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

• Published in: Review of Scientific Instruments
• Main Authors: Bryan, S., Ade, Peter, Tucker, Carole
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Institute of Physics 2016
• Subjects: QB Astronomy; QC Physics; Antarc*; Antarctica
• Online Access: https://orca.cardiff.ac.uk/id/eprint/104410/; https://doi.org/10.1063/1.4939435

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∘. The system performed well in Spider during its successful 16 day flight.