Abstract
Infrared astronomy is crucial for exploring the universe, but due to limitations in detector performance and the scarcity of suitable sites, China suffers from a severe shortage of astronomical observation equipment in the infrared band. Kunlun Station in Antarctica possesses unique natural conditions such as dryness, cold, and long polar nights, providing unparalleled advantages for near-infrared astronomical observations. However, it also faces challenges such as harsh environments, unmanned operation, limited energy, and snow and frost accumulation. This paper summarizes the advantages and challenges of near-infrared astronomical observation in Antarctica, the current status and plans for Antarctic infrared astronomy both domestically and internationally, and analyzes the key technologies of Antarctic infrared telescopes and existing technological accumulations (mainly including methods for suppressing radiation from the telescope itself and instruments, methods for de-icing mirrors, infrared detector technology, telescope control systems, and optimized dewar/cryostat design), providing technical references for the future development of Antarctic infrared telescopes.
0 Introduction
In astronomy, infrared radiation usually refers to electromagnetic waves with wavelengths ranging from about 0.75 to 350 μm [1], which can generally be divided into three sub-bands: near-infrared(0.75-2.5 μm), mid-infrared(2.5-25 μm) and far-infrared(25-350 μm) [2]. The transmission window of the Earth's atmosphere to infrared light is mainly located in the near-infrared and mid-infrared regions. However, due to the strong influence of atmospheric thermal radiation on the mid-infrared band, the background noise of the sky is extremely high, so ground-based infrared astronomical observations are mainly concentrated in the near-infrared band.
In recent years, infrared astronomical observation has received increasing attention internationally due to its unique advantages. In the study of the origin of life, the characteristic spectral lines of many molecules(such as methane, water, and carbon dioxide) that are closely related to extraterrestrial life are concentrated in the infrared band. By analyzing these spectral lines, we can explore key issues such as the atmospheric composition of exoplanets and the formation mechanism of planets. In the study of the origin of the universe, due to the cosmological redshift, the wavelengths of ultraviolet and visible light radiation emitted by galaxies formed in the early universe have shifted to the infrared band when they reach Earth. Therefore, infrared observation is the most effective means to discover these distant early galaxies and characterize their properties [3].
As the driest and coldest continent on Earth, Antarctica's unique geographical location and extreme climate conditions create excellent opportunities for near-infrared astronomical observations. This paper first discusses the advantages of conducting near-infrared astronomical observations in Antarctica, the challenges in instrument development and operation, and then analyzes the key technologies of the Antarctic Near-Infrared Survey Telescope from multiple perspectives.
1 Advantages and challenges of Antarctic near-Infrared astronomy
Dome A, the highest point of the Antarctic inland ice sheet, is located at 80°22'00"S, 77°21'11"E, at an altitude of 4093 m. Actual data show that the summer temperature is between-30 ℃ and-50 ℃, and the relative humidity is 55% to 70%; the winter temperature is between-45 ℃ and-80 ℃, and the relative humidity is 30% to 55%; the winter wind speed is slightly higher than the summer wind speed; within the 14 m height range, the higher the height, the greater the average wind speed; the summer average wind speed at a height of 4 m is 4 m/s, and the winter average wind speed is 4.5 m/s [4], the atmospheric boundary layer thickness is about 14 m [5], the median value of free atmospheric seeing is 0.31", and the optimal value is 0.13" [6].
Such natural conditions have unique advantages for near-infrared astronomical observation. First, water vapor is one of the main sources of infrared radiation absorption. As the driest continent on Earth, Antarctica has extremely low water vapor content in its atmosphere, which allows infrared radiation to penetrate the atmosphere more effectively. Second, the background noise of infrared astronomical observation mainly comes from the thermal radiation of the atmosphere and the telescope itself, while the low temperature environment of Antarctica significantly reduces the background radiation of the sky and the thermal radiation noise of the instruments, thus improving the sensitivity and signal-to-noise ratio of infrared observation. In addition, the continuous polar night period in Antarctica for several months and the high clear night rate of more than 80% [7] provide astronomers with the opportunity for long-term continuous observation.
