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Proceedings Paper

Support for site testing of the European Extremely Large Telescope: precipitable water vapor over Paranal
Author(s): Florian Kerber; Richard R. Querel; Reinhard W. Hanuschik; Arlette Chacón; Marta Caneo; Lissette Cortes; Michel Cure; Lizett Illanes; David A. Naylor; Alain Smette; Marc Sarazin; David Rabanus; Gregory Tompkins
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Paper Abstract

In support of characterization of potential sites for the European Extremely Large Telescope (E-ELT) the European Southern Observatory (ESO), the Institute for Space Imaging Science (ISIS) and the astrometeorology group of the Universidad Valparaiso have jointly established an improved understanding of atmospheric precipitable water vapour (PWV) above ESO's La Silla Paranal Observatory. In a first step, 8 years worth of high resolution near-IR spectra taken with VLT-UVES have been statistically analysed to reconstruct the PWV history above Paranal. To this end a radiative transfer model of Earth's atmosphere (BTRAM) developed by ISIS has been used. A median PWV of 2.1 mm is found for Paranal based on UVES data covering the period 2001-2008. Furthermore we conclude that Paranal can serve as a reference site for Northern Chile due to the stable atmospheric conditions in the region. The median offset between Paranal and Armazones is derived to be 0.3 mm, but local arbitrary variations of a few tenths of a mm between the sites have been found by measurement. In order to better understand the systematics involved two dedicated campaigns were conducted in August and November 2009. Several methods for determining the water column were employed, including radiosonde launches, continuous measurements by infrared radiometer, and VLT instruments operating at various wavelengths: CRIRES, UVES, VISIR and X-shooter. In a first for astronomical instruments all methods have been evaluated with respect to the radiosondes, the established standard in atmospheric research. Agreement between the radiosondes and the IR radiometer (IRMA) is excellent while all other astronomical methods covering a wavelength range from 700 - 20000 nm have also been successfully validated in a quantitative manner. All available observations were compared to satellite estimates of water vapour above the observatory in an attempt to ground-truth the satellite data. GOES can successfully be used for site evaluation in a purely statistical approach since agreement with the radiosondes is very good on average. For use as an operational tool at an observatory GOES data are much less suited because of significant deviations depending on atmospheric conditions. We propose to routinely monitor PWV at the VLT and to use it as an operational constraint to guide scheduling of IR observations at Paranal. For the E-ELT we find that a stand-alone high time resolution PWV monitor will be essential for optimizing the scientific output.

Paper Details

Date Published: 5 August 2010
PDF: 12 pages
Proc. SPIE 7733, Ground-based and Airborne Telescopes III, 77331M (5 August 2010); doi: 10.1117/12.856390
Show Author Affiliations
Florian Kerber, European Organisation for Astronomical Research in the Southern Hemisphere (Germany)
Richard R. Querel, Univ. of Lethbridge (Canada)
Reinhard W. Hanuschik, European Organisation for Astronomical Research in the Southern Hemisphere (Germany)
Arlette Chacón, Univ. de Valparaíso (Chile)
Marta Caneo, Univ. de Valparaíso (Chile)
Lissette Cortes, Univ. de Valparaíso (Chile)
Michel Cure, Univ. de Valparaíso (Chile)
Lizett Illanes, Univ. de Valparaíso (Chile)
David A. Naylor, Univ. of Lethbridge (Canada)
Alain Smette, European Organisation for Astronomical Research in the Southern Hemisphere (Chile)
Marc Sarazin, European Organisation for Astronomical Research in the Southern Hemisphere (Germany)
David Rabanus, European Organisation for Astronomical Research in the Southern Hemisphere (Chile)
Gregory Tompkins, Univ. of Lethbridge (Canada)

Published in SPIE Proceedings Vol. 7733:
Ground-based and Airborne Telescopes III
Larry M. Stepp; Roberto Gilmozzi; Helen J. Hall, Editor(s)

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