  The Atacama Cosmology Telescope Polarimeter (ACTPol) is a polarization
sensitive upgrade to the Atacama Cosmology Telescope. Located at an elevation
of 5190 m, ACTPol measures the Cosmic Microwave Background (CMB) temperature
and polarization with arcminute-scale angular resolution. Calibration of the
detector angles is a critical step in producing maps of the CMB polarization.
Polarization angle offsets in the detector calibration can cause leakage in
polarization from E to B modes and induce a spurious signal in the EB and TB
cross correlations, which eliminates our ability to measure potential
cosmological sources of EB and TB signals, such as cosmic birefringence. We
present our optical modeling and measurements associated with calibrating the
detector angles in ACTPol.
