LIQUID-CRYSTAL POLARIMETER - A SOLID-STATE IMAGER FOR SOLAR VECTOR MAGNETIC-FIELDS

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 birefr ingent filter and 2 CCDs, the system provides an essentially simultane ous polarization-spectral measurement by modulating input and output f erroelectric liquid crystals in phase at up to 31.5 kHz. Each measurem ent determines a single magnetic-field physical parameter unaffected b y atmospheric seeing. The sequence of four Stokes parameters spectrall y sampled in even and odd components in the solar line is obtained in successive reads of the CCDs with different settings of the nematic li quid crystals. The LCP provides solid state imaging of the vector magn etic flux, Doppler velocity, intensity, and equivalent width in a sola r spectral line. Our differential method with 2 CCDs allows for automa tic CCD gain and dark-current correction. The design provides a consid erable simplification to previous magnetograph designs for greatly imp roved speed, sensitivity, and flexibility. Some of our tests of the li quid crystals and of the LCP are described.