Polarimeter for coronal magnetic field measurement: design and evaluation of polarization cross talk

Stokes V, the circular polarization of light, from the solar corona is weak of the order of 10(-4) times the intensity (Stokes I). Measuring weak source polarization, for a faint source such as corona, is difficult and requires long integration time. To obtain long uninterrupted measurements, a space-based polarimeter would be preferable over a ground-based observatory. A full-Stokes polarimeter is designed such that the modulation matrix provides high efficiency in measuring the weak Stokes V signal, while minimizing the cross talk from Stokes Q and Stokes U. The prototype polarimeter consists of a single crystal retarder as the modulator and a Wollaston prism as a dual beam analyzer. The modulation is performed by rotating the retarder continuously. An optimum modulation matrix is derived taking into account the systematics and polarization cross talk due to satellite jitter. We present the results of cross talk simulation and the steps taken to obtain the modulation matrix. A polarimeter designed to observe the solar corona at Fe XIII 1074.6-nm emission line is presented. Jitter and drift from a low earth orbit satellite are taken to simulate and experimentally verify the cross talk and polarimetric efficiency of the polarimeter. A plane parallel crystal retarder produces polarized fringes. F-ratio of the beam incident on the retarder is one of the factors that effects the contrast of the polarized fringes. Results from a simulation performed to compare the polarizance produced by the retarder, when placed in a converging beam and a collimated beam of light, is presented. (C) 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)