Indian Astronomical Observatory, Hanle

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. Detailed characterization of the site began in January 1995, and has continued till date.

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 square 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 altitude of 4500 meters above msl.

The background of the early 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. The extinction and night sky brightness estimates based on data obtained since the 2m HCT was released for observations in 2003 are published in Bull. Astr. Soc. India, 36, 111-127, 2008.

  • Site Characteristics
  • Latitude

    32d46m46s N


    78d57m51s E


    4500 meters above msl



    Wind Speed

    Median 2.2 m/s (8 kmph) at night

    Wind Direction

    Prevailing south-south-easterly

    Low ambient temperature and very low humidity

    Annual precipitation of rain & snow

    < 7 cm

    Precipitable water vapour

    < 2 mm between October and April

    Number of spectroscopic nights

    ~ 260 per year

    Number of photometric nights

    ~ 190 per year

    Median seeing

    < 1 arcsec


    Uniform distribution of useful nights round the year

    Longitudinal advantage

    (79d E) Canary Islands (20d W); Eastern Australia(157d E)

    Good accessibility round the year

  • Extinction and Sky Brightness
  • Mean Extinction Values (2003-2008)


    0.36 +/- 0.07


    0.21 +/- 0.04


    0.12 +/- 0.04


    0.09 +/- 0.04


    0.05 +/- 0.03

    Mean (moonless) night sky brightness (2003-2008)


    22.14 +/- 0.32


    22.42 +/- 0.30


    21.28 +/- 0.20


    20.54 +/- 0.37


    18.86 +/- 0.35

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Last updated on: November 20, 2020