The Simons Observatory: Overview of the Cryogenic Half-wave Plate Polarization Modulators

The Simons Observatory (SO) is a ground-based Cosmic Microwave Background (CMB) experiment that is located in the Atacama plateau. The Small Aperture Telescopes (SATs) of SO are optimized for polarimetry on the degree scale. Atmospheric $1/f$ contamination of the CMB signal poses a significant challenge for observations at this angular scale. In order to control the $1/f$ noise, the SATs utilize a Cryogenic Half-Wave Plate (CHWP) in their optics. The CHWP modulates the polarization signal to a higher frequency to separate it from the unpolarized atmospheric noise. Precision measurements of the CHWP rotation angle are required to successfully recover the target polarized signals. We present a method to reconstruct the CHWP rotation angle that achieves a noise level of 0.16 $\mu\mathrm{rad\sqrt{s}}$, meeting our requirement.