Measurements of 220 GHz atmospheric opacity on Mt Fuji with a radiometer/radome system

We measured atmospheric opacity at 220 GHz at the summit of Mt. Fuji (alt. 3776 m) about one year in order to explore a feasibility of submillimeter-wave astronomical observations. For this purpose, a 220 GHz radiometer system enclosed in a radome (51 x 51 x 62 cm(3)) has been developed. The 220 GHz opacity was lower than 0.06 for a significant fraction (similar to 45%) of time from November 1994 to March 1995. Diurnal variation of the opacity at the summit of Mt. Fuji is so small that continuous observation at submillimeter-wave is possible through day and night. Yearly variation of the opacity is studied from water vapor pressure data measured at the weather station for the past 3 years. To prevent accumulation of ice and snow on the Gore-Tex membrane in the radome, the outer membrane was supplied with a thermal flux of 0.63 kW m(-2) and the adjacent metal radome surfaces were supplied with a flux of similar to 0.9 kW m(-2). We evaluate from the 220 GHz transmission data that this heat flux is sufficient to keep the membrane on the radome free of ice and snow during 83% of the time in 5 winter months. The summit of Mt. Fuji appears to be a promising site for submillimeter-wave observations.