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BICEP Array (BA) is the latest generation multi-frequency instrument of the BICEP/Keck program, which specifically targets degree-scale primordial B-modes in the CMB. In its final configuration, BA will consist of four small-aperture receivers, spanning six different frequency bands. The BA4 receiver is designed to characterize Galactic dust at 220/270 GHz. This receiver is currently undergoing commissioning at Stanford and is scheduled to deploy to the South Pole during the 2024-2025 austral summer. Here, we will provide an overview of this high frequency receiver, discussing the integration status and test results as it is being commissioned.
Design and characterization of the Cosmology Large Angular Scale Surveyor (CLASS) 93 GHz focal plane
Variable-delay polarization modulators (VPMs) are used in the Cosmology Large Angular Scale Surveyor (CLASS) telescopes as the first element in the optical chain to rapidly modulate the incoming polarization. VPMs consist of a linearly polarizing wire grid in front of a movable flat mirror. Varying the distance between the grid and the mirror produces a changing phase shift between polarization states parallel and perpendicular to the grid which modulates Stokes U (linear polarization at 45°) and Stokes V (circular polarization). The CLASS telescopes have VPMs as the first optical element from the sky; this simultaneously allows a lock-in style polarization measurement and the separation of sky polarization from any instrumental polarization further along in the optical path.
The CLASS VPM wire grids use 50 μm copper-plated tungsten wire with a 160μm spacing across a 60 cm clear aperture. The mirror is mounted on a flexure system with one degree of translational freedom, enabling the required mirror motion while maintaining excellent parallelism with respect to the wire grid. The wire grids and mirrors are held parallel to each other to better than 80 μm, and the wire grids have RMS flatness errors below 50 μm across the 60 cm aperture. The Q-band CLASS VPM was the first VPM to begin observing the CMB full time, starting in the Spring of 2016. The first W-band CLASS VPM was installed in the Spring of 2018.
The cosmology large angular scale surveyor (CLASS): 38-GHz detector array of bolometric polarimeters
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