The World’s Largest Liquid-Mirror Telescope Comes Online

Ask any astronomer, astrophysicist, or cosmologist, and they’ll probably tell you that a new age of astronomy is upon us! Between breakthroughs in gravitational-wave astronomy, the explosion in exoplanet studies, and the next-generation ground-based and space-based telescopes coming online, it’s pretty obvious that we are on the verge of an era of near-continuous discovery! As always, major discoveries, innovations, and the things they enable inspire scientists and researchers to look ahead and take the next big step.

Take, for example, the research into liquid mirrors and advanced interferometers, which would rely on entirely new types of telescopes and light-gathering to advance the science of astronomy. A pioneering example is the newly-commissioned International Liquid Mirror Telescope (ILMT) telescope that just came online at Devasthal Peak, a 2,450 m (8,040 ft) tall mountain located in the central Himalayan range. Unlike conventional telescopes, the ILMT relies on a rapidly-rotating 4-meter (13 ft) mirror coated with a layer of mercury to capture cosmic light.

Like other observatories, the ILMT is located high above sea level to minimize the distortion caused by atmospheric water vapor (a phenomenon known as atmospheric refraction). Much like the ESO’s Paranal Observatory in northern Chile or the Mauna Kea Observatories in Hawaii, the ILMT telescope is part of the Devasthal Observatory located in the remote mountains of Uttarakhand province in Northern India (west of Nepal). The telescope is designed to survey the sky and identify objects like supernovae, gravitational lenses, space debris, asteroids, and other transient and variable phenomena.

An image of the ILMT mirror taken during testing at Liege in Belgium. Credit: ILMT Collaboration/University of Liege

Dr. Paul Hicksonhas UBC Physics and Astronomy Professor and a liquid mirror technology pioneer, has been perfecting the technology over the years at the Large Zenith Telescope (LZT). Located at UBC’s Malcolm Knapp Research Forest east of Vancouver, BC, the LZT was the largest liquid-metal mirror before the ILMT was commissioned. Because of their expertise, Dr. Hickson and his colleagues played a pivotal role in designing and creating the ILMT air system. The facility gathered its first light this past May and will temporarily cease operations in October due to India’s monsoon season.

While it may sound like something out of science fiction, the basics of this technology are quite simple. The technology comes down to three components, including a dish containing a reflecting liquid (like mercury), a rotating section the Liquid Mirror (LM) sits atop (powered by air compressors), and a drive system. When powered up, the LM takes advantage of the fact that the rotational force causes the mirror to take on a parabolic shape, which is ideal for focusing light. Meanwhile, the liquid mercury is protected by an extremely thin layer of optical-quality mylar that prevents small waves from forming (due to wind or the rotation).

Liquid mercury provides a low-cost alternative to glass mirrors, which are very heavy and expensive to produce. The reflected light passes through a sophisticated multi-lens optical corrector while a large-format electronic camera at the focus records the images. As Dr. Hockson explained in a UBC Science press release:

“Rotating once every eight seconds, the mirror floats on a film of compressed air about 10 microns thick. By way of comparison, a human hair is approximately 70 microns thick. The air bearings are so sensitive that even smoke particles can damage them. A second air cushion prevents the rotor from moving sideways. The rotation of the Earth causes the images to drift across the camera, but this motion is compensated electronically.

“The camera has a corrector lens that was specially designed to remove star trail curvature. Stars go in circles around the north celestial pole, around the North Star. If you take a time exposure, the stars don’t go in straight lines, they go in arcs or circles. But this corrector is designed to correct for that to remove the curvature to straighten out the star trails, giving us sharp images.”

The 3.6m Devasthal Optical Telescope at night. Credit: ARIES

Regular science operations are scheduled to begin later this year. At this point, the ILMT is expected to gather about 10 GB of data every night that will be analyzed for stellar sources. These sources will then be selected for follow-up observations using the 3.6-meter (11.8 ft) Devasthal Optical Telescope (DOT) and its sophisticated spectroscopic instruments. As part of a facility overseen by the Aryabhatta Research Institute of Observational Sciences (ARIES) – which includes the ILMT and the ancient Devesthal Temple – the DOT has the distinction of being the largest optical telescope in India.

In particular, the ILMT will search for astronomical phenomena that are at the forefront of astronomical research today. This includes variable objects, stars that vary in brightness over time due to changes in their physical properties, or objects obstructing them (planets, dust rings, etc.). Transient phenomena, on the other hand, refer to short-lived events such as supernovae, Fast-Radio Burts (FRBs), gamma-ray bursts (GRBs), gravitational microlensing, etc. The study of these objects will lead to breakthroughs in the fields of astrophysics and cosmology.

In addition to ARIES and UBC, other organizations that make up the ILMT collaboration include the Indian Space Research Organization (ISRO), the Ulugh Beg Astronomical Institute (part of the Uzbek Academy of Sciences), the University of Liege, the Royal Observatory of BelgiumPoznan Observatory in Poland, Laval University, the University of Montreal, the University of Toronto, York University, and the University of Victoria in Canada.

Further Reading: UBC

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