In 2025, the Near-Infrared Camera 2 (NIRC2) at the 10-meter Keck II telescope will undergo an upgrade that will enable a suite of new polarimetric observing modes. This includes coronagraphic imaging, standard imaging, and spectropolarimetry in observing bands J through L′. The new modes will open up a wide range of studies such as investigations of exoplanets, the Galactic center, active galactic nuclei, and measurements of the solar system. The upgrade includes a Wollaston prism that was installed in NIRC2 in 2019 and a pair of half-wave plates (one for JHK bands and one for L′ band) to be installed in February 2025. The functionality of these waveplates must be evaluated before installation by characterizing their transmission and retardance. In this paper, we present results from characterizing the L′ band half-wave plate over the 1.1 to 1.95 μm range using a dual-rotating retarder polarimeter. In addition, we tested an anti-reflective nano-texture treatment that can be applied to the quartz JHK band waveplate in production. This treatment was found to improve transmission without significantly altering the retardance of a waveplate.
The differential polarization visibilities RQ and RU of an object are the ratios of its visibilities corresponding to orthogonal polarizations, the interferometric analogs to Stokes Q and U intensity images. The measurement of differential polarization visibilitites can be used for constraining inner parts of circumstellar envelopes of young or evolved stars at the diffraction limited resolution of the feeding telescope. We demonstrate the estimation of both amplitude and phase of RQ and RU from data obtained using SCExAO VAMPIRES through the full pupil of the 8-m Subaru telescope using the differential speckle polarimetry technique. The correction for biases arising due to instrumental polarization effects is discussed. The accuracy of RQ and RU measurement with VAMPIRES is limited by imperfect knowledge of instrumental polarization and amounts to 5 × 10 − 3.
The differential polarization visibilities RQ and RU of an object are the ratios of its visibilities corresponding to orthogonal polarizations, the interferometric analogs to Stokes Q and U intensity images. The measurement of differential polarization visibilitites can be used e.g. for constraining inner parts of circumstellar envelopes of young or evolved stars at the diffraction limited resolution of the feeding telescope. Here we demonstrate the estimation of both amplitude and phase of RQ and RU from data obtained using SCExAO VAMPIRES through the full pupil of the 8-m Subaru telescope using the Differential Speckle Polarimetry technique. The correction for biases arising due to instrumental polarization effects is discussed. The accuracy of RQ, RU measurement with VAMPIRES is limited by imperfect knowledge of instrumental polarization and amounts to 5 x 10-3.
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