A unique telescope that focuses light with a slowly rotating bowl of liquid mercury instead of a solid mirror has opened its eyes to the skies above India. Such telescopes have been built before, but the 4-meter-wide International Liquid Mirror Telescope (ILMT) is the first large one to be purpose-built for astronomy, at the kind of high-altitude site-watcher price – the Devasthal of 2450 meters. Observatory in the Himalayas.
Although astronomers have to content themselves with looking straight up, the $2 million instrument, built by a Belgian, Canadian and Indian consortium, is much cheaper than glass-mirror telescopes. A stone’s throw from ILMT is the 3.6-meter steerable Devasthal Optical Telescope (DOT), built by the same Belgian company around the same time, but for $18 million. “The simple things are often the best,” says project director Jean Surdej from the University of Liège. Some astronomers say liquid mirrors are the perfect technology for a giant telescope on the Moon that could see the time of the very first stars in the universe.
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 conventional telescope mirror, but without the expense of molding a mirror. made of glass, grinding its surface into a parabola, and covering it with reflective aluminum.
The ILMT was envisioned in the late 1990s. The dish-shaped container that holds the mercury was delivered to India in 2012, but construction of the telescope enclosure has been delayed. Then the researchers discovered that they didn’t have enough mercury. As they waited for more, the COVID-19 pandemic hit, making it impossible to travel to India. Finally, in April, the team spun 50 liters of mercury, creating a parabolic layer 3.5 millimeters thick. After such a long gestation, “we’re all very happy,” says team member Paul Hickson of the University of British Columbia in Vancouver.
Looking straight up, the rotating mirror will see a swath of sky almost as wide as the Full Moon as the Earth’s rotation sweeps it across the dusk-to-dawn skies. “Just turn it on and let it go,” Hickson says. Objects appear as long streaks in the image; the separate pixels can be added together later to create a single long exposure. Because the telescope sees roughly the same band of sky on successive nights, exposures from multiple nights can be added together to obtain extremely 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 waxing and waning as black holes supermassives consume matter. Surdej wants to chase away gravitational lenses, in which the gravity of a galaxy or cluster of galaxies deflects light from a more distant object like a giant magnifying glass. The sensitive measurement of the object’s brightness by the ILMT reveals the mass of lens galaxies and can help estimate the rate of expansion of the universe. One study suggested that up to 50 lenses could be visible in the ILMT band of sky.
Conventional surveying telescopes, such as the Zwicky Transitional Facility in California and the upcoming Vera C. Rubin Observatory in Chile, cover much more of the sky. But they’re unlikely to go back to the same patch every night to look for changes. “We have to have a niche,” Hickson says. The ILMT has the added strength of sitting next to the DOT, which is equipped with instruments capable of quickly examining any fleeting objects discovered by its next-door neighbour. This team approach “is more comprehensive and scientifically richer,” says Dipankar Banerjee, director of the Aryabhatta Research Institute of Observational Sciences, which runs the Devasthal Observatory.
If ILMT is successful, Surdej says the technology could be extended 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, resulting from the rotation of the planet, would distort the movement of mercury in mirrors over 8 meters. But the Moon rotates more slowly, allowing much larger liquid mirrors, but no mercury. It is too heavy to be transported to the Moon and would freeze at night and evaporate during the day. But more than a decade ago, liquid mirror pioneer Ermanno Borra of Laval University showed that “ionic liquids,” light molten salts with low freezing points, would survive lunar conditions and could be made reflective. with a thin layer of silver.
In the 2000s, NASA and the Canadian Space Agency commissioned studies of lunar liquid mirror telescopes, but went no further. Astronomers hope the current interest in lunar exploration and cheap launches offered by private space companies such as SpaceX will spur a revival. In 2020, a team from the University of Texas at Austin proposed the Ultimately Large Telescope, a 100-meter liquid mirror that would stare at the same piece of sky constantly for years from one of the Moon’s poles. Such a giant could collect the faint trickle of photons from the very first stars that lit up the universe, before galaxies even existed. Veteran mirror-maker Roger Angel of the University of Arizona says there is “a unique niche for a great [liquid] mirror that goes beyond what others can do.
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