Updating the SCExAO/CHARIS polarimetric calibration following the Nasmyth beam-switcher upgrade

Abstract

Subaru/SCExAO/CHARIS enables near-infrared integral field spectropolarimetry. Quantitative polarimetry is useful for a variety of science cases, particularly measurements related to dust grain properties in circumstellar disks. This capability requires correcting for polarization effects from the optical path via a Mueller matrix model. We present an updated model accounting for the recently installed SCExAO near-infrared wavefront sensor YJH50 dichroic beamsplitter and the major Subaru Nasmyth beam-switcher upgrade. Using internal light source measurements from before and after the beam switcher installation, we find an elliptical retarder model for the image derotator improves polarimetric accuracy over the previous linear retarder model. We additionally find the YJH50 dichroic produces faint polarization effects that we cannot characterize with our Mueller matrix modeling capabilities, and that the Nasmyth beam-switcher has minimal polarization effects other than inducing a sign flip in Q and V polarized light. Using unpolarized standard star calibration measurements, we fit the diattenuation of Subaru’s tertiary mirror as a function of wavelength and find that the diattenuation has increased since the previous CHARIS calibration. We calculate that the polarimetric accuracy of the model in the degree of linear polarization ranges from 0.02% to 0.12% for a 1% polarized target. This model update will soon be incorporated into CHARIS’s data processing pipeline, and should be used for any polarimetric data taken after the Nasmyth beam-switcher update in October 2025. Additionally, we provide the code for this calibration as part of an open-source Python package for polarimetric calibration called pyPolCal, enabling straightforward re-calibration of the system after any future changes, e.g. the recent recoating of M3.