A cryogenic rotation stage with a large clear aperture for the half-wave plates in the Spider instrument

TL;DR

This paper details the design and successful deployment of a cryogenic rotation stage with a large aperture for half-wave plates in the Spider telescope, enabling precise polarization measurements of the Cosmic Microwave Background during a balloon flight.

Contribution

It introduces a novel cryogenic rotation mechanism with a large aperture and high-precision angle monitoring for balloon-borne CMB experiments.

Findings

System operated successfully during the 16-day flight.

Achieved absolute angle accuracy of +/- 0.1 degrees.

Supported precise polarization measurements in a cryogenic environment.

Abstract

We describe the cryogenic half-wave plate rotation mechanisms built for and used in Spider, a polarization-sensitive balloon-borne telescope array that observed the Cosmic Microwave Background at 95 GHz and 150 GHz during a stratospheric balloon flight from Antarctica in January 2015. The mechanisms operate at liquid helium temperature in flight. A three-point contact design keeps the mechanical bearings relatively small but allows for a large (305 mm) diameter clear aperture. A worm gear driven by a cryogenic stepper motor allows for precise positioning and prevents undesired rotation when the motors are depowered. A custom-built optical encoder system monitors the bearing angle to an absolute accuracy of +/- 0.1 degrees. The system performed well in Spider during its successful 16 day flight.