Worldwide site comparison for submillimetre astronomy

Abstract

Aims. The most important limitation for ground-based submillimetre (submm) astronomy is the broad-band absorption of the total water vapour in the atmosphere above an observation site, often expressed as the precipitable water vapour (PWV). A long-term statistic on the PWV is thus mandatory to characterize the quality of an existing or potential site for observational submm-astronomy. In this study we present a three-year statistic (2008)(2009)(2010) of the PWV for ground-based telescope sites all around the world and for stratospheric altitudes relevant for SOFIA (Stratospheric Observatory for Far-Infrared Astronomy). The submm-transmission is calculated for typical PWVs using an atmospheric model. Methods. We used data from IASI (Infrared Atmospheric Sounding Interferometer) on the Metop-A satellite to retrieve water vapour profiles for each site (11 in total, comprising Antarctica, Chile, Mauna Kea, Greenland, Tibet). The use of a single instrument to make the comparison provides unbiased data with a common calibration method. The profiles are integrated above the mountain/stratospheric altitude to get an estimation of the PWV. We then applied the atmospheric model MOLIERE (Microwave Observation and LIne Estimation and REtrieval) to compute the corresponding atmospheric absorption for wavelengths between 150 μm and 3 mm. Results. We present the absolute PWV values for each site sorted by year and time percentage. The PWV corresponding to the first decile (10%) and the quartiles (25%, 50%, 75%) are calculated and transmission curves between 150 μm and 3 mm for these values are shown. The Antarctic and South-American sites present very good conditions for submillimetre astronomy. The 350 μm and 450 μm atmospheric windows are open all year long, whereas the 200 μm atmospheric window opens reasonably for 25% of the time in Antarctica and the extremely high-altitude sites in Chile. Potential interesting new facilities are Macon in Argentina and Summit in Greenland, which show similar conditions to for example, Mauna Kea (Hawaii). For SOFIA, we present transmission curves for different altitudes (11 to 14 km), PWV values, and higher frequencies (up to 5 THz) in more detail. Though the atmosphere at these altitude is generally very transparent, the absorption at very high frequencies becomes more important, partly caused by minor species. The method presented in this paper could identify sites on Earth, with great potential for submillimetre astronomy, and guide future site testing campaigns in situ.

Estimating and forecasting the precipitable water vapor from GOES satellite data at high altitude sites

“…The agreement is not as good on days with large PWV values, since large negative biases (GOES/GFS -APEX) are shown. Tremblin et al (2012) also show large negative PWV biases during large PWV episodes in Chajnantor in September 2009 from Infrared Atmospheric Sounding Interferometer (IASI) polar satellite data (see Appendix A in Tremblin et al 2012). On the other hand, Tremblin et al (2011) found larger but positive biases between PWV values obtained from IASI satellite data and ground-based PWV observations at Dome C, in the South Pole.…”

“…A number of site testing studies have been conducted to find good places for submillimeter/millimeter astronomy (Peterson et al 2003;Yang et al 2010;Radford et al 2011;Tremblin et al 2011Tremblin et al , 2012, and they have identified Mauna Kea in Hawaii, the Chajnantor plateau in the north of Chile, and some regions in the south pole as very good sites.…”

“…Kurlandczyk & Sarazin (2007) used Medium Resolution Imaging Spectrometer (MERIS) data onboard the polar Environmental Satellite (Envisat) to estimate the cloud cover and PWV to select the best sites for the installation of the European Extremely Large Telescope (E-ELT). Yang et al (2010) and Tremblin et al (2011Tremblin et al ( , 2012 used polar satellite data for comparative studies among different sites to conduct submillimeter/millimeter astronomy. Nevertheless, the main drawback of estimating the PWV from polar satellites is their low temporal resolution since they pass every two or three days over a specific spot on Earth at almost the same hour of the day.…”

A worldwide comparison of the best sites for submillimetre astronomy

“…Schneider et al 2009;Tremblin et al 2011;Matsushita et al 1999;Peterson et al 2003) already showed that a few sites are well-suited for submm, mid-IR, and FIR astronomy and their transmission properties are rather well determined (for example by Fourier Transform Spectrometer obervations in the 0.5-1.6 THz range at Mauna Kea/Hawaii (Pardo et al 2001)). The high-altitude ( 5000 m) Chilean sites are known for dry conditions (see Matsushita et al 1999;Peterson et al 2003), and site testing is now carried out at the driest place on Earth, Antarctica (see Chamberlin et al 1997;Yang et al 2010;Tremblin et al 2011). Comparisons between Antarctic and Chilean sites are difficult and uncertain since they rely on ground-based instruments that use different methods and calibration techniques (see Peterson et al 2003, for example).…”

“…A meaningful comparison is possible if several independent instruments are used at each place. An example of such a study is the one of Tremblin et al (2011) that obtained transmission data at Dome C thanks to radio-soundings and the radiometers HAMSTRAD (Ricaud et al 2010) and SUMMIT08. However, it is rare to have many instruments at one site.…”