ar X iv : a st ro - p h / 01 11 49 2 v 1 2 7 N ov 2 00 1 220 GHZ ZENITH ATMOSPHERIC TRANSPARENCY

We present 220 GHz (1.36 mm) measurements of zenith optical depth obtained to characterise the Indian Astronomical Observatory, Hanle (Ladakh, India) during the period from late December 1999 to early May 2000 and early October 2000 to September 2001. The data were sampled at an interval of 10 minutes. We describe the automated 220 GHz tipping radiometer used, its basic principle, operation, data acquisition method and data reduction scheme in detail. The 220 GHz opacity is found to be less than 0.06 for a significant fraction (40%) of the time during the winter months, indicating that Hanle is one of the good observing sites for submillimeter-wave astronomy. We make a preliminary correlation with the precipitable water vapour derived from surface relative humidity and air temperature measurements made during the same period with a weather station installed at the site. We also compare the Hanle site with other high-altitude sites like Mauna Kea and Atacama desert. keywords: atmospheric opacity, precipitable water vapour (pwv), surface water vapour pressure(Po), superheterodyne receiver, sub-mm astronomy and mm-wave astronomy ∗ Hanle : Latitude 32◦46′46′′ N; Longitude 78◦57′51′′ E; Altitude 4500 m c © 2008 Kluwer Academic Publishers. Printed in the Netherlands. radiometer.tex; 1/02/2008; 20:42; p.1 2 P.G. Ananthasubramanian et al.