Giant Magellan Telescope site testing: PWV statistics and calibration
2010
Cerro Las Campanas located at Las Campanas Observatory (LCO) in Chile has been selected as the site for the Giant
Magellan Telescope. We report results obtained since the commencement, in 2005, of a systematic site testing survey of
potential GMT sites at LCO. Atmospheric precipitable water vapor (PWV) adversely impacts mid-IR astronomy
through reduced transparency and increased background. Prior to the GMT site testing effort, little was known regarding
the PWV characteristics at LCO and therefore, a multi-pronged approach was used to ensure the determination of the
fraction of the time suitable for mid-IR observations. High time resolution monitoring was achieved with an Infrared
Radiometer for Millimeter Astronomy (IRMA) from the University of Lethbridge deployed at LCO since September of
2007. Absolute calibrations via the robust Brault method (described in Thomas-Osip et al . 1 ) are provided by the
Magellan Inamori Kyocera Echelle (MIKE), mounted on the Clay 6.5-m telescope on a timescale of several per month.
We find that conditions suitable for mid-IR astronomy (PWV < 1.5 mm) are concentrated in the southern winter and
spring months. Nearly 40% of clear time during these seasons have PWV < 1.5mm. Approximately 10% of these nights
meet our PWV requirement for the entire night.
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