The Simons Observatory: A fully remote controlled calibration system with a sparse wire grid for cosmic microwave background telescopes

TL;DR

This paper introduces a fully remote-controlled calibration system with a sparse wire grid for CMB telescopes, enabling precise detector polarization angle calibration with minimal systematic error.

Contribution

It presents a novel remote-controlled calibration system with a rotatable sparse wire grid, allowing regular and precise polarization calibration for CMB telescopes.

Findings

Achieved systematic error of 0.08 degrees in detector angle calibration.

Developed a remote control system using electric linear actuators.

System is suitable for small-aperture telescopes at the Simons Observatory.

Abstract

For cosmic microwave background (CMB) polarization observations, calibration of detector polarization angles is essential. We have developed a fully remote controlled calibration system with a sparse wire grid that reflects linearly polarized light along the wire direction. The new feature is a remote-controlled system for regular calibration, which has not been possible in sparse wire grid calibrators in past experiments. The remote control can be achieved by two electric linear actuators that load or unload the sparse wire grid into a position centered on the optical axis of a telescope between the calibration time and CMB observation. Furthermore, the sparse wire grid can be rotated by a motor. A rotary encoder and a gravity sensor are installed on the sparse wire grid to monitor the wire direction. They allow us to achieve detector angle calibration with expected systematic error of $0.08^{\circ}$. The calibration system will be installed in small-aperture telescopes at Simons Observatory.