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

We report precipitable water vapor (PWV) measurements made at Las Campanas Observatory using optical spectra of H2O lines obtained with the Magellan echelle spectrograph, and calculated using a robust technique that is accurate to 5%-10%. Calibration of the relationship between our PWV measurements and opacity values at 225 GHz was made possible by simultaneous observations with a tipping radiometer. Based on this calibration, we present Las Campanas Observatory wintertime precipitable water vapor statistics, measured using the tipping radiometer, during a 1.5 month campaign. The median value of 2.8 +/- 0.3 mm is consistent with that measured at the nearby La Silla Observatory during the VLT site survey. We conclude that in the Southern hemisphere winter months, we can expect good conditions for infrared observing (less than or similar to 1.5 mm) approximately 10% of the time at Las Campanas Observatory.