A unique telescope that focuses light with a slowly spinning bowl of liquid mercury instead of a solid mirror has opened its eyes to the sky above India. Such telescopes have been built before, but the 4 meter wide International Liquid Mirror Telescope (ILMT) is the first large telescope specially built for astronomy, at the type of observer price at high altitude – 2450 meters Devasthal Observatory in the Himalayas.
Although astronomers have to content themselves with just looking straight up, the $ 2 million instrument, built by a consortium of Belgium, Canada and India, is much cheaper than telescopes with glass mirrors. A stone’s throw from ILMT is the 3.6 meter long, steerable Devasthal Optical Telescope (DOT) – built by the same Belgian company at the same time – but for 18 million dollars. “Simple things are often the best,” says project manager Jean Surdej at the University of Liège. Some astronomers say that floating mirrors are the perfect technology for a giant telescope on the moon that can look 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 perfectly parabolic shape, just like a conventional telescopic mirror – but without the cost of casting a glass mirror, grinding its surface into a parabola, and coating it with reflective aluminum .
ILMT was originally created in the late 1990s. The bowl-shaped vessel containing the mercury was delivered to India in 2012, but the construction of the telescopic housing was delayed. Then the researchers found that they did not have enough mercury. As they waited for more, the covid-19 pandemic struck, making it impossible to travel to India. Finally, in April, the team put 50 liters of mercury in spinning, creating a parabolic layer 3.5 millimeters thick. After such a long pregnancy, we are all very happy, says team member Paul Hickson at the University of British Columbia, Vancouver.
Staring straight up, the rotating mirror will see a stretch of sky almost as wide as the full moon while the earth’s rotation scans it across the sky from dusk to dawn. “You just turn it on and let it go,” Hickson says. Objects appear as long lines in the image; the separate pixels can be added together afterwards to create a single long exposure. Because the telescope sees roughly the same strip of sky on consecutive 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 such as supernovae and quasars, the bright hearts of distant galaxies that grow and decay as supermassive black holes consume matter. Sourdough wants to hunt for gravitational lenses, where the gravity of a galaxy or galaxy cluster bends the light from a more distant object like a giant magnifying glass. ILMT’s sensitive measurement of the object’s brightness reveals the mass of the lens galaxies and can help estimate the rate of expansion of the universe. One study suggested that as many as 50 lenses may be visible on ILMT’s sky strip.
Conventional survey telescopes, such as Zwicky Transient Facility in California and the upcoming 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 search for changes. “We have to have a niche,” says Hickson. ILMT has the extra strength to sit next to the DOT, which is equipped with instruments that can quickly examine all volatile objects discovered by its neighbor. This tag-team strategy “is more comprehensive and scientifically richer,” said Dipankar Banerjee, director of the Aryabhatta Research Institute of Observational Sciences, which operates the Devasthal Observatory.
If ILMT is a success, Surdej says the technology can be scaled up to build much larger liquid levels on the moon, an attractive place for future giant telescopes because it is less seismically active than the Earth and has no atmosphere. On Earth, the Coriolis effect, from the rotation of the planet, would distort the motion of mercury in mirrors larger than 8 meters. But the moon rotates more slowly and allows much larger liquid levels – even if they are not made of mercury. It is too heavy to transport 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”, easily melted salts with low freezing points, would survive the conditions of the moon and could be made reflective with a thin coating of silver.
In the 2000s, both NASA and the Canadian Space Agency ordered studies of the moon’s floating mirror telescope but did not go any further. Astronomers hope that the current interest in lunar exploration and the cheap launches offered by private space companies such as SpaceX will stimulate a revival. In 2020, a team at the University of Texas, Austin, proposed the Ultimately Large Telescope, a 100-meter liquid mirror that would constantly stare at the same spot in the sky for several years from one of the moon’s poles. Such a giant could collect the faint stripes of photons from the very first stars that illuminated the universe, before galaxies even existed. Veteran mirror maker Roger Angel of the University of Arizona says there is “a unique niche for a large [liquid] a mirror that goes beyond what others can do. ”
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