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Experimental Astronomy

, Volume 29, Issue 3, pp 207–225 | Cite as

Characterization of the atmosphere above a site for millimeter wave astronomy

  • Francesco Tony Nasir
  • Franco Buffa
  • Gian Luigi Deiana
Original Article

Abstract

The Sardinia Radio Telescope (SRT) is a challeging scientific project managed by the National Institute for Astrophysics (INAF), it is being developed at 30 km North of the city of Cagliari, Italy. The goal of the SRT project is to build a general purpose, fully steerable, 64 m diameter radio telescope, capable of operating with high efficiency in the centimeter and millimeter frequency range (0.3–100 GHz). In portions of this frequency range, especially towards the high end, astronomical observations can be heavily deteriorated by non-optimal atmospheric conditions, especially by water vapor content. The water molecule permanent electric dipole in fact, leads to pressure broadened rotational transitions around the 22.23 GHz spectral line. Furthermore, water vapor’s continuum absorption and emission may influence higher frequency observations too. To a lower degree, cloud liquid black body radiation can also affect centimeter and millimeter observations. In addition to this, inhomogeneities in water vapor distributions can cause signal phase errors which introduce a great amount of uncertainty to VLBI mode observations. The Astronomical Observatory of Cagliari (OA-CA) has obtained historical timeseries of radiosonde profiles conducted at the airport of Cagliari. Through the radiosonde measurements and an appropriate radiative transfer model, we have performed a statistical analysis of the SRT site’s atmosphere which accounts for atmospheric opacity at different frequencies, integrated water vapor (IWV), integrated liquid water (ILW) and cloud cover distributions during the year. This will help to investigate in which period of the year astronomical observations at different frequencies should be performed preferably. The results show that, at the SRT site, K-band astronomical observations are possible all year round, the median opacity at 22.23 GHz is 0.10 Np in the winter (Dec-Jan-Feb) and 0.16 Np in the summer (Jun-Jul-Aug). Integrated water vapor during winter months ranges, on average, between 7 and 15 mm. Cloud cover is usually not present for more than 36% of the time during the year. The atmospheric opacity study indicates that observations at higher frequencies (50–100 GHz) may be performed usefully: the median opacity at 100 GHz is usually below or equal to 0.2 Np in the period that ranges from January to April.

Keywords

Opacity Integrated water vapor Integrated liquid water Radiative transfer model Radiosondes Radiotelescope 

Notes

Acknowledgements

The authors are grateful to Dr. Andrea Tarchi, to Dr. Alessandro Orfei and to Dr. Alessandro Delitala for their useful suggestions. We are also thankful to Dr. Mauro Sorgente for his contributions to the data processing of the GPS receiver.

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Copyright information

© Springer Science+Business Media B.V. 2011

Authors and Affiliations

  • Francesco Tony Nasir
    • 1
  • Franco Buffa
    • 1
  • Gian Luigi Deiana
    • 1
  1. 1.Osservatorio Astronomico di CagliariCapoterra (CA)Italy

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