The emission lines of atmospheric glow, especially those of hydroxyl(OH) groups in the near-infrared band, have a significant impact on infrared astronomical observations by ground-based telescopes. As shown in Figure 1, in the range of 1 to 2.3 μm band, the OH spectral lines are densely distributed, while the Kdark band of 2.25-2.5 μm is located in the region of low atmospheric glow intensity [8], which can significantly reduce the influence of atmospheric glow, effectively improve the signal-to-noise ratio, and thus enhance the detection performance of the telescope. As shown in Figure 2, in the Kdark band, the atmospheric transmittance in Antarctica is high, and the infrared background radiation is about 1/40 of that at mid-latitude sites. The Kdark band of 2.25-2.5 μm is the optimal band for infrared observation in Antarctica [8].
Fig.
2
Antarctic Kdark band:(a) high atmospheric transmittance;(b) low infrared background radiation [8]
In recent years, several infrared telescopes have been or will be deployed in Antarctica(see Table 1). Around 2008, the 80 cm infrared telescope developed by Italy—the International Robotic Antarctic Infrared Telescope(IRAIT) [9]—was transported to Dome C in Antarctica for installation. Due to problems with some systems, IRAIT only completed engineering testing and was not put into regular scientific operation. In 2012, the ASTEP South telescope was successfully deployed at Dome C in Antarctica [10]. This telescope is used for high-precision photometric observations of exoplanets and variable stars. It has an aperture of 40 cm, a field of view of 1 deg2, and its main working band is the R-band of visible light. Australia and the United States have also been iterating on related technologies on several infrared telescopes. In 2018, the Palomar Gattini-IR telescope was put into trial observation at the Palomar Observatory in the United States. The telescope has a diameter of 30 cm, a field of view of 25 deg2, and operates in the J band [11]. In 2023, the DREAMS telescope [12] was tested at the Siding Spring Observatory in Australia. The telescope has a diameter of 50 cm, a field of view of 3.8 deg2, and operates in the Y, J, and H bands. It is equipped with a high-sensitivity infrared detector(7.8 megapixels). Australia and the United States recently plan to develop a fully cooled telescope with an aperture of 26 cm-the Cryoscope Pathfinder, which will be installed at Dome C in Antarctica in 2026. Its focal ratio is f/2; it adopts an innovative double-lunar corrector design with a field of view of 16 deg2; it uses a 2048×2048 mercury cadmium telluride detector with a pixel pitch of 18 μm, a pixel resolution of 7.1"/pixel, and a working band of 2.25-2.5 μm in the Kdark band [13]. This telescope is a precursor to the planned Cryoscope telescope(with an aperture of 1-1.2 m and a field of view of 50 deg2) [14].
Table
1
Performance parameters of foreign Antarctic infrared telescopes
The Chinese astronomical community has been actively promoting the development of Antarctic infrared astronomy. In 2019, the Near-Infrared Sky Brightness Monitor(NISBM) developed by the University of Science and Technology of China was installed at Kunlun Station in the Dome A region of Antarctica, and obtained for the first time the night sky background data of the J, H and Ks bands of Kunlun Station in Antarctica(using an indium gallium arsenide detector) [14]. In 2023, the Antarctic Infrared Binocular Telescope(AIRBT) of Sun Yat-sen University was successfully installed at Dome A. The telescope also uses an indium gallium arsenide detector and consists of two 15 cm aperture infrared telescopes, which can simultaneously conduct dual-band infrared observations in the J and H bands. In 2024, the 40th Antarctic Expedition successfully installed the Antarctic Near-Infrared Telescope at Dome A [15]. The telescope has an aperture of 15 cm, a field of view of 0.87°×0.69°, and operates in the J band. It uses a SONY-IMX990 indium gallium arsenide detector with a pixel resolution of 2.5"/pixel and an average image quality of 1.4 pixels across the entire field of view. The team used the telescope to complete daytime star observation, near-infrared sky brightness background measurement, and daytime observation experiments of near-Earth orbit satellites and space debris [16]. The Nanjing Institute of Astronomical Optics and Technology of the Chinese Academy of Sciences plans to deploy the Antarctic Survey Telescope AST3-3 at Kunlun Station in Antarctica in 2026. Its effective aperture is 50 cm and its focal ratio is 4.48. It uses four 640×512 mercury cadmium telluride detectors with a pixel pitch of 15 μm, a pixel resolution of 1.38"/pixel, and a field of view of 0.98°×0.78°. It uses a Stirling cooler to cool the chip to 77 K and the secondary optical elements to 150 K.
