A unique telescope focusing light with a slowly rotating bowl of liquid mercury rather than a solid mirror opened its eyes to the sky over India. Such telescopes have been built before, but the 4m wide International Liquid Mirror Telescope (ILMT) is the first large telescope built for the purpose of astronomy, in a sort of prize for high-altitude position observers – the 2,450m Devasthal Prize Observatory in the Himalayas.
Although astronomers have to satisfy themselves by looking only straight, the $2 million instrument, built by a consortium of Belgium, Canada, and India, is much cheaper than glass-mirror telescopes. A stone’s throw from the ILMT is the 3.6-meter Devasthal Optical Steering Telescope (DOT) – built by the same Belgian company at the same time – but for $18 million. “Simple things are often the best,” said project manager Jean Sorge of the University of Liege. Some astronomers say liquid mirrors are an ideal technology for a giant telescope on the Moon that could go back to the time of the universe’s first stars.
When a bowl of reflective liquid mercury is rotated, the combination of gravity and centrifugal force pushes the liquid into a perfect parabolic shape, just like a traditional telescope mirror — but not counting pouring an empty glass mirror, grinding its surface into a parabola, and coating it with reflective aluminum.
ILMT’s dream was originally in the late 1990s. The dish-shaped vessel containing the mercury was delivered to India in 2012, but the construction of the telescope container was delayed. Then the researchers found that they did not have enough mercury. With more waiting for them, the COVID-19 pandemic hit, making travel to India impossible. Finally, in April, the team mapped 50 liters of mercury spinning, creating a 3.5 mm-thick equivalent layer. After such a long pregnancy, “we are all very happy,” says team member Paul Hickson of the University of British Columbia, Vancouver.
The rotating mirror, staring straight up, will see a patch of sky roughly equal to the width of the full moon as the Earth’s rotation scans it across the sky from dusk to dawn. “Just turn it on and let it go,” says Hickson. Objects appear as long lines in the image; The separate pixels can then be added together to create a single long exposure. Because the telescope sees roughly the same strip of sky on successive nights, exposures from several nights can be added together to obtain highly sensitive images of faint objects.
Alternatively, one night’s image can be subtracted from the next to see what has changed, revealing transient objects such as supernovae and quasars, the bright cores of distant galaxies that dwindle and fade as supermassive black holes devour matter. Surdej wants to look for gravitational lenses, in which the gravity of a galaxy or galaxy cluster bends the light of a distant object like a giant magnifying glass. Sensitive ILMT measurements of object brightness reveal the mass of lens galaxies and can help estimate the expansion rate of the universe. A study suggested that up to 50 lenses may be visible in the sky band at ILMT.
Conventional surveying telescopes, such as the Zwicky Transit Facility in California and the upcoming Vera C. Rubin Observatory in Chile, cover a much larger area of the sky. But they are unlikely to return to the same patch every night to look for changes. “We’re forced to have a niche,” says Hickson. The ILMT has the added power of sitting next to the DOT, and is equipped with instruments that can quickly scan any passing objects that its next-door neighbor detects. This marker team’s approach is “more comprehensive, and more scientifically rich,” says Dipankar Banerjee, director of the Aryapatta Research Institute of Observational Sciences, which operates the Devastal Observatory.
If the ILMT is successful, Surdej says the technology could be scaled up to build much larger liquid mirrors on the Moon, an attractive location for future giant telescopes because it is less seismically active than Earth and has no atmosphere. On Earth, the Coriolis effect, from the planet’s rotation, would distort the motion of Mercury in mirrors larger than 8 meters in size. But the Moon rotates more slowly, allowing for much larger liquid mirrors – albeit not of Mercury. It is too heavy to be carried to the moon and freezes at night and evaporates during the day. But more than a decade ago, liquid mirror pioneer Ermanno Porra of Laval University demonstrated that “ionic liquids,” lightweight molten salts with low freezing points, would withstand lunar conditions and could be made reflective with a thin layer of silver.
In the 2000s, both NASA and the Canadian Space Agency commissioned studies of liquid-mirror telescopes on the lunar surface, but they did not go further. Astronomers hope the current interest in lunar exploration and the cheap launches offered by private space companies such as SpaceX will spur a revival. In 2020, a team from the University of Texas, Austin, proposed the ultimate large telescope, a 100-meter liquid mirror that would constantly stare at the same patch of sky for years on end from one of the moon’s poles. Such a giant could collect faint droplets of photons from the first stars that lit up the universe, even before galaxies existed. There is a “unique niche for a great company .” says veteran mirrormaker Roger Engel from the University of Arizona [liquid] A mirror beyond what others can do.”