A unique telescope that focuses light with a slowly rotating bowl of liquid mercury instead of a solid mirror has opened his eyes to the skies above India. These telescopes have been built before, but the 4-meter-wide International Liquid Mirror Telescope (ILMT) is the first large one to be built specifically for astronomy, in the high-altitude location-observer prize type – the 2,450-meter Devasthal. Observatory in the Himalayas.
While astronomers must be satisfied just looking up, the $2 million instrument, built by a consortium of Belgium, Canada and India, is much cheaper than telescopes with glass mirrors. A few steps away from the ILMT is the 3.6-meter, steerable Devasthal Optical Telescope (DOT), built by the same Belgian company at the same time, but for $18 million. “Simple things are often the best,” says Project Director Jean Surdej at the University of Liège. Some astronomers say liquid mirrors are the perfect technology for a giant telescope on the Moon that can see the epoch 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 conventional telescope mirror – but without the expense of flipping a blank glass mirror, grinding its surface. in a parabola. , and coating it with reflective aluminum.
The ILMT was originally conceived in the late 1990s. The dish-shaped container that contains the mercury was delivered to India in 2012, but construction of the telescope’s case was delayed. Then the researchers found they didn’t have enough mercury. As they waited for more, the COVID-19 pandemic struck, making travel to India impossible. Finally, in April, the team spun 50 liters of mercury around, creating a 3.5 millimeter-thick parabolic layer. After such a long pregnancy, “we are all very happy,” says Paul Hickson, a member of the team at 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 across the skies from dusk to dawn. “You just turn it on and off,” says Hickson. Objects appear as long stripes in the image; separate pixels can be added later to create a single long exposure. As the telescope sees approximately the same swath of sky on successive nights, exposures from many 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 like supernovae and quasars, the glowing hearts of distant galaxies that wax and wane as supermassive black holes consume matter. Surdej wants to hunt for gravitational lensing, in which the gravity of a galaxy or cluster of galaxies bends the light of a more distant object like a giant magnifying glass. The sensitive measurement of the object’s brightness by the ILMT reveals the mass of the lensing galaxies and can help estimate the expansion rate of the universe. One study suggested that up to 50 lenses may be visible in the ILMT sky band.
Conventional survey telescopes such as the Zwicky Transient Facility in California and the nearby Vera C. Rubin Observatory in Chile cover much more of the sky. But they are unlikely to return to the same patch every night to look for changes. “We are forced to have a niche,” says Hickson. The ILMT has the added advantage of sitting next to the DOT, which is equipped with instruments that can quickly scan any elusive object discovered by its neighbor. This tag team approach “is more comprehensive and scientifically richer,” says Dipankar Banerjee, director of the Aryabhatta Research Institute of Observational Sciences, which manages the Devasthal Observatory.
If the ILMT is a success, 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 of the planet’s rotation would deform the movement of mercury in mirrors larger than 8 meters. But the Moon rotates more slowly, allowing for much larger liquid mirrors – albeit not mercury ones. 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 melting salts with low freezing points, would survive lunar conditions and could be reflected with a thin layer of silver.
In the 2000s, both NASA and the Canadian Space Agency commissioned studies of lunar liquid mirror telescopes, but they went no further. Astronomers hope the current interest in moon exploration and the cheap launches offered by private space companies like SpaceX will spur a renaissance. In 2020, a team at the University of Texas, Austin, proposed the Great Final 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 gather the faint string of photons from the 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.”