Packing a PUNCH: Wide-field polarimetry of the heliosphere

Accurate measurements of linear polarization in wide-field solar imagers are critical for characterizing coronal and heliospheric structures. The PUNCH mission employs a Narrow Field Imager and three Wide Field Imagers (WFIs) equipped with linear polarizers at −60°, 0°, and +60°. Each WFI has a 40° field of view; across such wide fields, geometric projection effects cause the effective polarizer angles to vary spatially, with deviations reaching up to ±2.5°. The PUNCH data pipeline takes into account such geometrical effects and conversion of polarization among various bases including total brightness (B), polarized brightness (pB), Stokes parameters, and radial and tangential components (Br/Bt) using the open-source package solpolpy. We discuss the strategy of attaining accurate polarization measurement combining all instrument data from the rotating frame, separation of polarized background from F corona and stars, and present an overview of the visualization tools developed for interpretation.