The Atacama B-Mode Search: CMB Polarimetry with Transition-Edge-Sensor
  Bolometers
  The Atacama B-mode Search (ABS) experiment is a 145 GHz polarimeter designed
to measure the B-mode polarization of the Cosmic Microwave Background (CMB) at
large angular scales. The ABS instrument will ship to the Atacama Desert of
Chile fully tested and ready to observe in 2010. ABS will image
large-angular-scale CMB polarization anisotropies onto a focal plane of 240
feedhorn-coupled, transition-edge sensor (TES) polarimeters, using a cryogenic
crossed-Dragone design. The ABS detectors, which are fabricated at NIST, use
orthomode transducers to couple orthogonal polarizations of incoming radiation
onto separate TES bolometers. The incoming radiation is modulated by an
ambient-temperature half-wave plate in front of the vacuum window at an
aperture stop. Preliminary detector characterization indicates that the ABS
detectors can achieve a sensitivity of 300 $\mu K \sqrt{s}$ in the field. This
paper describes the ABS optical design and detector readout scheme, including
feedhorn design and performance, magnetic shielding, focal plane architecture,
and cryogenic electronics.
