Instead of a glass mirror, this telescope uses a spinning pan of liquid mercur

Unique liquid-mirror telescope comes online in India

Instead of a glass mirror, this telescope uses a spinning pan of liquid mercury.

Three observatories sit across a tree-covered mountain, with peaks stretching into the distance.

India Ministry of Science & Technology

At 8,040 feet above sea level (2,450 meters) in Uttarakhand, India, lies a prime site for astronomical observations — Devasthal, 31 (52 kilometers) miles east of the resort town Nainital. Surrounded by the scenic beauty of the lofty Himalayas, Devasthal Observatory already hosts two research-grade optical telescopes.

Now, a novel instrument has joined the race to unravel cosmic mysteries — the International Liquid Mirror Telescope (ILMT), which uses a rotating pan of liquid mercury as its primary mirror, not a solid sheet of polished glass.

ILMT is the first telescope of its kind for India, the largest in Asia, and made solely for astronomical surveys. Although the idea of a liquid mirror is not new, no modern instrument has ever been constructed in a location as suited for astronomy as Devasthal.

“The site has a very dark sky and a good number of clear nights,” says Paul Hickson, an astronomer at the University of British Columbia (UBC) in Vancouver who has worked on other liquid mirror telescopes and visited Devasthal on multiple occasions.

An old idea

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 and plagued by vibrations.

Eventually, LMTs took a back seat as solid-mirror technology advanced. Then, in the 1980s, scientists began to resurrect the technology, addressing its limitations with modern technology. From 1994 to 2002, NASA operated a 3-meter LMT to scan Earth’s orbit for space debris. Later, UBC reused some parts to construct the 6-meter Large Zenith Telescope — the largest of its kind. However, the weather at its site was not ideal for astronomy and it was decommissioned in 2016.

Today, the concept may be poised for a mainstream resurgence. “In 1997, a consortium of astronomers interested in the 4-meter-wide ILMT was formed. But construction took almost 25 years due to liquid requirements and other delays,” says Jean Surdej, ILMT’s project director. India, Belgium, Canada, Poland and Uzbekistan did the work of telescope construction.

A mercurial mirror

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 liters) of liquid weighing 1,540 pounds (700 kg) have been used to create a 0.14-inch-thick (3.5 millimeters) 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 wind; from the bottom, it sits on an air bearing system — a 10-micron-thick cushion of compressed air (human hair is 70 microns). It is so delicate that even smoke particles can harm its performance.

Observing the zenith

Because the shape of a liquid-mirror telescope depends on gravity, it can only point straight up at the zenith of the sky. However, this is not as much of a disadvantage as it might appear, as the zenith slews across the night sky with Earth’s rotation. “Over the year, the telescope can observe nearly 120 square degrees of sky — 600 times the area of the Full Moon. This area corresponds to about 1 percent of the entire sky and is large enough to contain thousands of interesting objects,” explains Hickson, an astronomer at the University of British Columbia (UBC).

Those objects could range from supernovae explosions, luminous quasars, elusive stars, and gravitational lenses to solar system objects like asteroids, comets and even space debris.

Observing the same patch of 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.

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.

Researchers at the University of Texas in Austin have even proposed installing a Ultimately Large Telescope with a 100-meter liquid mirror on the Moon. Like to the James Webb Space Telescope, such a telescope could observe infrared light to peer straight into the early years of the universe. But a ULT would have vastly more light-gathering power than JWST, and be capable of directly observing the first stars ever created in the universe composed of primordial gas, known as Population III stars.

There’s a catch: The extreme lunar conditions would freeze liquid mercury. However, ionic liquids would stay in liquid form at frigid temperatures and could be coated with silver for a reflective surface.

NASA has already been explore the possibilities of constructing liquid mirrors in space with a project called the Fluidic Telescope Experiment (FLUTE). For it, the crew of the private Axiom-1 mission to the International Space Station conducted several experiments on how liquids take shape in microgravity.

Meanwhile, ILMT has already seen first light and will kick off science observations in October, after monsoon season. “We feel very confident that ILMT will deliver interesting data in the future,” says Surdej.

Is dark matter real? Astronomy’s multi-decade mystery

Modern astronomy is in a bit of turmoil. Astronomers understand how stars form, burn, and die, and they are improving their understanding of how planets assemble themselves into planetary systems like our own. 

But astronomers have a problem: They don’t understand how galaxies can exist — a problem that has remained unsolved after decades of research.