Table
2
China's Antarctic Ice Domes Performance parameters of an infrared telescope
Building a telescope in Antarctica is challenging. The natural conditions at Kunlun Station in Antarctica are harsh: the temperature is extremely low during the polar night(-50℃ to-80 ℃), and there are strong winds with wind speeds exceeding 10 m/s many times throughout the year, with an air pressure of only about 580 hPa [17]. These extreme environments pose a severe test to the operation of the equipment: low temperatures can cause ice to form on the surface of the equipment or cables, increasing the load on the equipment; the combined effect of low temperatures and strong ultraviolet radiation can cause materials to degenerate, such as the outer sheath of cables becoming hard, brittle, or even cracking; the viscosity of lubricants increases or even solidifies at low temperatures, leading to lubrication failure; strong winds can cause snow to accumulate in the gaps of the equipment or penetrate into the equipment, causing moving parts to jam or electronic equipment to fail; in addition, snowfall, snow blowing, and temperature changes can also cause the optical mirrors to freeze or frost, seriously affecting the quality of observation, or even making observation impossible.
Kunlun Station in Antarctica are also extremely harsh: during the polar night that lasts for several months, the power supply mainly relies on diesel generators, and the current total power supply is only 1 kW, which puts extremely high demands on the energy consumption management of the equipment; the scientific expedition team can only come to the station for maintenance for about 20 days each year, and the rest of the time is completely unattended, which also puts extremely high demands on the reliability of the equipment [18]. Since 2025, Zhongshan Station has begun to deploy high-speed network communication of China's independent satellites, and Kunlun Station in Antarctica is about to enter the era of high-speed network; but before that, the communication between China and Kunlun Station in Antarctica is still mainly based on low-bandwidth, high-cost satellite communication, such as Iridium communication and maritime satellite communication systems. These satellite networks have problems of high instability and large delay, which limits the efficiency of real-time interaction and data acquisition.
These unfavorable conditions place extremely high technical demands on the astronomical observation equipment at Kunlun Station in Antarctica, while also providing important opportunities for the innovation and development of related technologies.
2 Key technologies of the Antarctic near-infrared survey telescope
2.1 Methods for suppressing infrared radiation from the telescope itself and the instrument
According to Planck's law of blackbody radiation, any object with a temperature above absolute zero(-273.15 °C) continuously radiates electromagnetic waves, and the intensity and wavelength distribution of this radiation depend on the object's own temperature. Therefore, in addition to atmospheric infrared background radiation, the thermal radiation from the Antarctic telescope and its instruments is also received by the infrared detectors, thus reducing image quality. Consequently, the telescope's optical system needs to strictly control the impact of the thermal radiation from the imaging front-end devices on the observations.
The infrared radiation of the Antarctic infrared telescope itself and the instrument mainly includes the radiation of the telescope mirror and structure, the radiation of the sealed windows and surfaces inside the cold box, and stray light inside the instrument. When the Antarctic infrared telescope is working, its temperature is usually close to the ambient temperature(-40 ℃ to-80 ℃). The self-radiation of the telescope and the internal components of the instrument corresponding to this temperature range can be calculated by three physical quantities: the absolute temperature T of the blackbody radiation spectrum determined by the Planck function Bλ(T), the radiation coefficient ε(λ) of each component that determines the proportion of blackbody radiation contribution, and the solid angle pointing to the detector plane. Figure 3(a) shows the comparison of the self-radiation intensity of the telescope at different temperatures and in different bands. Among them, the self-thermal radiation intensity of the K-band telescope is 104 to 106 times that of the J-band [19]. As shown in Figure 3(b), in the Kdark band, the self-thermal radiation of the Antarctic telescope is higher than the background radiation of the sky [8]. Therefore, the K-band telescope is more severely affected by self-radiation, and special attention must be paid to the suppression of the infrared background.
Fig.
