Calibration of the relationship between precipitable water vapor and 225 GHz atmospheric opacity via optical echelle Spectroscopy at Las Campanas Observatory

We report on the calibration of the relationship between precipitable water 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 [21 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 13]. Furthermore, in the Southern hemisphere winter months, we can expect good conditions for infrared observing (less than or similar to 1.5 mm) at the tenth percentile level. Further details can be found in Thomas-Osip et al. [4].