We report on the calibration of the relationship between precipitable water vapor vapor and opacity at 225 GHz at Las Campanas Observatory as measured by a Tipping Radiometer. This relationship is a function of altitude and temperature and thus is highly dependent on location. We determine the relationship applicable at Las Campanas Observatory by using high-resolution Magellan Echelle spectra to measure the precipitable water vapor independently and absolutely. Temperature insensitive (between 220-300 K) lines allow the use of a single temperature atmospheric model as long as the lines are unsaturated. Absolute calibration was achieved by measuring the humidity in the path length of the McMath Solar telescope with a psycrometer [1]. We have expanded the method presented by Brault et al. (1975) with improved partition functions and additional lines. Based on this calibration, we present Southern hemisphere winter-time precipitable water vapor statistics for Las Campanas Observatory as measured during a two month campaign. We find that the median winter value of 2.8 ± 0.3 mm is consistent with that measured at the nearby La Silla Observatory during the VLT site survey [2] and inconsistent (lower by a factor of approximately two) with estimates, also for La Silla, derived from GOES-8 satellite imagery and the European Centre for Medium-Range Weather Forecasting (ECMWF) meteorological numerical model [3]. Furthermore, in the Southern hemisphere winter months, we can expect good conditions for infrared observing (≲1.5 mm) at the tenth percentile level. Further details can be found in Thomas-Osip et al. [4]. © 2008 Springer-Verlag Berlin Heidelberg.
CITATION STYLE
Thomas-Osip, J., Mcwilliam, A., Phillips, M., & Osip, D. (2008). Calibration of the relationship between precipitable water vapor and 225 GHz atmospheric opacity via optical echelle spectroscopy at Las Campanas observatory. In ESO Astrophysics Symposia (Vol. 2008, p. 447). https://doi.org/10.1007/978-3-540-76963-7_61
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