Hail A was selected as the "Best Stargazing Point in the World"! Go to the South Pole and see the clearest stars on the earth.

As the second generation telescope in China, the aperture of the Antarctic Sky Survey Telescope AST3 has increased to 50 cm, making it the largest optical telescope in Antarctica at present.

Looking up at the vast starry sky on earth, where can I see it most clearly? The answer is South Pole. People living on the "seabed" of the atmosphere are looking for observation points with clear and quiet atmosphere in order to see the starry sky as truly as possible. Recently, scientists in China discovered that Dome A, located at the highest point in the South Pole, is the best star-watching point in the world. There, the observation ability of the 2.5-meter optical telescope is comparable to that of other 6-meter telescopes. This will bring new opportunities for the development of Antarctic astronomy in China.

Where are the stars in the telescope's ideal home that don't like to blink?

Since ancient times, human beings have observed the vast starry sky with the naked eye. 1609, Galileo aimed his telescope at the night sky for the first time and discovered craters, sunspots and Jupiter's four largest satellites on the moon. This has greatly broadened the horizons of mankind and opened the door to modern astronomy.

The starry sky in Dome A, photographed by the Aurora Monitor of the National Astronomical Observatory of Chinese Academy of Sciences, contains a 360-degree panorama from the zenith to the ground, and the green light band near the ground is aurora.

After more than 400 years, with the progress of science and technology, many changes have taken place in optical telescope technology. In terms of aperture, the telescope in Galileo's era was only 4 cm in aperture, but the largest optical telescope that has been built so far has reached 10 m, and the aperture of ELT, a large telescope of the European Southern Observatory under construction, has reached 39 m-while the pupil of the human eye can only be enlarged to about 8 mm in the dark.

According to theoretical calculation, the resolution of a telescope with a diameter of 1 m can reach 0. 1 angular second (equivalent to looking at a coin at a distance of 10 km), but it has not been achieved for a long time. Why?

This is because of the existence of the atmosphere. We have all had this experience: the quiet water surface is crystal clear, but the turbulent current distorts the underwater scene. So is the air, full of turbulence.

Turbulence in the atmosphere will cause constant changes in the refractive index of the atmosphere in different places, just as the atmosphere is covered by a small deflection mirror. For example, atmospheric turbulence at high altitude will cause stars to "blink".

This group of atmospheric turbulence has added additional restrictions to the resolution of the telescope, and the actual resolution of the telescope can only be "compromised" to the larger of these two values-when the aperture of the telescope is smaller than the size of atmospheric turbulence, the actual resolution is the telescope resolution; On the contrary, even if the resolution of the telescope itself is higher, the actual resolution can only be the resolution of atmospheric turbulence.

Because the aperture of the optical telescope has reached the meter level, the atmospheric turbulence with a diameter of several centimeters to more than ten centimeters is always much smaller than the aperture of the telescope, and the corresponding visibility ranges from 1 to several seconds, which greatly limits the actual performance of the telescope. Therefore, in order to give full play to the maximum potential of telescopes, finding suitable homes for them has become the primary task of astronomical observation. The influence of atmospheric turbulence on the imaging resolution of telescope is called "atmospheric visibility", which is often used as one of the important bases for astronomical site selection.

After decades of hard site selection, the best sites in the world are now concentrated in the middle and low latitudes such as Hawaii, northern Chile and the Canary Islands of Spain. The altitude of these places ranges from more than 2,000 meters to more than 4,000 meters, the visibility is 0.6-0.8 seconds, and the proportion of sunny days at night reaches 70%-80%. Therefore, most of the world's 8 8- 10/0m telescopes and three 30m telescopes under construction are located in these areas.

Countries in the Antarctic Plateau are competing to find the best visibility.

Kunlun diopter telescope. Shanggongtu

The Antarctic continent is covered with thick ice all the year round, and the average thickness of the Antarctic ice sheet exceeds 2000 meters, forming the Antarctic inland plateau.

Due to its unique geographical location, the Antarctic Plateau has always been considered as an excellent observation site. However, due to its harsh natural environment and relatively weak logistical support, it was not until the 1990s that scientists began to make astronomical site selection at the Amundsen-Scott scientific research station in the Antarctic. The measurable results are quite disappointing: the visibility in Antarctica is poor, reaching 1.8 seconds, but the atmospheric turbulence is mainly concentrated in the boundary layer near the ground 100 meters, and the visibility in the free atmosphere above the boundary layer is only about 0.3 seconds.

The boundary layer is the lowest layer in the atmosphere, which is greatly influenced by the ground and concentrates most of the atmospheric turbulence. The boundary layer is called free atmosphere, and there is almost no turbulence. At excellent stations in the middle and low latitudes, the visibility of the free atmosphere is about 0.4 seconds, but the thickness of the boundary layer is about several hundred meters, and its influence is inevitable.

