Solar spectropolarimetry of Ca II 8542 Å line: polarimeter development, calibration, and preliminary observations

Abstract

Chromospheric magnetic fields are of paramount importance in understanding the dynamics of energetic events in the solar atmosphere. At the Kodaikanal Solar Observatory, several polarimeters were developed in the past to study the active region magnetic fields. A new polarimeter has been developed and installed at Kodaikanal Tower-tunnel Telescope to study the active regions at Chromospheric level, in Ca II $8542~\AA$ spectral line. Design aspects of the instrument and polarimetry strategy are discussed. Telescope instrumental polarization has been revisited and possible ways to reduce it have been proposed. Telescope polarization model developed in Zemax to examine the analytical instrumental polarization model is discussed. The polarimeter control unit, and the software developed to operate the polarimeter are briefly described. Polarimetric calibration of the instrument, observations, corrections for instrumental polarization and the sample Stokes profiles are presented. Polarimetric accuracy and sensitivity are estimated to be better than $3\times10^{-2}$ and $3\times 10^{-3}$ respectively.