Atmospheric transmission at the Chajnantor plateau. Normal conditions of 1.0 mm PWV are regularly observed during several months of the year. Outstanding conditions, below 0.5 mm PWV, are available on average during 890 hours per year. In the same plot, we show the AMKID filter-stack transmission for the two arrays and the Ruze efficiency of a telescope with 15–20 µm RMS surface quality, a typical value measured via holography at APEX.