The most powerful telescope in the Northern Hemisphere is now online! What can you see through it?

The universe is vast and boundless, and celestial bodies are constantly moving. Repeated observations of the same region of the universe to find objects whose brightness or position changes over time are called "time-domain surveys."

On September 17, 2023, the Mozi Survey Telescope, jointly developed by the University of Science and Technology of China and the Purple Mountain Observatory of the Chinese Academy of Sciences, was officially put into use. This telescope is currently the most powerful device in the northern hemisphere for optical time-domain surveys, and will significantly enhance my country's time-domain astronomical research capabilities. Let's follow the steps of Starlight to learn about it!

Mozi Survey Telescope

Source: University of Science and Technology of China

1What does the main mirror of the Mozi Survey Telescope look like?

The Mozi Survey Telescope is located at the Saishiteng Mountain Observatory at an altitude of 4,200 meters in Lenghu Town, Haixi Mongolian and Tibetan Autonomous Prefecture, Qinghai Province. It is a large-field optical imaging telescope. The telescope's main mirror has a diameter of 2.5 meters and is equipped with a 765-million-pixel large-target prime camera. It has a large light-transmitting area, little stray light, and a high system detection sensitivity.

The primary mirror is one of the most important and core components of a telescope.

Source of the main mirror of the Mozi Sky Survey Telescope丨News screenshot

The Mozi Sky Survey Telescope has a large dome with a skylight on it. The distant starlight shines through this skylight onto the 12-centimeter-thick primary mirror, which is coated with a layer of silver aluminum reflective film.

Source: News screenshot

The observation band of the optical telescope is between 360 nanometers and 920 nanometers. The shorter the wavelength, the higher the requirement for the flatness of the mirror. Gravity, weak airflow, thermal expansion and contraction, etc. may slightly change the shape of the mirror, and the larger the mirror, the more difficult it is to maintain its accuracy. In order to enable the mirror to perfectly image under any circumstances, the scientific research team has developed a hard-core technology-thin mirror surface active correction technology. There are 54 active optical support points behind the mirror, which can change the size of the force in real time according to the shape of the mirror. Every time a photo is taken, the mirror actively changes its shape. What kind of flatness can it achieve? For example, it is equivalent to an area the size of a football field, with fluctuations no more than a strand of hair.

Source: News screenshot

Every three nights, the Mozi Survey Telescope can survey the entire northern celestial sphere . With such a large workload, how to maintain this primary mirror? Chief Designer Kong Xu said that if there is less dust, it can be blown away with wind; if it is particularly dirty, it can be re-coated.

2The largest target camera in China

The primary mirror will continue to reflect the starlight, passing through the seven lenses inside and reaching the prime camera directly above. It looks very similar to a camera lens, but it has a big feature: its target surface is very large, which can also be understood as a large imaging surface.

The imaging surface (side length) of a general camera is about 2 cm, while the Mozi Survey Telescope is 32.5 cm, making it the camera with the largest target surface in China and also one of the world's leading cameras.

How is the imaging effect of such a large camera?

A photo taken by it requires 90 4K ultra-high-definition TV screens to fully display, and even a ping-pong ball 10 kilometers away can be seen clearly.

The detector of the Mozi Sky Survey Telescope is composed of 9 10-cm square charge-coupled devices (CCDs). Each CCD has 9,000 rows and 9,000 columns of pixels. Thanks to them, the image can maintain extremely high clarity despite its huge size.

3Why was the site chosen in Mangya City, Haixi Prefecture, Qinghai Province?

On the day when the Micius Sky Survey Telescope was officially put into use, a photo of the Andromeda Galaxy taken by the telescope during a test observation was released. The Andromeda Galaxy is the largest galaxy in the Local Cluster where the Milky Way is located. It is a huge spiral galaxy, numbered M31, with a diameter of about 220,000 light-years and a distance of about 2.54 million light-years from the Earth.

The Andromeda Galaxy photographed by the Mozi Survey Telescope

Source: Chinese Academy of Sciences

Not every location on Earth is suitable for cosmic observation, and the selection of an observatory site is very strict: first, it must be visible, that is, the cloud cover must be small; second, it must be able to see through, that is, the atmosphere must be thin; third, it must be able to see clearly, that is, the transparency must be good; fourth, it must be able to see clearly, that is, the seeing must be good; fifth, it must be able to see deeply, that is, there must be no light pollution or little light pollution; sixth, it must be able to see for a long time, that is, there must be many clear nights.

Source: News screenshot

Although populous cities can provide great convenience for scientific research, for telescopes, prosperous areas mean large impacts of artificial light sources, large impacts of human activities, and serious air pollution, which are very unfavorable for observation activities. The area around Lenghu Town in Qinghai is sparsely populated. It takes more than 100 kilometers to drive from Lenghu Town to reach the Saishiteng Mountain where the Lenghu Astronomical Base is located. The home of the Mozi Survey Telescope is on the top of this mountain.

A distant view of the Mozi Survey Telescope Observation Base Source: Chinese Academy of Sciences

The climate in Saishiteng Mountain is dry and the weather is clear. There are nearly 10 hours of sunshine every day, and the available time for astronomical observation is up to 300 days per year. At the working platform of the Mozi Sky Survey Telescope at an altitude of 4,200 meters, you can see a very obvious dividing line in the sky, which is the boundary between the floating dust in the Qaidam Basin and the pure sky. The dividing line is about 3,200 meters above sea level. The sky below the mountain is gray, while the top of the mountain is still blue, almost unaffected by dust.

So far, 12 telescope projects from 11 scientific research units have been implemented at the Qinghai Lenghu Astronomical Observatory. Once all are completed, it will become the largest astronomical observation base in Asia.

In the future, the Micius survey telescope will be able to monitor moving celestial bodies and light-variable objects, and be used for efficient search and monitoring of astronomical dynamic events. It can play an important role in high-energy time-domain astronomy, solar system celestial body census, Milky Way structure and near-field cosmology, provide a large amount of data for my country's space exploration and promote my country's scientific and technological progress.

The article is compiled from: Chinese Academy of Sciences Purple Mountain Observatory official account, People's Daily official account, China Youth Network, Beijing Daily