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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.
Design and characterization of the Cosmology Large Angular Scale Surveyor (CLASS) 93 GHz focal plane
We measure CE7 to a) superconduct below a critical transition temperature, Tc, ~1.2 K, b) have a thermal contraction profile much closer to Si than metals, which enables simple mating, and c) have a low thermal conductivity which can be improved by Au-plating. Our investigations also demonstrate that CE7 can be machined well enough to fabricate small structures, such as #0-80 threaded holes, to tight tolerances (~25 μm) in contrast with pure silicon and similar substrates. We have fabricated CE7 baseplates being deployed in the 93 GHz polarimetric focal planes used in the Cosmology Large Angular Scale Surveyor (CLASS).1 We also report on the development of smooth-walled feedhorn arrays made of CE7 that will be used in a focal plane of dichroic 150/220 GHz detectors for the CLASS High-Frequency camera.
The cosmology large angular scale surveyor (CLASS): 38-GHz detector array of bolometric polarimeters
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