Liquid mirror telescope starts operating


Technological Innovation Website Editor – 06/14/2022


The International Liquid Mirror Telescope uses as a mirror a turntable containing 50 liters of mercury.
[Imagem: ILMT]

liquid mirror

The International Liquid Mirror Telescope (ILMT: International Liquid Mirror Telescope).

Instead of a conventional crystalline mirror, the ILMT uses a 4 meter diameter liquid mercury turntable.

The combination of gravity and centrifugal force makes the liquid assume a perfect parabolic shape, just like a conventional telescope mirror. Mercury takes the parabolic form because a liquid surface always defines its local surface perpendicular to the acceleration it experiences, in this case becoming stronger and steeper with distance from the central axis.

And that mirror is created just by turning on the motor that turns the plate, without the expense of casting a glass mirror mold, sanding it into a parabola, polishing the surface and finally coating it with a reflective material such as aluminum or silver. .

The practical result is that the telescope is much cheaper: US$ 2 million, against US$ 18 million for a telescope with a conventional glass mirror of similar size (3.6 meters), built at the same time and right next to the ILMT, in the Devasthal Mountains, India.

“Simple things are usually the best,” said Jean Surdej, from the University of Lige, in Belgium, who coordinates the consortium that built and will operate the observatory, which also has collaborators from Canada and India.

In fact, because it is simpler and smaller, the technology of mirrors or liquid lenses is considered ideal for the construction of telescopes on the Moon – except that, on the Moon, the telescope could not be made of mercury, which would solidify, but of a ion liquid.


First light of the ILMT.
[Imagem: ILMT]

Weak objects and transient phenomena

The telescope’s liquid mirror is formed by rotating 50 liters of mercury, a metal that is liquid at room temperature, making it form a 3.5 mm thick parabola, creating a mirror with exceptional reflectivity, without any need for polishing.

A large-format camera, installed at the focus of the mirror, records the images. “The rotation of the Earth causes images to move through the camera, but this movement is electronically compensated for by the camera. This mode of operation increases observation efficiency and makes the telescope particularly sensitive to faint and diffuse objects,” explained Professor Paul Hickson. , from the University of British Columbia, Canada.

Thus, although it captures at once an area of ​​the sky equivalent to a full moon, the celestial bodies appear in the image as long wavy lines. It sounds strange, but this allows pixels to be added to create each point of the final image, which is equivalent to a long exposure. And because the telescope sees approximately the same swath of the sky on successive nights, exposures from several nights can be added together to generate extremely sensitive images of dimly lit objects.

Another possibility is to compare the images, subtracting one night’s image from observations made the night before, to see what has changed, revealing not only transient objects and phenomena such as quasars, supernovae, and black holes engulfing matter and emitting jets of radiation, but Also the much sought after gravitational lens, which shows objects hidden behind those that are visible.


This section is the first scientific image obtained by the liquid-mirror telescope.
[Imagem: ILMT]

How does a liquid mirror telescope work?

Liquid mirror telescopes are zenith pointing telescopes: They see only a small field of view around the zenith, the area along the local vertical axis. And, because of the Earth’s rotation, the telescope sweeps a band of constant declination equal to the observatory’s latitude.

So, you can’t keep pointing at random celestial bodies, as in lens telescopes and glass mirrors, because if you tilt the telescope, the mirror is destabilized by the action of gravity.

Images are also not obtained directly, as in a photograph. Instead, a real-time imaging technique known as time-delayed integration (TDI: Time Delayed Integration).

Capturing an image with a CCD, the sensor of a digital camera, is usually done in two steps. First, the sensor’s pixel array is exposed to the light of the object being photographed, with each pixel storing its corresponding amount of photons, which is proportional to the flux of light arriving at that pixel during exposure. Second, the CCD is read, which is done by counting the number of photons that hit each pixel.

In a common camera, the reading is done by successively shifting each column of the CCD pixel matrix to an associated electronic device, called a register, which counts the number of photons in each pixel of each column.

In TDI (time-delayed integration) mode, the displacement of the columns is decelerated. Thus, as a star passes through the telescope’s field of view, its image crosses the sensor. The transfer of columns to the recorder is slowed down such that the photons generated by the star are displaced over the sensor at the same speed as the image of the star travels through the focal plane.

Consequently, as soon as a star leaves the field of view, the number of photons it generated is counted, creating a real-time imaging technique: At every moment, ILMT will create an image of the sky field passing the zenith.

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