New Delhi, Sep 30 The advantages of more clear nights, minimal light pollution, background aerosol concentration, extremely dry atmospheric condition, and no interruption by monsoon are amongst the reasons that have made the Indian Astronomical Observatory (IAO) at Hanle in Ladakh as one of the promising observatory sites globally, as per a recent study.
Researchers from India and their collaborators carried out a detailed study of the night-time cloud cover fraction over eight high-altitude observatories, including three in India. They used re-analysis data, combined from assimilation and observation extending over 41 years, along with 21 years of data from satellites, a Science and Technology Ministry statement said on Thursday.
The study classified the quality of observable nights for different astronomical usages like photometry and spectroscopy on a daily basis. They analysed datasets for the Indian Astronomical Observatory (IAO) in Hanle and Merak (Ladakh), and Devasthal (Nainital) in India, Ali Observatory in the Tibet Autonomous Region in China, South African Large Telescope in South Africa, University of Tokyo Atacama Observatory and Paranal in Chile, and the National Astronomical Observatory in Mexico.
The team found that the Hanle site is as dry as Atacama Desert in Chile and much drier than Devasthal, has around 270 clear nights in a year, and is also one of the emerging sites for infrared and sub-mm optical astronomy. "This is because water vapour absorbs electromagnetic signals and reduces their strength," the release said.
The research led by Dr Shantikumar Singh Ningombam of Indian Institute of Astrophysics (IIA), Bengaluru, and scientists from Aryabhatta Research Institute of Observational Sciences (ARIES) in Nainital, and collaborators from St Joseph's College, Bengaluru, and the National Institute of Meteorological Sciences, South Korea, University of Colorado and Chemical Sciences Laboratory, NOAA, US has been published in the Monthly Notices for Royal Astronomical Society (MNRAS), it said.
"In order to plan for future observatories, such detailed analysis of data from multiple sites over many years and the prediction of their variation with time are crucial," said Ningombam.
Researchers found Paranal, located in a high-altitude desert in Chile, to be the best site in terms of clear skies with around 87 per cent clear nights in a year. IAO-Hanle, and Ali observatories, which are located around 80 km from each other, are similar to each other in terms of clear night skies. Devasthal has a slightly larger number of clear nights compared to the other sites in the sub-continent but is affected by monsoons for about three months in a year. However, night observations at IAO-Hanle from 2m-Himalayan Chandra Telescope (HCT) are possible throughout the year without any interruption due to monsoons.
"Due to the advantages of more clear nights, minimal light pollution, background aerosol concentration, extremely dry atmospheric conditions, and (the place being) uninterrupted by monsoon, this region is becoming one of the promising sites globally for the next generation of astronomical observatories," the study said.
On the other hand, the cloud cover fraction for Hanle, Merak, and Devasthal in India and Ali in China were 66-75 per cent, 51-68 per cent, 61-78 per cent, and 61-75 per cent, respectively at various time resolutions using satellite-era and reanalysis data.
After examining several years of data of various Astro-climatological parameters, IIA had installed the 2-metre aperture Himalayan Chandra Telescope (HCT) at Indian Astronomical Observatory (IAO), Hanle, during 2000. Thereafter, due to the uniqueness of this site, several astronomical telescopes operating at optical and infrared wavebands have been installed at Hanle by several Institutes in the country.
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