We present an evaluation of systematic effects associated with a continuously-rotating , ambient-temperature half-wave plate ( HWP ) based on two seasons of data from the Atacama B-Mode Search ( ABS ) experiment located in the Atacama Desert of Chile .
The ABS experiment is a microwave telescope sensitive at 145 GHz .
Here we present our in-field evaluation of celestial ( CMB plus galactic foreground ) temperature-to-polarization leakage .
We decompose the leakage into scalar , dipole , and quadrupole leakage terms .
We report a scalar leakage of \sim 0.01 \% , consistent with model expectations and an order of magnitude smaller than other CMB experiments have reported .
No significant dipole or quadrupole terms are detected ; we constrain each to be < 0.07 % ( 95 % confidence ) , limited by statistical uncertainty in our measurement .
Dipole and quadrupole leakage at this level lead to systematic error on r \lesssim 0.01 before any mitigation due to scan cross-linking or boresight rotation .
The measured scalar leakage and the theoretical level of dipole and quadrupole leakage produce systematic error of r < 0.001 for the ABS survey and focal-plane layout before any data correction such as so-called deprojection .
This demonstrates that ABS achieves significant beam systematic error mitigation from its HWP and shows the promise of continuously-rotating HWPs for future experiments .