Katabatic wind prevails in the Antarctic Plateau, that is, the wind blows from high to low, and the Antarctic is only 2835 meters above sea level, which is in the gentle slope of the Antarctic Plateau, so the wind is still relatively strong, resulting in more atmospheric turbulence. But at several vertices of the Antarctic plateau, the wind speed will be very small, so there will be little turbulence, and the visibility is expected to be good. These vertices include dome a (80 22 ′ s, 77 21′ e, with an altitude of 4093m), dome c (75 06 ′ s,123 20 ′ e, with an altitude of 3233m) and dome f (77 30 ′).

Relying on the French-Italian scientific research site in Dome C, the international team carried out a large number of observation site surveys.

In 2004, the team of the University of New South Wales in Australia published the measurement results of nighttime visibility of Dome C in the British magazine Nature. At a height of 30 meters above the ground, the visibility reaches 0.27 arc seconds, which means that the boundary layer thickness of dome C is only about 30 meters.

Since then, the French and Italian teams have been monitoring with different instruments for many years, further confirming that the boundary layer thickness of hail C is about 30 meters, and the visibility of free atmosphere is about 0.3 seconds. Since Dome C is a wintering station, there are also explorers on duty in the station at midnight, and the power supply is sufficient. On this basis, the French-Italian team installed small-caliber telescopes in Dome C, namely ASTEP- South with a diameter of 10 cm and ASTEP-400 with a diameter of 40 cm, and carried out many searches for variable stars and extrasolar planets during the extreme night. They also proposed a 2-meter optical infrared telescope, but it was not approved or funded.

Japan has a stop in Dome F, but it is a summer stop. Japanese astronomers made a four-day visibility measurement at 20 1 1. 1. Because the height of the telescope is only 2 meters, it is greatly influenced by the turbulence in the atmospheric boundary layer, and the visibility obtained is 1. 1 arc second. On June 20 13, 13, they set up a 9-meter-high tower and observed it for more than 20 days. The median visibility during the day was about 0.5 seconds. When the thickness of the boundary layer is lower than that of the telescope, excellent visibility of about 0.3 seconds is also measured. However, so far, there is no measurement result of the visibility of Dome F at night ... In addition, Japanese astronomers have also proposed the project of building a telescope in Dome F for astronomical observation, but it has not been approved.

In addition, although the Antarctic is not a good optical observation site, it is called "white desert" because the air in the Antarctic Plateau is extremely dry, and it is an excellent radio and (sub) millimeter wave observation site.

Strong inversion layer and dome A were selected as "the best star-watching spots in the world"

From the point of view of scientific research, the Antarctic continent has four most important points: pole, freezing point, magnetic point and high point.

Kunlun diopter telescope. Shanggongtu

Before 1980s, the United States, Russia and France established scientific research stations at the first three points respectively, and only the highest point in the Antarctic inland, Dome A, was still "blank", which was called "the inaccessible pole".

Starting from 1997, China Antarctic expedition team has been advancing from Zhongshan Station to Dome A. After years of hard exploration, it finally planted the five-star red flag on the highest point of the Antarctic ice sheet in 65438+2005 10.

After learning this exciting news, China astronomers realized that this was a great opportunity in the history of astronomical observation in China. For a long time, China's astronomical observation has lagged behind the international advanced level. If we can make use of the excellent site conditions of Dome A and choose a unique telescope design, we can reach or even exceed the world-class level in some fields.

For this reason, China astronomers, together with the international astronomical team, and with the strong support of China Antarctic expedition, carried out the survey of observation point A of the ice dome. In June 5438 +2008 10, China astronomers reached Dome A for the first time, and then reached Dome A several times in the subsequent Antarctic expedition.

On June 5438+ 10, 2009, China established the Kunlun Station in the area of Hail A. During years of site survey, astronomers installed a variety of site survey instruments in the preliminary astronomical observation site established in the area of Kunlun Station, and obtained a lot of valuable site data. These data prove the superiority of Dome A as an observation point: the background of nightglow is dark, which is equivalent to or even darker than the best observation point in the middle and low latitudes; The proportion of sunny nights is high, and the proportion of cloudless is as high as 83%, which is slightly better than the best venue in the middle and low latitudes. The wind speed is low. The average wind speed at the height of 4m is only 1.5-4m/s, which is about the second-class wind, while the windy weather above 10m/s (about the fifth-class wind) is very rare.

