Hanle Dark Sky Reserve Was Notified in December 2022 Around a 4,500-Metre Observatory Whose 2-Metre Telescope Has Run Since 2000 and Whose 21-Metre Cherenkov Telescope Was Inaugurated on 4 October 2024

The Hanle Dark Sky Reserve in eastern Ladakh was notified by the Government of Ladakh in December 2022, the Press Information Bureau (PIB) records. The Department of Science and Technology had announced it in September 2022 as India’s first “Night Sky Sanctuary”. It is an area of roughly 22 km radius around the Indian Astronomical Observatory, which the Indian Institute of Astrophysics (IIA) places at 4,500 metres above sea level.

Inside that circle stand a 2-metre optical telescope that has run since 2000, a seven-telescope gamma-ray array, and the 21-metre MACE telescope that the Department of Atomic Energy inaugurated on 4 October 2024. For visitors, 24 trained villagers lead night-sky tours with their own telescopes. Hanle is in Leh district, inside the Changthang Wildlife Sanctuary. Ladakh itself, its councils and its budget are covered in our profile of the Union Territory.

How a cluster of villages agreed to dim its lights

The IIA’s account of the reserve starts in July 2021, with the first talks with the Union Territory administration. In October 2021 a team from the observatory, the Wildlife Department and SECURE Himalayas met villagers and explained the idea. The villagers raised concerns, the IIA says, and proposed some operating principles that were taken on board. On 30 November 2021 the village elders signed the La Palma Declaration, which the IIA calls a historic milestone for the reserve. A tripartite agreement between UT Ladakh, the Ladakh Autonomous Hill Development Council in Leh and the IIA followed on 16 June 2022.

PIB quotes Union Minister Jitendra Singh on 3 September 2022 saying Hanle suited the project because it lies in a cold desert, away from human disturbance, with clear skies and dry weather. The reserve rests on two pillars, PIB’s 2025 release says: cutting light pollution, and astro-tourism that pays local people. UT Ladakh funds both. The IIA bought 24 small telescopes with that money and gave them to selected villagers, who were trained as “Astronomy Ambassadors”.

The 2-metre telescope on Digpa Ratsa Ri

The Himalayan Chandra Telescope (HCT) stands on Digpa Ratsa Ri, a hilltop that PIB gives as 4,517 metres. The IIA was named the nodal agency for finding an Indian site for optical and infrared telescopes in 1989. From 1993 its teams explored six locations in the Himalayas before choosing Hanle. The foundation stone was laid in 1997, the telescope saw first light on 26 September 2000, and it was dedicated to the nation on 29 August 2001. It is named after the Nobel laureate Subrahmanyan Chandrasekhar, whose white dwarf limit has its own article on tuput.

The IIA says the telescope was made by EOS Technologies of Tucson, Arizona. It has been operated in real time from the IIA’s campus at Hosakote near Bengaluru since June 2001, over a dedicated satellite link from ISRO, according to the institute’s director, Annapurni Subramaniam. It carries three instruments: HFOSC, which takes images and spectra in visible light, TIRSPEC, for near-infrared, and HESP, a high-resolution spectrograph.

Two gamma-ray telescopes lower down the slope

The IIA puts both gamma-ray instruments at about 4,250 metres, below the optical telescope. HAGAR, the High Altitude Gamma Ray array, is a joint project of the IIA and the Tata Institute of Fundamental Research. It has seven telescopes, one at the centre and six on a circle of 50 metres radius, with about 31 square metres of mirror in all. It is aimed at gamma rays between roughly 10 and 250 GeV, an energy range the IIA calls largely unexplored. The IIA’s page says all seven units are installed and calibration tests are under way.

MACE, the Major Atmospheric Cherenkov Experiment, was built indigenously by the Bhabha Atomic Research Centre (BARC), which is named after Homi Bhabha (his life is covered here), with support from ECIL and other Indian companies. DAE secretary Ajit Kumar Mohanty inaugurated it on 4 October 2024. DAE calls it the largest imaging Cherenkov telescope in Asia and the highest of its kind in the world, at about 4,300 metres. The team’s own paper gives 4,270 metres. It has a 21-metre diameter and 356 mirror panels. It detects gamma rays indirectly, by collecting the Cherenkov light, a faint blue flash, from the shower of particles a gamma ray sets off in the air. The paper says it has observed the Crab Nebula regularly since September 2021.