3
(a)The intrinsic radiation intensity of the telescope at different temperatures and in different bands [19]；(b) Comparison of the intrinsic thermal radiation of the Antarctic telescope instruments with the backgroundradiation of the sky [8]
The infrared background suppression of the telescope optical system mainly adopts the following two methods:(1) Based on the design of the secondary imaging system, the exit pupil position is controlled near the front of the detector, and a cold stop is placed at the pupil surface to ensure that the optical engine part behind the cold stop is in a low temperature environment;(2) The optical components of the imaging optical system and its lens barrel are used as an integrally cooled optical system.
Secondary imaging optical systems typically employ cold apertures and light shields to suppress direct or multiple reflections of the telescope's own thermal radiation from entering the detector, thereby reducing background noise. Based on the analysis results of the telescope's own thermal radiation and ghost images, and taking into account the effectiveness of thermal radiation suppression, cost, and the complexity of stray light suppression methods, special design optimizations are performed.
The Antarctic Near-Infrared Survey Telescope(AST3-3) employs a modified Schmidt-type optical system. As shown in Figure 4, the telescope exit pupil is formed within the secondary imaging optical path, and a cold aperture is placed at the exit pupil position to prevent all out-of-field scattered light and thermal radiation from warm surfaces from reaching the detector plane, thereby reducing the impact of thermal radiation on the system. A drawback of this design is that the system's field of view is constrained due to the limitations imposed on the optical path design by the aperture position control. To further reduce the impact of thermal radiation stray light on the system's detection capability, AST3-3 suppresses system stray light by adding a field of view aperture. Since the focal plane of the modified Schmidt-type optical system is not behind the primary mirror, primary and secondary mirror shields are no longer the most effective means of stray light suppression for this type of system. Adding a field of view aperture is a more direct and effective method that can significantly reduce the system's stray light level. By placing the field of view aperture at the primary imaging plane position, it is possible to effectively prevent light from outside the field of view from reaching the image plane directly without being scattered by optical elements or support elements, thus reducing the impact of stray light. In summary, the Antarctic Near-Infrared Survey Telescope did not adopt a traditional sunshade design. Instead, it achieved significant results in suppressing infrared background radiation and stray light by combining a cold aperture with a field aperture, thereby improving the system's detection performance.
Fig.
4
Secondary Imaging Optical System of AST3-3 Telescope
Partfinder telescope [13] is an example of an integrated cooling optical system. As shown in Figure 5, the telescope body adopts a sealed design, which reduces the heat radiation of the primary mirror and the tube itself through integrated cooling. At the same time, the corrector mirror located outside the telescope body is not cooled, and its heat radiation is reflected out of the telescope system through the complex ellipsoidal hood design, reducing the heat radiation entering the detector. The surface of the hood also uses a dichroic coating to absorb light within the band and reflect long-wave radiation, further reducing the interference of stray light. In order to achieve rapid cooling of the optical system, the primary mirror adopts a lightweight aluminum primary mirror with diamond single-point turning, and the hood adopts a petal-shaped multi-layer structure. This integrated design makes the Cryoscope Partfinder telescope perform well in suppressing heat radiation and stray light, significantly improving the detection performance of the system.
Fig.
5
(a) Mechanical layout of the Cryoscope Pathfinder telescope; (b) Stray light suppression analysisof the petal-shaped hood [13]
2.2 Mirror defrosting
Antarctic optical/infrared telescopes typically have at least one or more optical mirrors exposed to extreme environments. When the relative humidity-saturated air of Antarctica passes over an optical mirror with a temperature below its freezing point, water vapor will sublimate and adhere to form a frost layer, as shown in Figure 6 [20]. The frost layer will significantly reduce the light transmittance of the mirror and greatly reduce the telescope efficiency; the uneven distribution of the frost layer will also introduce randomly varying phase, light scattering and distortion, which will greatly reduce the resolution and imaging quality of the optical system; the frost layer will cause damage to the quality of observation data, affecting high-precision photometry and spectral analysis, especially negatively impacting the data quality of cutting-edge research such as dark matter and exoplanets.
Fig.
