INDIAN ASTRONOMICAL OBSERVATORY SITE
Digpa-ratsa Ri, Hanle, was chosen as the prospective site for a National Observatory after a study of meteorological conditions over the Indian subcontinent, study of topographic maps of high-altitude areas in the Himalayan and trans-Himalayan regions, and a simultaneous reconnaisance survey of six candidate sites in September 1993. Further visits were made by scientists and engineers of the Institute in January and June 1994. The permanent site survey camp was established at the edge of Nilamkhul Plain, due north of Digpa-ratsa Ri in 1994 December. There has been a continuous presence of personnel belonging to the Indian Institute of Astrophysics at the site since then.
The highest peak in Digpa-ratsa Ri is at an altitude of 4517 meters, and has been renamed Mt. Saraswati after the Hindu Goddess of learning. The surrounding Nilamkhul Plain is at an altitude of 4240 meters above msl. The range measures 2 km east-west and 1 km north-south with the top providing about half sqaure km of flat area. The peak contains a few rocky mounds which have been levelled by a few meters. The location of the 2-m Himalayan Chandra Telescope (HCT) is to the east of the peak at an i altitude of 4500 meters above msl.
Characterization of the site for astronomy has been going on since January 1995. The site characterization activity includes hourly monitoring of cloud cover, ambient temperature and relative humidity, wind speed and wind direction, solar radiation and rainfall. Occasional measurements of seeing, sky brightness, extinction and atmospheric water vapour have also been undertaken.
The background of the site survey and results are published in Bull. Astr. Soc. India, 24, 859-869, 1996, and Proc. XIX Meeting of the ASI. 1999 Feb 1-4.
Site Characteristics :
Extinction and Sky Brightness
Mean Extinction Values (2000 December - 2003 January) :
Mean night sky brightness (1995 & 2002) :