An ambient-temperature Continuously Rotating Half-Wave Plate (CRHWP) modulates the input polarization signal thereby enabling removal of low-frequency (1/f) noise from polarized flux measurements. This 1/f noise arises from atmospheric turbulence as well as from effects intrinsic to certain detectors. Here, we describe the design and performance of the half wave plate rotator and achromatic half-wave plate for the the new imaging polarimeter, TolTEC. These components are mounted in front of the cryostat window and operate at ambient temperature. The Half-Wave Plate Rotator (HWPR) spins the half-wave plate at 2 revolutions per second. The rotation mechanism consists of nine air bearings to provide low-friction motion and a frameless torque motor to directly drive rotation. The orientation of the rotor and half-wave plate are recorded using a high-precision optical encoder. We review the experimental requirements and technical design of the rotator as well as the associated electronics, pneumatics, and software.
The Observatorio Astron´omico Nacional on the Sierra San Pedro M´artir (OAN-SPM) in Baja California, Mexico is currently undergoing a substantial expansion in its observational infrastructure. The OAN-SPM’s three principal telescopes were installed in the 1970s. In 2015, the BOOTES-5 telescope was installed and is now operational (partners: Mexico, Spain, South Korea). In 2011 the construction of the TAOS-II project begun and its three telescopes are now in commissioning (partners: Taiwan, Mexico, USA, Canada). Also undergoing commissioning are the COATLI and DDOTI projects (both: Mexico, USA). Two projects, COLIBR´I and SAINT-EX are about to begin construction (COLIBR´I: Mexico, France; SAINT-EX: Switzerland, Mexico, UK). Finally, the Telescopio San Pedro M´artir project is advancing through its design phase (partners: Mexico, USA). All save the TSPM are fully funded, so the OAN-SPM will host 11-12 telescopes by the 2020’s, ranging in size from 28cm to 6.5m.
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