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

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. (C) 2016 AIP Publishing LLC.