| Summary: | 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. © 2016 AIP Publishing LLC. Received 6 October 2015; accepted 18 December 2015; published online 8 January 2016. SPIDER is supported in the U.S. by National Aeronautics and Space Administration under Grant Nos. NNX07AL64G and NNX12AE95G issued through the Science Mission Directorate, with support for A.S.R. from NESSF NNX10AM55H, and by the National Science Foundation through No. PLR-1043515. Logistical support for the Antarctic deployment and operations was provided by the NSF through the U.S. Antarctic Program. The collaboration is grateful for the generous support of the David and Lucile Packard Foundation, which has been crucial to the success of the project. Support in Canada is provided by the National Sciences and Engineering Council and the Canadian Space Agency. Published - 1.4939435.pdf Submitted - 1510.01771v2.pdf
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