The problem is relatively simple. Galaxies are collections of stars held together by gravity. Like our solar system, they rotate, with stars marching in stately paths, orbiting the galactic center. At any fixed distance from the center of the galaxy, stars moving faster require stronger gravity to hold them in that orbit. When astronomers measure the orbital speed of stars in galaxies at a range of distances from the center, they find that the stars are moving so fast that galaxies should be torn apart. 

The most common explanation for this observational conundrum is a so-far undiscovered form of matter: dark matter. If it exists, dark matter exerts gravity, but it doesn’t emit light or any form of electromagnetic radiation. This means it can’t be seen by telescopes or any instrumentation that astronomers use to observe the cosmos.  However, this invisible dark matter would add to any galaxy’s gravitational pull, explaining why the stars orbit the galaxy so quickly.

The problem with the dark matter hypothesis is that nobody knows what form dark matter takes. When the term was first proposed back in 1933 by the Swiss-American astronomer Fritz Zwicky, it was possible that the extra mass was simply clouds of hydrogen gas. Interstellar hydrogen gas is largely invisible to telescopes. However, as technology has improved, astronomers found ways to measure the amount of hydrogen gas in galaxies and, while there’s a lot of it out there, there’s not enough to explain the galaxy rotation mystery.

Other explanations that have been proposed include things like burned out stars, black holes, and other objects that are known to exist within galaxies but don’t emit light.  However, astronomers searched for such objects (called MACHOs, short for MAssive Compact Halo Objects) in the 1990s and, again, while they found examples of MACHOs, there weren’t enough to explain the motion of stars in galaxies.


With some of the simpler explanations ruled out, scientists began to think that perhaps dark matter exists as a kind of a “gas,” or as never-before-seen particles. These particles are generically called “WIMPs,” short for “Weakly Interacting Massive Particles.” WIMPs, if they exist, are basically stable subatomic particles, with a mass somewhere in the range of the mass of a proton up to 10,000 protons, or even more.  

Like all dark matter particle candidates, WIMPs interact gravitationally, but that “W” in the name means that they also interact via the weak nuclear force. The weak nuclear force is involved in some forms of radioactivity. much stronger than gravity, but unlike gravity’s infinite range, the weak nuclear force only acts over tiny distances — distances much smaller than a proton. If WIMPs exist, they pervade galaxies, including our Milky Way, and even our own solar system. Depending on the mass of the WIMPs, astronomers estimate that if you make a fist, one dark matter particle could be found inside it.

Scientists have been looking for direct and compelling evidence for the existence of WIMPs for many decades. They do this in several ways. For example, some WIMP theories suggest that WIMPs can be made in particle accelerators, like the Large Hadron Collider in Europe. Particle physicists look at their data, hoping to see the signature of WIMP production. No evidence has been observed so far.

Another way in which researchers look for WIMPs is directly observing dark matter particles that waft through the solar system. Scientists build very large detectors and cool them to very cold temperatures so the atoms of the detectors are moving slowly.  They then put these detectors a half-mile or more underground to shield them from radiation from space. Then they wait, hoping that a dark matter particle will interact in their detector, disturbing one of the nearly stationary atoms.  

But despite decades of efforts, no WIMPs have been observed. Predictions in the 1980s suggested researchers could expect to detect WIMPs at a particular rate. When no WIMPs were detected, researchers built a series of detectors with much greater sensitivity, all of which failed to find WIMPs. Current detectors are 100 million times more sensitive than the ones of the 1980s, and no definitive observation of WIMPs have occurred, including a very recent measurement by the LZ experiment, which employs 10 tons of xenon to achieve unparalleled sensitivity to WIMPs.

Four Indian physicists bag top awards at International Astronomy Meet

The four Indian PhD thesis winners at the 31 IAU meet at Busan, South Korea

Four Indian researchers were among the seven international students who bagged top awards for their PhD work at the 31st International Astronomical Union (IAU) meet at Busan, South Korea.

All four researchers specialise in solar physics. Prantika Bhowmik from Centre of Excellence in Space Sciences India at the Indian Institute of Science Education and Research, Kolkata, Gopal Hazra of the Indian Institute of Sciences and Souvik Bose, formerly with the Indian Institute of Astrophysics, Bengaluru and Reetika Joshi from Kumaun University and Aryabhatta Research Institute of Observational Sciences, Nainital claimed the top honours.

“It is rare for a country to win so many PhD thesis awards at a global forum. Even more rare is that the prizes are in the field of solar physics. This is proof of high quality work to understand the Sun and its impact on the space environment being done in India in recent years,” said Professor Dipankar Banerjee, president, Astronomical Society of India.