On the performance of a dual-beam polarimeter at Kodaikanal tower telescope

Abstract

Calibration and characterization of a dual-beam polarimeter installed at Kodaikanal Tower Telescope (KTT) for spectropolarimetric observations are presented in this paper. It was found that a slit width of 48 μ is optimum for the spectrograph setup at KTT and corresponding spectral resolution is 32.47 m\AA. It was demonstrated that the precision in polarization measurement can be achieved better than 0.1 % by increasing the exposure time. However, the polarimetric calibration accuracy is limited to 0.35% for Stokes Q and U parameters and 0.2% for Stokes V parameter, mainly due to the uncertainty in the retardance of the calibration retarder. A comparison of the magnetic field measurement between the Solar Optical Telescope (SOT) onboard Hinode and KTT for an active region was made and a good match was found after spatial smearing of Hinode/SOT measurements by 5.12''.