Polarization Calibration of the Chromospheric LAyer Spectro-Polarimeter (CLASP2)

The Chromospheric LAyer Spectro-Polarimeter (CLASP2) is a NASA sounding rocket experiment that was successfully launched on April 11, 2019. The CLASP2 measured the four Stokes parameters in the 280 nm wavelength range (Mg II h & k lines) caused by the joint action of the scattering processes and the Hanle and Zeeman effects with 0.1% polarization accuracy. A unique polarization calibration of the instrument is required to ensure such a polarization accuracy. To this end, we determined the response matrix of the CLASP2 by only performing the polarization calibration of the spectro-polarimeter (SP). This is because it is estimated that the instrumental polarization of the CLASP2 telescope, due to the non-uniformity of the mirror coating and off-axis incidence, is negligible. Our polarization calibration was carried out in two steps as follows. First, we determined the response matrix of the SP by using the pre-flight polarization calibration data. To derive the response matrix elements, we assumed the spurious polarization term to be zero based on un-polarized light measurements, and determined the scale factor and crosstalk between the Stokes parameters. After the CLASP2 launch, we confirmed the level of spurious polarization by using the in-flight data taken at the solar disk. Here, we present our methodology of the polarization calibration and discuss the final CLASP2 response matrix based on our tolerance requirements.