Polarization calibration of the Solar Magnetic Activity Research Telescope(SMART)

One of the four telescopes of the Solar Magnetic Activity Research Telescope (SMART: UeNo et al. 2004) is a partial disk (320”x240”) filter magnetograph SMART-T4(Nagata et al. 2014). SMART-T4 consists of a rotating wave plate, tunable tandem Fabry-Perot filters which scan four wavelength points around the Fe I 630.25nm (-1.6nm, -0.8nm, 0.8nm and 1.6nm) with 1.3nm bandwidth, a polarizing beam splitter, and two CCD cameras simultaneously take orthogonally polarized light with a frame rate of 30 frames per second. SMART-T4's polarization optical system is expressed by a simple combination of a polarization modulator and a polarization analyzer. The system collects 10^4 photons at each pixel and exposure, by integrating as many as 700 images, SMART-T4 achieves the polarimetric sensitivity of 5x10^-4, with maximum temporal resolution of ~20 seconds. We have carried out two types of polarization calibration of the SMART-T4; one is with linear and circular sheet polarizers placed in front of the polarization modulator, and the other is with linear and circular sheet polarizers placed in front of the telescope. Based on the measurement of the first experiment, we have evaluated the polarimeter response matrix of the SMART-T4. As a result of this analysis, we found the following things. First, the most dominant modulation component in each Stokes Q, U, or V input was consistent to the polarization optical system model. Second, we found the significant spacial variation of the polarimeter response matrix, which means that the deduced matrix cannot be regarded as uniform through the SMART-T4 field of view. For crosstalk from the linear to circular polarizations, the spacial variation was two order larger in magnitude than the required uncertainty of 0.007, and for circular to linear polarization, it was one order larger than that of 0.005. Thus, we need to calibrate the SMART-T4 stokes profiles pixel by pixel. We then have examined the vector magnetic field derived from the calibrated stokes profiles. In this presentation, we explain the development of the polarization demodulation code to deduce Stokes vectors from the observed data, and the Stokes inversion code to derive the vector magnetic field from Stokes vector as well as the comparison of the vector magnetic field obtained with SMART-T4, Helioseismic and Magnetic Imager (HMI) aboard Solar Dynamics Observatory (SDO), and Hinode Solar Optical Telescope (SOT).