  The Advanced Atacama Cosmology Telescope Polarimeter (AdvACT) \cite{thornton}
is an upgrade for the Atacama Cosmology Telescope using Transition Edge Sensor
(TES) detector arrays to measure cosmic microwave background (CMB) temperature
and polarization anisotropies in multiple frequencies. The low frequency (LF)
array was deployed early 2020. It consists of 292 TES bolometers observing in
two bands centered at 27 GHz and 39 GHz. At these frequencies, it is sensitive
to synchrotron radiation from our galaxy as well as to the CMB, and complements
the AdvACT arrays operating at 90, 150 and 230 GHz. We present the initial LF
array on-site characterization, including the time constant, optical efficiency
and array sensitivity.
