Polarimetric calibration of the Tunable Magnetograph for the Sunrise III mission

The Tunable Magnetograph (TuMag) onboard Sunrise III is an imaging spectropolarimeter designed to ultimately infer the solar vector magnetic field and line-of-sight velocity. It includes Liquid Crystal Variable Retarders for polarimetric modulation and hence requires accurate polarimetric calibration to achieve its scientific goals. This work reports on the pre-flight calibration carried out on ground at different stages during the TuMag assembly, integration and verification phase. It includes the polarization modulator characterization, the verifications at instrument level, and, finally, the full system calibration after its integration in SUNRISE within the Post-Focus Instrumentation platform.The calibration includes determining the field-dependent demodulation matrices (pixel-by-pixel calibration) and evaluating polarimetric efficiencies, ensuring compliance with the mission sensitivity requirements. The results of this work provide the calibrated demodulation matrices that will be used for the scientific exploitation of the mission data.