Mark Megahan
Why would astronomers want to climb 8,000 feet up the Indian side of the Himalayas to put 13 gallons of mercury in a dish, then spin it? One thing that researchers at Devasthal Observatory in Nainital, Uttarakhand, can say for absolute certain is that their fancy new mirror will never get a chip in it, as happened to the James Webb Space Telescope recently. They built the “world’s first liquid-mirror telescope to be solely set up for astronomy.” It’s also “the only one of its kind to be operational anywhere in the world.”
Astronomers dizzy with results
The equipment is pulling in one record after another already. India’s first liquid-mirror telescope “also happens to be Asia’s largest telescope.” It’s mercurial eye immediately opened on a vast expanse of sky, peering out from a mile and a half up the Himalayas, benchmarked at an altitude of 2,450 meters.
Astronomers working at the Devasthal Observatory campus of the Aryabhatta Research Institute of Observational Sciences got the gear comfortably nestled in and on the job in Nainital, Uttarakhand. The International Liquid-Mirror Telescope, or ILMT for short, has “entered the commissioning phase.” After passing systems checks, “ILMT will observe asteroids, supernovae explosions, space debris, and other celestial bodies.”
The concept has been tried before but not often. “Only a handful of liquid-mirror telescopes have been previously built but were majorly used either for tracking satellites or military purposes,” Dr. Kuntal Misra, project investigator of ILMT informs.
His colleague at ARIES, Professor Dipankar Banerjee, adds that ILMT is the third telescope to set up at Devasthal, “considered as one of the best sites for astronomical observations.” The international team of astronomers will crank up full-scale operations this October, after the monsoons blow through. The other two operational devices are the 3.6-meter Devasthal Optical Telescope, and the 1.3-meter Devasthal Fast Optical Telescope.
There’s a real good reason why only a handful have ever been put into action. They’re extremely limited in what they can do. Astronomers have to rethink everything about the way they work with it.
Under those limited application ranges, it does things that ordinary lenses don’t. Experts thought that the ARIES site would be a perfect place to get the most out of the revolutionary concept.
A truly ‘revolutionary’ idea
Normal optical telescopes used by astronomers are connected to devices which compensate for the rotation of the Earth by slowly and steadily tracking the target across the night sky. It’s not easy to move a rapidly spinning dish of mercury.
“Unlike the conventional telescopes that can be steered to track specific stellar source objects, the ILMT will be stationary. It will carry out observations and imaging at the zenith, that is, of the overhead sky.” As Dr. Misra explains, “This is a survey telescope having high potential for discovering newer objects.”
They start with about 13 gallons or 50 liters of mercury. That weighs about 700 kilos or a ton-and-a-half. Without breathing the fumes, astronomers pour it into a “container that will be rotated at a fixed constant speed along the vertical axis of the ILMT.”
Centripetal force spreads the liquid metal into a thin, lens shaped layer. Instant mirror. It ends up having a four meter diameter and has “an aperture of f/2 defined by the speed of rotation.” It’s already produced it’s first color composite image.
Instead of targeting specific objects, this setup watches the exact same patch of sky night-after-night. “A large-format electronic camera located at the focus records the images. The rotation of the earth causes the images to drift across the camera, but this motion is compensated electronically by the camera.”
“This mode of operation increases observing efficiency and makes the telescope particularly sensitive to faint and diffuse objects.” Astronomers expect it to suck in “about 10 GB of data at night, which will be quickly analyzed to reveal variables and transient stellar sources.” They’re thrilled because “ILMT data will be ideally suited to perform deep photometric and astrometric variability surveys over a period of five years.“
