Ring for KECK NIRC2 Polarimetry Mode Installation

NIRC2 polarimetry mode is set to be installed in March 2025 at the 10m Keck II Telescope in Mauna Kea, Hawaii. This mode will use polarized light to detect and characterize protoplanetary disks around low-mass and cool M-type stars, a population of stars previously inaccessible to other ground-based high-contrast polarimetric imagers. The installation requires the addition of two near-infrared half-waveplates at Keck’s NIRC2 camera, corresponding to the JHK and L band wavelengths. These half-waveplates can rotate incoming polarized light, allowing the camera to analyze various components of an astrophysical source’s polarization state, which can reveal details about disk structure and composition. In turn, this will inform us about exoplanet formation mechanisms and environments. For proper functionality, we need to perform a full quantitative analysis of the half-waveplates with an infrared CMOS camera, C-RED2 ER. To ensure accurate wave plate measurements, we must first determine the noise properties and detector responsivity of C-RED2 ER. We measure bias level and structure, read noise, dark current, and linearity of the detector by taking images at various temperatures, frame rates, exposure times, and experimental configurations. Our findings indicate that C-RED2 ER exhibits an average bias of 1090 ± 90  ADU, stable and low read noise levels around 3 ADU, minimal dark current at cooler temperatures (9008 ADU/s at -40 ℃), and a consistent linear response over a range of exposure times from 1 ms to 125 ms. With our calibration of C-RED2 ER, we ensure that later polarimetric measurements are accurate and robust.