The Roman Coronagraph Community Participation Program: calibration strategy for the Mueller matrix using on-sky sources

The Coronagraphic Instrument on the Roman Space Telescope has a visible polarimetric mode. Accurate calibration of its polarization response is essential for characterizing debris disks, detecting planetary polarization, and diagnosing polarization aberrations. We will observe two unpolarized standards and two polarized standards with intrinsic polarization angles separated by about 45 degrees. These four stars provide the necessary constraints to solve the Mueller matrix such that the known polarization degrees and angles of the standards are correctly recovered. Standards must be single, bright enough for high S/N without saturating, and sufficiently polarized; visibility and operational constraints also apply. From optical catalogs we identified 72 candidates and began a precursor campaign targeting 21 northern objects. Multi-band HONIR polarimetry on the Kanata 1.5 m telescope measures wavelength dependence and enables Serkowski-law extrapolation to the Roman filters.