Most importantly, the median thickness of the atmospheric boundary layer in dome A is only 14m, which is only half that of dome C! In other words, you only need to build a tower about 14 meters and install a telescope to get excellent free atmospheric visibility. In 20 19, the measurement records of the visibility measuring telescope KL-DIMM of the National Astronomical Observatory confirmed this point, and the visibility of the free atmosphere was directly measured around 0.3 1 arc second. The recently published Nature magazine published this achievement, saying that "Dome A is the best star-watching point on the ground". Compared with the excellent sites in the middle and low latitudes, the visibility of Hail A is better. Building a 2.5-meter optical telescope there can be compared with other 6-meter telescopes in other sites. Compared with the dome C in Antarctica, the construction difficulty and cost of dome A are greatly reduced.

The thin boundary layer of dome A is due to its unique meteorological conditions. Because the Antarctic plateau is covered with ice, it is dark for several months at night. Without the sun's irradiation and heating, the snow surface will rapidly radiate and cool down, which will also cool the air near the snow surface, resulting in the lower the air height near the ground and the lower the temperature. This is the so-called "inversion layer" Due to the existence of inversion layer, the Antarctic air is very stable as a whole. How strong is the inversion layer of dome A? Astronomers set up a meteorological tower with a height of 14 meters, and placed thermometers every 2 meters. At the height of only 2m from the snow surface, the temperature is nearly 15℃ higher than the ice surface, and at the height of14m from the ice surface, the temperature is 20℃ higher than the ice surface. Such a strong inversion layer forms the thinnest boundary layer on the ground. The actual measurement data also confirmed that the greater the temperature difference, the lower the height of the boundary layer.

Watching the sky day and night at Kunlun Station, China's Antarctic astronomical research is at the forefront of the world.

At present, the astronomical station of China Antarctic Kunlun Station has begun to take shape. In addition to field survey, some astronomical observations were made.

There are two major challenges in operating the telescope at Kunlun Station: first, the extremely harsh natural environment, the average temperature in winter is around -60℃, and the lowest is over -80℃, during which the water vapor in the air is always close to saturation, which is very easy to frost; Second, Kunlun Station is still a summer station. Every year in Antarctic summer, only more than 20 days of scientific research personnel are present for necessary maintenance, and the rest of the time is unattended, so all equipment must run automatically and reliably.

Astronomers in China installed two generations of optical telescopes at Kunlun Station. The first generation is China Star small telescope array CSTAR, with a diameter of only 14cm, but the field of view is 20 square degrees (about 8 1 full moon is arranged in a square), and the telescope is fixed to the south celestial pole. The second generation telescope is the Antarctic survey telescope AST3, and its aperture has been increased to 50 cm. It is the largest optical telescope in Antarctica, with a field of view of 4.3 square degrees and complete pointing and tracking functions.

Using the observation conditions of Dome A and the dark night in the polar night for several months, CSTAR and AST3 have carried out research mainly on time-domain astronomy, that is, to study the changes of various celestial bodies with time.

China Satellite Telescope Array (left), yellow is the instrument cabin of Plato -A support platform for China-Australia cooperation.

In the quiet night sky, there are stars changing all the time-some are changing their positions, such as asteroids, comets, artificial celestial bodies, and some asteroids may even crash into the earth, seriously threatening mankind; Some brightness changes, brightens, darkens and even bursts out of thin air.

If a star also has a planet, when the planet orbits in front of our eyes, it will block the light of its parent star, making the star look very dark, and the darkening range is the area ratio of the planet to the parent star, which is less than 1%, or even110000,1/kloc. AST3 is the first large-scale exoplanet search conducted by astronomers in China, and more than 100 high-quality candidates have been successfully found, which is under further study.

In addition to these small changes, there are violent explosions in the universe, such as supernovae when stars die. Similar to AST3, there are a large number of telescopes in the world monitoring the night sky, looking for and studying changing celestial bodies. In order to improve the efficiency, the search method mainly adopts the image subtraction technology, that is, taking an image of a sky area, and subtracting it from the previous image (template), the celestial bodies with constant brightness will be basically eliminated, and after careful screening by the computer, the truly changed celestial bodies will be finally confirmed by manual inspection.

On August 17, 2065438, the event of double neutron stars merging GW 1708 17 was observed for the first time, and it was observed in both gravitational waves and electromagnetic waves, which opened the era of multi-messenger astronomy. AST3 also participated in the follow-up observation of the signal, providing important data for studying its physical mechanism.

China's Antarctic astronomy started late, but after 10 years of rapid development, it has been at the forefront of the world. Astronomers in China have accumulated a lot of valuable experience in the development technology of telescopes in extreme environments and unattended automatic observation technology, which has laid a solid foundation for the development and operation of large Antarctic telescopes in China in the future.

Author: Ma Bin

Photos: Unless otherwise specified, all are provided by Ma Bin.