odd components in the solar line is obtained in successive reads of the CCDs with different settings of the nematic liquid crystals. The LCP provides solid state imaging of the vector magnetic flux, Doppler velocity, intensity, and equivalent width in a solar spectral line. Our differential method with 2 CCDs allows for automatic CCD gain and dark-current correction. The design provides a considerable simplification to previous magnetograph designs for greatly improved speed, sensitivity, and flexibility. Some of our tests of the liquid crystals and of the LCP are described.