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What the world’s largest liquid mirror telescope means for astronomy

Updated: Dec 9, 2022

The International Liquid Mirror Telescope, located in Uttarakhand, India, high in the Himalayas, started making observations in August this year.

The telescope, commissioned at Devasthal, a hill in Uttarakhand, will help in surveying the sky making it possible to observe several galaxies and other astronomical sources just by staring at the strip of sky that passes overhead. It is the first liquid mirror telescope in the country and the largest in Asia.

Built by astronomers from India, Belgium and Canada, the novel instrument employs a 4-meter-diameter rotating mirror made up of a thin film of liquid mercury to collect and focus light. It is located at an altitude of 2450 metres at the Devasthal Observatory campus of Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institute under the Department of Science and Technology (DST), Govt. of India in Nainital district, Uttarakhand.

Such telescopes have benefits over conventional ones. Most importantly, they are much cheaper to build. But although the idea of a liquid telescope has been around for centuries, creating a viable one has proven fiendishly tricky. The ILMT was in the works for more than a decade. This year, it opened its eye for the first time. It is the largest of its kind, and the first built to carry out astronomical observations.

The telescope scans the night sky in the hope of spotting new phenomena – when it isn’t raining, that is.

Observing the same patch of the sky also has its advantages, especially in detecting transient objects. Scientists can look for changes by subtracting images taken on different nights.

“ILMT will generate a huge 10–15 GB of data nightly. So, advanced computational tools, artificial intelligence, and machine learning will be implemented to classify space objects,” Kuntal adds.

When it does discover objects, the steerable 3.6-meter Devasthal Optical Telescope next door will be able to take a quick follow-up observation.

Liquid mirrors have a long but mixed record in astronomy. Over 300 years ago, Isaac Newton noted that a liquid in a rotating container would take on the shape of a parabola — precisely the shape needed by a telescope mirror to focus light to a single point. In 1850, Italian astronomer Ernesto Capocci further conceptualized this idea, but couldn’t build a working model.

During the rest of that decade, London-born astronomer Henry Skey investigated the concept independently and experimented with building one. He emigrated to New Zealand in 1860 and published an account of a working liquid-mirror telescope in 1872.

In the early 20th century, Robert Wood, a physicist at Johns Hopkins University, played a pivotal role by constructing LMTs of different sizes to observe astronomical objects passing over the zenith (the point in the sky directly overhead). But despite his best attempts, the technology was still not precise.

ILMT uses shiny mercury in liquid form to collect and focus light. Mercury has strong reflective power and stays in a liquid form at room temperature. And it’s much cheaper than highly prized glass mirrors. Grinding mirrors into a parabolic shape is an arduous and expensive task. The total cost of ILMT comes in at $2 million, while a conventional solid-mirror telescope of its size could reach hundreds of millions.

“One problem is that mercury is hazardous to humans, so proper care needs to be taken,” says Kuntal Misra, Project Investigator of ILMT at the Aryabhatta Research Institute of Observational Sciences (ARIES), which operates Devasthal Observatory and is located in Nainital.

13.2 gallons (50 litres) of liquid weighing 1,540 pounds (700 kg) have been used to create a 0.14-inch-thick (3.5 millimetres) layer in a bowl that slowly spins every eight seconds via motors. As a result, the liquid takes a parabolic shape under the influence of gravity and centrifugal force — that’s what Newton stated.

The liquid surface must be smooth and rotate at a constant speed, as any distortions could lead to warped images. To avoid deformities, the mercury is protected on both sides. On top, a thin mylar sheet protects the liquid from the wind; from the bottom, it sits on an air-bearing system — a 10-micron-thick cushion of compressed air (a human hair is 70 microns). It is so delicate that even smoke particles can harm its performance.

The future of liquid mirrors

LMTs could play an expanded role as the current era of space exploration picks up. With their lightweight and simple design, astronauts could easily deploy one on the Moon, where there is no atmosphere to get in the way of observations.

CAA Team


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