The observatory also hosts the GROWTH-India Telescope, a 0.7-metre robotic instrument run with IIT Bombay to monitor transient events such as exploding stars.

Why astronomers chose Hanle

Several measures point the same way, though they do not give the same number. The IIA’s site page counts about 260 spectroscopic nights and about 190 photometric nights a year, two grades of observing night, the photometric being the more demanding. Rain and snow total under 7 centimetres a year, the median blur of starlight (the seeing) is under 1 arc-second, and precipitable water vapour, the water in the air above the telescope, stays under 2 millimetres from October to April.

At the July 2026 conference Subramaniam spoke of more than 250 usable nights a year and under 2.5 millimetres of water vapour. A 2021 study led by Shantikumar Singh Ningombam of the IIA compared eight high-altitude sites using 21 years of ground and satellite records and 41 years of reanalysis data. PIB’s release on that study says Hanle is as dry as Chile’s Atacama Desert, with around 270 clear nights a year, and that the Himalayas keep the monsoon from interrupting HCT observing. The paper puts Hanle at 70 to 75 per cent spectroscopic nights a year, against 87 per cent for Paranal in Chile, the best site in the study. Hanle, Ali in Tibet and Devasthal near Nainital fell between 68 and 78 per cent.

The IIA’s page also compares Hanle’s longitude, 79 degrees east, with the Canary Islands at 20 degrees west and eastern Australia at 157 degrees east.

What the instruments have produced

The HCT’s first scientific result, in 2001, was a paper on the optical afterglow of a gamma-ray burst, Subramaniam said in 2026. PIB lists comet spectroscopy, studies of exoplanet atmospheres, work on novae and supernovae, and participation in the discovery of the exoplanet TRAPPIST-1b. In a 2024 paper Athira Unni, Thirupathi Sivarani and colleagues used HFOSC for the first transmission spectroscopy done with the HCT, watching one transit each of three giant planets. They report the first low-resolution transmission spectrum for KELT-18b and a confirmed signature of Rayleigh scattering in the atmosphere of WASP-127b.

HAGAR watched the blazar Mrk 421 flare to 6 or 7 times the brightness of the Crab Nebula on 17 February 2010, at energies above 250 GeV, as Amit Shukla and colleagues reported. MACE’s team analysed about 15 hours of data from November 2022 to February 2023 and detected the Crab Nebula above 80 GeV, with a spectrum reaching about 5 TeV that agrees with earlier measurements within uncertainties.

Visiting the reserve

Hanle is about 270 km east of Leh, a trip of around six hours, with a scheduled bus twice a week each way, the IIA says. It advises resting for 48 hours in Leh first, because altitude sickness can affect healthy people, and gives Hanle’s height as about 4,250 metres. Our first-visit guide to Ladakh covers permits and acclimatisation in general.

The observatory is open to visitors by day only. Visitors may enter the HCT dome area, vehicles must stop at the parking area, and there are no telescopes for visitors at the observatory. Stargazing is through the Astronomy Ambassadors, who bring 8-inch Dobsonian telescopes and charge a fee. The IIA says local homestays and other service providers have benefited as astro-tourism has grown.

The reserve’s rules ask visitors to keep lights to a minimum, use red-filtered light when needed, carry out all rubbish, keep noise down and keep a distance from wildlife. Three star parties for experienced amateurs have been held, with about 30 participants in October 2023, more than 45 in the 2024 edition and a third in September 2025. PIB reports more than 450 tourists during the 2024 party and more than 300 people at the 2025 open night. A participant quoted by UT Ladakh in 2023 described the sky as Bortle-1, the darkest class on the scale amateur astronomers use.

The light plan and the next telescopes

IIA’s light management page, updated in July 2024, records that UT Ladakh funded warm-colour bulbs, lamp shades and black curtains for every house in the reserve, and that all roads now have retro-reflectors. A ban on high-beam headlights is to apply once the plan is complete. The plan names the remaining sources of stray light: villages and the monastery, vehicle headlights, floodlights at army camps, which senior army staff are working with UT Ladakh to reduce, and tourists’ torches.

PIB reported on 29 July 2026 that a recent budget announcement included a 3.7-metre upgraded HCT and a 13.7-metre National Large Optical-infrared Telescope at Hanle, both to be led by the IIA.