The Simons Observatory Small Aperture Telescopes feature cryogenic half-wave plates (HWP) consisting of a sapphire disk filter rotating at over 100 rpm, levitating on superconducting YBCO magnets with a transition temperature of about 92 Kelvin. The HWP has its own power and controller module with custom software to monitor temperature and rotational velocity. Solenoids grip the plate when warm and center it until the transition temperature is reached. Post Doctoral Researcher Peter Ashton from UC Berkeley designed and assembled the HWP for the SATs.

The advanced design of the SO HWP stands on the shoulders of the HWPs for the Simons Array Polarbear-2 telescopes also designed and fabricated at UC Berkeley, led by PhD. candidate Charlie Hill.

The continuously rotating HWPs help ensure that the SATs will deliver a data product that is highly robust, and along with other optical components, allow researchers to filter the signal in order to observe subtle polarizations in the cosmic microwave background.