Explained: World's Largest Liquid Mirror Telescope, Exclusively For Astronomy, Inaugurated In India

A new telescope to help identify transient or variable objects or phenomena was launched in Uttarakhand on 21 March.

The International Liquid Mirror Telescope (ILMT) is stationed about 50 kilometres (km) from Nainital city, at the Devasthal Observatory campus of the Aryabhatta Research Institute of Observational Sciences (ARIES).

ARIES is an autonomous institution under the Department of Science and Technology, Government of India. Its Devasthal site already hosts the 1.3 m and 3.6 m aperture telescopes, and now the 4 m ILMT has made quite the powerful addition.

The ILMT now becomes the largest-aperture telescope available in the country, and is also the first optical survey telescope in India.

Incidentally, the second-largest aperture telescope is also with ARIES — the 3.6 m optical telescope.

India, Belgium, Canada, Poland, and Uzbekistan are the collaborators on the ILMT project.

The funding, estimated to be about Rs 30-40 crore, came from Canada and Belgium. The operations and upkeep of the telescope is India’s responsibility.

The telescope was designed and built by the Advanced Mechanical and Optical Systems Corporation and the Centre Spatial de Liège in Belgium.

“ILMT is the first liquid-mirror telescope designed exclusively for astronomical observations,” Professor Dipankar Banerjee, Director of ARIES, has said.

The ILMT uses a 4-m-diameter rotating mirror made up of a thin film of liquid mercury to collect and focus light.

“It is designed to survey the strip of sky passing overhead each night, allowing it to identify transient or variable objects such as supernovae, gravitational lenses, space debris, asteroids, etc,” ARIES says.

The telescope uses liquid-mirror technology; the primary mirror of the telescope is a rotating container with a highly-reflective liquid in it. In the case of the ILMT, that liquid is mercury.

At room temperature, mercury is in liquid form and also highly reflective, thus making it a good choice.

The basis for the ILMT is simple: when a liquid is poured into a container and spun, it assumes a parabolic shape. “In the ILMT, this principle is used to form a primary mirror using liquid mercury,” ARIES explains.

As for getting clear images, the time-delay integration (TDI) technique helps out with that. ARIES says clear images are formed by “matching the parallel charge transfer rate of the CCD camera to the rotation of the earth.”

A “CCD camera” is any type of digital camera with a charge-coupled device (CCD).

The International Liquid Mirror Telescope scans a strip of width 22 arcmin in the sky at zenith. The size of the full moon, for comparison, is 31 arcmin. “Per night, the telescope has access to around 47 sq. degree of the sky,” ARIES says. 

Unlike a conventional telescope, which can be made to point in different directions, the ILMT just points upwards, to the zenith, constrained by the use of liquid mercury. The tracking is done artificially by using the TDI technique.

Until 1996, several liquid-mirror telescopes were used for Lidar (light detection and ranging) applications, aimed at the study of Earth’s atmosphere.

A conference on liquid mirrors in 1997 led to the idea of a combined effort to build a 4 m international liquid-mirror telescope entirely dedicated to astronomical observations.

Belgium, Canada, England, and France initially came together for the project, but budgetary constraints saw England and France pull out.

India later joined the effort, through ARIES, and eventually became a full partner on the ILMT project in 2010. Poland and Uzbekistan, too, later got on board.

Thus, the idea from 1997 has finally materialised in Uttarakhand, India, in 2022-23.

Some of the stated science goals of the ILMT are: statistical determination of the cosmological parameters by surveying quasars and supernovae, detection of near-Earth objects and space debris, and detection of faint extended objects, like low-surface-brightness and star-forming galaxies, galactic nebulae, and supernovae.

It is also the goal of the project to create a unique database that can be data-mined for various science outputs, with, perhaps, machine learning and artificial intelligence coming into play in a big way, and testing the next generation of image processing algorithms.

The ILMT achieved ‘first light’ on 29 April 2022. ‘First light’ refers to the moment a new telescope opens its eyes to the universe, or when it makes its first observation.

The colour-composite image, the ILMT’s first, featured the NGC 4274 galaxy, among other objects, prominently.

The plan is for ILMT to be used for ​continuous zenith ​monitoring ​within ​a ​narrow ​27 ​arcminutes ​strip ​of ​sky ​for ​at ​least ​five ​years.

About 10-15 GB of telescope data is expected every night after the commencement of regular science operations.

And the data “will be quickly analyzed to reveal variable and transient stellar sources,” according to Dr Brajesh Kumar, the ILMT project scientist at ARIES.

The existing 3.6 m optical telescope will get to work quickly to make follow-up observations of the newly detected transient sources using ILMT.

The first commissioning phase of the telescope in October-November 2022 already led to multispectral band observations. 

“Precise astrometric and photometric calibration of the data has been accomplished and hundreds of known asteroids, nebulae, planetary nebulae, distant galaxies, stellar clusters have already been identified,” the ILMT website states.

“There is no doubt that the ILMT constitutes a unique survey instrument that will lead soon to new discoveries,” it adds.