International Liquid-Mirror Telescope (ILMT)

Published
14th Jun, 2022

Context

A new telescope facility has been launched in top of the Himalayan range that will keep a watch on the overhead sky.

It can identify transient or variable objects such as supernovae, gravitational lenses, space debris, and asteroids.

About

Highlights of the event

It is the first liquid mirror telescope in the country and the largest in Asia.
The novel instrument employs a 4-meter-diameter rotating mirror made up of a thin film of liquid mercury to collect and focus light.
It has been developed by the scientists of India, Belgium and Canada.
It is located at an altitude of 2450 metres at the Devasthal Observatory campus of Aryabhatta Research Institute of Observational Sciences (ARIES).

What is liquid mirror telescope?

Liquid-mirror telescopesare telescopes with mirrors made with a reflective liquid.
The most common liquid used is mercury, but other liquids will work as well (for example, low-melting alloys of gallium).
The liquid and its container are rotated at a constant speed around a vertical axis, which causes the surface of the liquid to assume a paraboloidal shape.
This parabolic reflector can serve as the primary mirror of a reflecting telescope.
The rotating liquid assumes the same surface shape regardless of the container's shape; to reduce the amount of liquid metal needed, and thus weight, a rotating mercury mirror uses a container that is as close to the necessary parabolic shape as possible.
Liquid mirrors can be a low-cost alternative to conventional large telescopes.
Compared to a solid glass mirror that must be cast, ground, and polished, a rotating liquid-metal mirror is much less expensive to manufacture.

How does it work?

It has spun a pool of mercury which is a reflective liquid, so that the surface curved into a parabolic shape which is ideal for focusing light.
A thin transparent film of mylar protects the mercury from wind.
The reflected light passes through a sophisticated multi-lens optical corrector that produces sharp images over a wide field of view.
A large-format electronic camera located at the focus records the images.
The rotation of the earth causes the images to drift across the camera, but this motion is compensated electronically by the camera itself.
This mode of operation increases observing efficiency and makes the telescope particularly sensitive to faint and diffuse objects.

Different space telescopes

This list of space telescopes (astronomical space observatories) is grouped by major frequency ranges:
- Gamma ray: Gamma ray telescopes collect and measure individual, high energy gamma rays from astrophysical sources. These are absorbed by the atmosphere, requiring that observations are done by high-altitude balloons or space missions.
- X-ray: X-ray telescopes measure high-energy photons called X-rays. These cannot travel a long distance through the atmosphere, meaning that they can only be observed high in the atmosphere or in space.
- Ultraviolet: Ultraviolet telescopes make observations at ultraviolet wavelengths, i.e. between approximately 10 and 320 nm. Light at these wavelengths is absorbed by the Earth's atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space.
- Objects emitting ultraviolet radiation include the Sun, other stars and galaxies.
- Visible: The oldest form of astronomy, optical or visible-light astronomy, observes wavelengths of light from approximately 400 to 700 nm.
- Positioning an optical telescope in space eliminates the distortions and limitations that hamper that ground-based optical telescope, providing higher resolution images.
- Infrared: Infrared light is of lower energy than visible light, hence is emitted by sources that are either cooler, or moving away from the observer (in present context: Earth) at high speed.
- Microwave: Microwave space telescopes have primarily been used to measure cosmological parameters from the Cosmic Microwave Background.
- Radio: As the atmosphere is transparent for radio waves, radio telescopes in space are most useful for Very Long Baseline Interferometry: doing simultaneous observations of a source with both a satellite and a ground-based telescope and by correlating their signals to simulate a radio telescope the size of the separation between the two telescopes.

Significance

ILMT is the first liquid-mirror telescope designed exclusively for astronomical observations.
Devasthal Observatory now hosts two four-meter class telescopes – the ILMT and the Devasthal Optical Telescope (DOT).
Both are the largest aperture telescopes available in the country.
It is based on the application of Big Data and Artificial Intelligence/Machine Learning (AI/ML) algorithms that will be implemented for classifying the objects observed.
The data collected from ILMT will be ideally suited to perform a deep photometric and astrometric variability survey over a period of typically 5 years.

Consequences

Cost: The greatest advantage of a liquid mirror is its small cost, about 1% of a conventional telescope mirror. This cuts down the cost of the entire telescope at least 95%.
Alignment problem: But the greatest disadvantage is that the mirror can only be pointed straight up.
Research is underway to develop telescopes that can be tilted, but currently if a liquid mirror were to tilt out of the line and it can lose its shape.
Health: Since mercury metal and its vapour are both toxic to humans and animals, there remains a problem for its use in any telescope where it may affect its users and others in its area.