Calibration of the Relationship between Precipitable Water Vapor and 225 GHz Atmospheric Opacity via Optical Echelle Spectroscopy at Las Campanas Observatory

TL;DR

This study calibrates the relationship between optical spectroscopy measurements of water vapor and 225 GHz atmospheric opacity at Las Campanas Observatory, providing winter-time PWV statistics and assessing observing conditions.

Contribution

It introduces a robust method to accurately measure PWV using optical spectra and calibrates it against 225 GHz opacity, enabling reliable atmospheric assessments.

Findings

Median PWV during winter is 2.8 mm.

Good IR observing conditions (<1.5 mm PWV) occur about 10% of the time.

PWV measurements are consistent with nearby La Silla Observatory.

Abstract

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 tipping radiometer observations. Based on this calibration, we present Las Campanas Observatory winter-time precipitable water vapor statistics, measured using the tipping radiometer, during a two 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.