Characterizing Atacama B-mode Search Detectors with a Half-Wave Plate
  The Atacama B-Mode Search (ABS) instrument is a cryogenic ($\sim$10 K)
crossed-Dragone telescope located at an elevation of 5190 m in the Atacama
Desert in Chile that observed for three seasons between February 2012 and
October 2014. ABS observed the Cosmic Microwave Background (CMB) at large
angular scales ($40<\ell<500$) to limit the B-mode polarization spectrum around
the primordial B-mode peak from inflationary gravity waves at $\ell \sim100$.
The ABS focal plane consists of 480 transition-edge sensor (TES) bolometers.
They are coupled to orthogonal polarizations from a planar ortho-mode
transducer (OMT) and observe at 145 GHz. ABS employs an ambient-temperature,
rapidly rotating half-wave plate (HWP) to mitigate systematic effects and move
the signal band away from atmospheric $1/f$ noise, allowing for the recovery of
large angular scales. We discuss how the signal at the second harmonic of the
HWP rotation frequency can be used for data selection and for monitoring the
detector responsivities.
