Measurements of the 492 GHz atmospheric opacity at Pampa la Bola and Rio Frio in Northern Chile

We have developed a transportable 492 GHz tipping radiometer to measure the atmospheric opacity at potential sites for future ground-based astronomical observations in the submillimeter-wave band. With this radiometer, we measured the atmospheric opacity at two sites in northern Chile, Pampa la Bola (elevation 4800 m) and Rio Frio (elevation 4100 m), each for a few days. The 492 GHz opacity mostly ranged from 0.5 to 1.5 during the measurements. The 220 GHz opacity was also measured at the same time. The 492 GHz opacity correlates well with the 220 GHz opacity, the ratio between the 492 and 220 GHz opacities being 21.2+/-0.4. This result supports the standard atmospheric model, and can be used to evaluate the observable fraction of time for submillimeter-wave observations on the basis of the long-term 220 GHz opacity data.