2007PASP..119..697T

We report precipitable water vapor (PWV) measurements made at Las Campanas Observatory using optical spectra of H₂ O 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 (≲1.5 mm) approximately 10% of the time at Las Campanas Observatory.