What are the benefits of using mercury to make a liquid telescope?

Astronomical telescopes are the eyes of human beings to observe the universe. Most of the cosmic information we know today comes from astronomical telescopes. It can be said that without astronomical telescopes, there would be no modern astronomy. The well-known Chinese Sky Eye FAST, Hubble Space Telescope, and James Webb Space Telescope are representatives of them. They have a common feature, that is, they look like a big pot.

The big pot is actually designed to collect as many faint signals from the distant universe as possible. The diameter of the pot is called the aperture of the telescope. The larger the aperture, the stronger the telescope's ability to collect signals, the longer the detection distance, and the clearer and brighter the image. Therefore, the big pot in the cross section can be said to be one of the most core components of the telescope.

The mirrors of traditional telescopes are made of solid materials. At the end of 2022, there was a news report that on the south side of the Himalayas, the world's first liquid mirror telescope has opened its eyes and started observing the universe. Why does this telescope use liquid as the mirror? What are the advantages and disadvantages of liquid mirror telescopes? What other important roles can liquid mirror telescopes play in the future in exploring the universe?

The name of this telescope is the International Liquid Mirror Telescope, or ILMT for short. It was jointly built by Belgium, Canada, India and other countries and is located at an altitude of 2,450 meters on the southern foot of the Himalayas. This is a 4-meter optical telescope, which has a larger aperture than the Hubble Space Telescope and the space station survey telescope that my country will launch in the future. Both of these telescopes have an aperture of 2.4 meters. Its aperture also exceeds the largest civilian optical telescope in my country - the 2.4-meter telescope at the Lijiang Base of the Yunnan Astronomical Observatory and the 2.16-meter telescope at the Xinglong Base of the National Astronomical Observatory.

Beijing Astronomical Observatory Radio Telescope (Photo from Tuchong.com)

For telescopes, the larger the aperture, the more complicated its processing and manufacturing process, and the higher the cost. Generally speaking, if the aperture of a telescope is doubled, the difficulty and cost of development may increase several times, or even dozens of times. This is why there are dozens of 1-meter optical telescopes in our country, but only a handful of civilian optical telescopes with an aperture of more than 2 meters.

A key difficulty in the development of telescopes is how to process the big pot? The big pot responsible for collecting light has extremely high processing requirements, requiring the mirror surface to be as close to the ideal parabolic configuration as possible. The surface accuracy of the optical telescope mirror is required to be at least 1/30 wavelength, which is about 20 nanometers. What is the concept? For example, if a 4-meter-diameter mirror is to be enlarged to the size of our Beijing city, its ground flatness needs to be less than 1 mm to meet the requirements. Factors such as thermal expansion and contraction and gravity will cause deformation of the mirror surface. The larger the aperture of the telescope, the more difficult it is to maintain the accuracy of the mirror surface. Therefore, the requirements for the processing technology of the telescope mirror are very high. It must go through a series of very complex process such as smelting, forging, grinding, polishing, coating, etc. to create an optical mirror with an ideal cross-section.

Liquid mirrors are much easier. Imagine that if you stir the water in a cup with a chopstick, the water in the middle of the cup will flow to the surroundings, forming a rotating picture. It is very similar to the cross-section of the telescope cauldron. The basic principle is that the liquid rotating under the action of gravity and centrifugal force will automatically form an ideal parabola, which is also the basic principle of the liquid mirror telescope. By rotating the mercury in a disk, an ideal cross-section can be formed to gather light and rotate the liquid mirror. Try to use a container that is close to the cross-sectional shape. At this time, the amount of liquid used is very small, and its thickness is less than 1 mm.

The biggest advantage of liquid mirrors is that they are very cheap, because they do not need to go through a difficult processing process like solid mirrors. They only need to rotate the mercury in a disk at a certain speed to form a very ideal parabola. The 4-meter International Liquid Mirror Telescope costs only $2 million, while a 3.6-meter optical radius not far away, built by the same Belgian company at the same time, costs about $18 million, a cost difference of almost 10 times.

However, liquid mirror telescopes are not omnipotent. The formation of an ideal parabola is the result of the balance between gravity and centrifugal force. Therefore, liquid mirror telescopes cannot point in any direction at will. They can only observe the zenith of the sky. If you want to manipulate the telescope to look everywhere, then liquid mirror telescopes may not be suitable for you.

In the future, humans also plan to build larger-aperture liquid optical mirror telescopes such as 8-meter ones in South America. Scientists are even considering building a 100-meter-diameter liquid mirror optical telescope on the moon. By then, they will be able to see the earliest stars formed since the Big Bang, providing brighter eyes for humans to explore the universe.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Li Mingtao

Reviewer: Zhou Binghong

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.