The World’s Highest Telescope at Hanle

hanle sky

In the north-eastern Himalayas, Hanle is a small hamlet in Changthang Ladakh near the border with Tibet. Snowbound peaks provide a majestic background to the vast Nilamkhul Plain. The eerie silence in the virtually empty place under the dark blue sky is forbidding. The sky turns blackish blue at increasing altitudes.

Those who stay up there for some days could spot an occasional wild yak, antelope and gazelle, sheep called Nyan and wild ass known as Kyang, besides jackals and hares, thought the famous snow leopard has yet to appear. As the Sun sets, even the rare black-necked Tibetan crane merges with the blackest of black in the sky, which begins to sparkle with its star-studded brilliance. One can see the stars of the sixth magnitude and beyond with naked eye, as haze and aerosols are absent. A fort-cum-monastery on a hill-top near Hanle overlooks a vast plain dotted with a few hamlets like Khuldo, Naga, Pugh and Pungok—all within a radius of ten kilometers.

In the center of the plain is a peak, which was until recently known as Digpa-ratsa Ri or Scorpion Mount, 4,517 metres above mean sea level. A little farther to the east of the peak, some four kilometers away from the monastery, is the Indian Astronomical Observatory (IAO) at Hanle, designed to probe cosmic wonders. The world’s highest ground-based optical stellar telescope of the IAO is on a flat land at 4,500 meters above sea level. But our measurement is for the record. Indeed, the mountain and the sky above seem to measure us and our knowledge.

In India’s scientific quest after Independence, the search for an astronomical observatory site makes fascinating story of a few dedicated scientists fighting their way against all odds to get the best facility in the field for their country. In the 1970s, Vainu Bappu undertook a site survey for locating a new telescope, as the existing telescope at Kodaikanal were limited by the country’s two monsoons in a year. He soon found out that Ladakh would offer much better advantages than any other place south of the Himalayas, but he had to settle for Kavalur in South India for reasons of logistics and infrastructure. After Bappu passed away in 1982, the site search was renewed.

Even as the telescope at Kodaikanal, Nainital, Mt. Abu and Kavalur continued to serve the Indian astronomical community, attempts were made to find the best Himalayan sites for future development. In the late 1980s a committee, chaired by the well-known scientist, Prof. B.V. Sreekantan, recommended a national large optical telescope to be taken up as a priority project

The search tool off in 1992 with the participation of all national centres of astronomy in the search under the leadership of the late Professor Arvind Bhatnagar, a leading astronomer. However, the surveyors concentrated only around Leh. It was at this time, the Indian Institute of Astrophysics, under the active leadership of its Director, Professor Ramanath Cowsik, succeeded in finding a site at Hanle. The choice was made after a sustained and highly scientific search evaluation. In fact, a team of 36 memebers of IIA undertook simultaneous visits to six sits: Tso Moriri and Hanle in Ladakh, Kaza, Lamdal and Kalpa in Himachal Pradesh and Kedarnath in Uttaranchal

IIA made the best use of all available data. The India Meteorological Department had accumulated climate data for well over a century. The Surveys of India’s topographical data were scanned to identify accessible high-altitude peaks in Uttar Pradesh, Himachal Pradesh and Jammu & Kashmir. Satellite imagery was also evaluated. The data indicated that the best sites would be found high up in the Himalayan region.

The region south of the Himalayas has a complex terrain, prone to frequent landslides and turbulent atmosphere. It was found that the cloudiness and precipitation decrease, as one proceeds northward. The driest sites with cloud-free skies are in Changthang, Ladakh region of Jammu & Kashmir. The intervening peaks of Greater Himalayas intercept most of the rain-bearing clouds. A place with minimum atmospheric turbulence and incidence of clouds was found to be in the vast plain near Hanle Monastery in the Digpa-ratsa Ri range of mountains. The region was also easily accessible from Leh all round the year, since snowfall in negligible in Ladakh. And Leh is linked to Delhi by a regular air service, though surface access in limited to summer months.

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