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

Understand

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.