A unique telescope that focuses light with a slowly spinning bowl of liquid mercury instead of a solid mirror has opened its eye 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 observers prize—the 2450-meter Devasthal Observatory in the Himalayas.
Although astronomers must satisfy themselves with only looking straight up, the $2 million instrument, built by a consortium from Belgium, Canada, and India, is much cheaper than telescopes with glass mirrors.
Some astronomers say liquid mirrors are the perfect technology for a giant telescope on the Moon that could see back to the time of the universe’s very 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, exactly like a conventional telescope mirror—but without the expense of casting a glass mirror blank, grinding its surface into a parabola, and coating it with reflective aluminum.
ILMT was originally dreamt up in the late 1990s. The dish-shaped vessel that holds the mercury was delivered to India in 2012, but construction of the telescope enclosure was delayed.
Then 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 set 50 liters of mercury spinning, 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, Vancouver.
Staring straight up, the rotating mirror will see a swath of sky almost as wide as the full Moon while Earth’s rotation scans it across the heavens from dusk to dawn.