First Measurements of the Atmospheric Transmission at Cerro SairecaburFollowing a 2-1/2 year run at the 5000m ALMA site near Cerro Chajnantor, in 2000 October the FTS was moved 35 km NNW to a site on Cerro Sairecabur, at 5500m altitude. So far, data have been recovered through mid-December 2000. Although this is a limited data set, covering only austral spring conditions, it is nevertheless possible to make some preliminary comparisons with Chajnantor. This is because, as indicated by the NRAO 225 GHz tipper data for November 1999 and November 2000 at Chajnantor, conditions in these two months happened to be very similar. The FTS was operating 73% of the time in 1999 November, and 57% of the time in 2000 November. Downtime occurred in stretches of several days, resulting in negligible time-of-day bias. Neither month exhibited significant variation in weather conditions.
Shown below are opacity statistics for the 1.03 THz, 1.3 THz, and 1.5 THz windows, from the FTS measurements made in 1999 November at Chajnantor, and in 2000 November at Sairecabur. The differences are significant--the atmospheric transmission corresponding to the first opacity quartile is improved by nearly a factor of two for all three windows (e.g. exp(-2.1) = 12.3%, versus exp(-2.7) = 6.7%). In all plots, the y-axis values refer to the black line.
The Complete FTS Dataset from Chajnantor
Since we have gathered data throughout the calendar year at the Chajnantor site, a more detailed review of the year-long opacity statistics is warranted. In the plot below, the time series of the 675 GHz opacity is presented, indicating the seasonal dependence of low opacity weather.
The cumulative distribution functions averaged over the entire dataset are presented below.