Characterizing half-wave plates for the NIRC2 polarimetry mode on the Keck Observatory

In 2024, the NIRC2 infrared camera at the Keck II 10-m telescope will undergo an upgrade that will enable a suite of new polarimetric observing modes in observing bands J to L’ including coronagraphic imaging, standard imaging, and spectropolarimetry. The upgrade includes a pair of half-wave plates which have been characterized at wavelengths between 1.4 μm and 2 μm. The transmission of each waveplate was measured directly and the retardance was found from the Mueller matrix by using a dual-rotating retarder polarimeter setup. In addition, an anti-reflective nano-texture coating was tested on a waveplate without creating a noticeable change in the retardance.