Characterizing the Instrumental Polarization of SCExAO VAMPIRES

The Visible Aperture Masking Polarimetric Imager for Resolved Exoplanetary Structures (VAMPIRES) is a visible light instrument on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system. To characterize the instrumental polarization (IP) and crosstalk of the system, measurements with a polarized internal calibration source were taken to provide diattenuation and retardance estimates of components downstream of the telescope mirrors. Using Markov Chain Monte Carlo (MCMC) simulations, we developed a Mueller matrix model of VAMPIRES in five wavelength bands. The process of deriving the model revealed degeneracies involving multiple possible sets of values for highly correlated instrument parameters. It also revealed a significant difference in the EM gain ratio between VAMPIRES’ two cameras and the highly non-ideal behaviour of the ferroelectric liquid crystal (FLC). Observations of unpolarized and polarized standard stars from two separate nights have been collected and are currently being analyzed to provide an estimate of M3’s diattenuation and retardance as well, and the polarized standard stars in particular will be used as a final metric to quantify the instrument model’s accuracy. The current Mueller matrix model has already been applied to preliminary data analysis for the circumstellar disk HD34700 and has helped confirm that an observed 17^◦ phase shift in differential visibilities for Mu Cephei is astrophysical in nature. The non-ideal behaviour of certain components highlights the need for a standard calibration procedure that should be performed regularly to track changes in VAMPIRES’ polarization qualities. Our laboratory and on-sky data collection and analysis procedures will be refined to be re-used to calibrate future versions of VAMPIRES, including its most recent upgrade in 2023.