Liquid reflecting telescope opens in India

A unique telescope that focuses light with a slowly spinning bowl of liquid mercury instead of a massive mirror has opened its eye to the skies over India. Such telescopes have been built before, but the 4-meter-wide International Liquid Mirror Telescope (ILMT) is the first large one purpose-built for astronomy, with the kind of high-altitude observer prize – the 2,450-meter-long Devasthal Observatory in the Himalayas.

While astronomers have to content themselves with just looking straight up, the $2 million instrument, built by a consortium from Belgium, Canada and India, 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, but for $18 million. “Simple things are often best,” says project director Jean Surdej of the University of Liège. Some astronomers say liquid mirrors are the perfect technology for a giant telescope on the moon that can look back to 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 casting a glass mirror, with the surface in a parabola. is crushed and covered with reflective aluminum.

The ARIES Observatory Complex, Devasthal, Uttarakhand, India
The International Liquid Mirror Telescope (bottom left) of the Devasthal Observatory in India sits next to the 3.6-meter-long Devasthal Optical Telescope (center).Anna and Jean Surdej

ILMT was originally conceived in the late 1990s. The saucer-shaped vessel containing the mercury was delivered to India in 2012, but construction of the telescope housing was delayed. Then researchers discovered they didn’t have enough mercury. While they waited for more, the COVID-19 pandemic struck, making travel to India impossible. Finally, in April, the team spun 50 liters of mercury, creating a parabolic layer 3.5 millimeters thick. After such a long pregnancy, “we are all very happy,” said team member Paul Hickson of the University of British Columbia, Vancouver.

Staring straight up, the revolving mirror will see a swath of sky nearly as wide as the full moon, as 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 streaks in the image; the individual pixels can then be added together to create a single long exposure. Because the telescope sees roughly the same strip of sky on consecutive nights, exposures from several nights can be added together to get 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 supernovas and quasars, the bright hearts of distant galaxies that increase and decrease as supermassive black holes matter. to consume. Surdej wants to hunt for gravitational lenses, where 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 ILMT reveals the mass of the lens systems and can help estimate the universe’s expansion rate. One study suggested that as many as 50 lenses could be visible in ILMT’s strip of sky.

Conventional survey telescopes, such as the Zwicky Temporary Facility in California and the upcoming Vera C. Rubin Observatory in Chile, cover much more of the sky. But it’s unlikely they’ll return to the same patch every night to look for changes. “We’re forced to have a niche,” Hickson says. ILMT has the extra power to sit next to DOT, which is equipped with instruments that can quickly investigate any volatile objects discovered by its neighbor. This tag-team approach “is more comprehensive and scientifically richer,” said Dipankar Banerjee, director of the Aryabhatta Research Institute of Observational Sciences, which runs the Devasthal Observatory.

If ILMT is a success, Surdej says the technology could be scaled up to build much larger liquid levels 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, which comes from the planet’s rotation, would distort the movement of the mercury in mirrors larger than 8 meters. But the moon spins more slowly, allowing for much greater levels of liquid, albeit not 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, Laval University liquid mirror pioneer Ermanno Borra showed that “ionic liquids,” lightweight, low-freezing molten salts, could survive 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 moon, but went no further. Astronomers hope the current interest in lunar exploration and the low-cost launches offered by private space companies like SpaceX will spark a resurgence. In 2020, a team from the University of Texas, Austin, proposed the Ultimate Large Telescope, a 100-meter-long liquid mirror that would stare at the same patch of sky from one of the moon’s poles for years. Such a giant could collect the faint trickle of photons from the very first stars to illuminate the universe, before galaxies even existed. Veteran mirror maker Roger Angel of the University of Arizona says there’s “a unique niche for a big one” [liquid] mirror that goes beyond what others can do.”

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