Water vapor in the atmosphere is the main obstacle in astronomical observations at millimeter and submillimeter wavelengths, degrading the sensitivity of the telescope. Because the water vapor is largely confined to the lower atmosphere, very dry air can be found at sites of high altitude. The Puna
region with altitudes above 4000 m meets the conditions favorable for this kind of astronomical observations.
During the past nine years the IAR has been carried out a series of almost uninterrupted monitoring campaigns to measure the transparency of the atmosphere in two locations in the northwestern region of Argentina, in Salta
province. These sites are Tolar Grande in Macón Mountains and Alto Chorrillos. In parallel, we analyzed and rejected sites near the towns of Mina Aguilar
(Province of Jujuy
). The instrument used for the measurements of transparency is a radiometer that operates at a frequency of 210 GHz ("tipper"
) provided by the Universidad Autónoma de México
(UNAM). The daily-base obtained data
are transmitted through a microwave link to San Antonio de los Cobres (SAC) and then from there via the Internet to IAR.
The site finally selected to install LLAMA es the region of Alto Chorrillos
. This site is about 4820 meters above sea level and about 16 km away from the town of San Antonio de los Cobres [(longitude λ, latitude φ) = (66º 28' 29.4" (W), -24º 11' 31.4"(S))], (see Figure 1). The data obtained so far indicate that the characteristics of this site are suitable for the installation of LLAMA. On the contrary, strong winds in the region of Tolar Grande at 4604 meters above sea level can distort the surface of the antenna and increase pointing errors.
|Figure 1: Google image showing the relative location of Alto Chorrillos from the ALMA interferometer. The distance between LLAMA and ALMA are indicated. The black line indicates the trace of the National Route RN 51.
|Figure 2: Opacity distribution function for autumn and winter obtained for Alto Chorrillos and Tolar Grande.
Alto Chorrillos experiences seasonal changes, especially the summer season is wetter which degrades the atmospheric opacity. Such degradation stems from the influence of the phenomenon known as "Bolivian winter", which translates into an increase in the frequency of rainfall for that season. This phenomenon affects the entire region, including the region of Chajnantor in northern Chile, site of the ALMA telescopes. However, even in that period, Alto Chorrillos remains potentially eligible for observation in the millimeter range of the spectrum, though not in the submillimeter.
The high altitude needed for this kind of astronomy observation, has its drawback in the lack of oxygen, which will require providing special equipped environments. For this reason, staff authorized to perform tasks on the site shall do so under strict security rules in order to prevent the symptoms of sickness. For the same reason, dormitories, laboratories and offices shall be installed at a lower altitude (city of Salta and / or San Antonio de los Cobres) and operation of the telescope should be preferably remotely.