6
Several forms of frost adhering to the mirror surface of an Antarctic telescope: (a) frost;(b) ice and snow; (c) snowstorm
Currently, the visible light telescopes(such as CSTAR, AST3, KL-DIMM, etc.) operating at Kunlun Station in Antarctica all use closed tubes to minimize the impact of ice and snow on the optical components inside the tubes. For the entrance pupil mirror, a layer of conductive transparent indium tin oxide(ITO) conductive film is coated [21]: when energized, it generates heat to eliminate ice and snow on the mirror and prevent frost formation. However, this technology cannot be applied to the entrance pupil mirror of infrared astronomy. First, the ITO film has good transmittance in the visible light band, but in the near-infrared band above 1.2 μm, the transmittance drops significantly [22], as shown in Figure 7. Second, the heat generated when the ITO film is heated will become an important source of noise in the observation. Analysis of the AST3-3 telescope shows that the ITO heating film is the largest source of its own heat radiation, accounting for 31% [8]. Therefore, optimizing heating methods or exploring other ways to remove ice and snow from the telescope and prevent frost formation are of vital importance for reducing the telescope's own thermal radiation and improving observation quality.
Fig.
7
Transmittance and absorptivity curves of ITO thin film in the visible and infrared bands [30]
Physical methods such as mirror blowing and mirror snow removal were tried on the AST 3-2 telescope which observes in the visible light band and the first near-infrared telescope in Antarctica [19]. The Cryoscope telescope in Australia also plans to use the method of blowing dry air to prevent frost on the entrance pupil mirror [13]. Figure 8(a) shows the mirror blowing scheme used by the AST 3-2 telescope, and Figure 8(b) shows the mechanical snow removal device of the near-infrared telescope. The results show that mirror blowing can effectively improve mirror seeing; the mechanical snow removal device can effectively alleviate the effect of floating snow on extinction [19]. Figure 9 shows the comparison of star image information before and after mechanical snow removal. The Antarctic Near Infrared Survey Telescope is designed with a mechanical snow removal scheme(see Table 3), combined with dynamic image quality detection, and the frequency of snow removal is controlled by the closed loop of system transmittance; at the same time, the blowing scheme is assisted to achieve the purpose of snow prevention and frost removal without heating the mirror.
Fig.
8
(a) AST3-2 mirror blowing system; (b) Mechanical snow removal system for the near-infrared telescope
Table
3
Comparison of de-icing methods for Antarctic telescope mirrors
2.3 Infrared detector
Infrared detectors are devices that can convert received infrared radiation energy into physical quantities that are easy to measure [2]. Compared with other fields, astronomical observation is characterized by its extreme requirements for detecting extremely weak signals; the requirements for parameters such as quantum efficiency, dark current, and readout noise of detectors are much higher than those for conventional applications. This is because the infrared radiation of celestial bodies is extremely weak, requiring detectors to have extremely high sensitivity and to maintain low noise during long-term integration observations in order to obtain clear and accurate signals. Celestial observation requires the readout noise of infrared devices to be a few to tens of electrons, and the dark current is generally at the level of a few electrons per second or even less. Infrared detectors used for astronomical observations can employ different material systems depending on their operating wavelength and detection requirements. In the 1–5 μm wavelength range, mercury cadmium telluride, indium telluride, and indium gallium arsenide detectors are commonly used. In longer wavelength ranges, doped materials such as silicon or germanium, such as Si:As and Ge:Ga, are more frequently employed.
As mentioned earlier, Antarctic astronomical observations primarily employ mercury cadmium telluride(HgCdTe) and indium telluride detectors, which require cooling to reduce noise generated by heat from internal components and the sensors themselves. Indium gallium arsenide(InGaAs) is also a common uncooled detector material, but since its operating band is typically the short-wave infrared band of 0.9-1.7 μm, the Antarctic Near-Infrared Survey Telescope, which focuses on observations in the 2.25-2.5 μm band, will use a cooled HgCdTe detector.
Mercury cadmium telluride is a pseudobinary solid solution semiconductor material composed of mercury telluride and cadmium telluride in a certain proportion, with a zinc blende crystal structure. Its response spectrum can be continuously adjusted by changing the cadmium content and can cover a fairly wide infrared band. It is an excellent material for preparing infrared detectors. MCT can grow large-area materials on heterogeneous substrates(such as silicon, germanium, gallium arsenide, and alumina) to meet the requirements of ultra-large-scale MCT infrared focal plane detectors [2]. At present, it is difficult to build large-scale MCT detector arrays. The mainstream single chip size internationally is 2048×2048. In many telescopes, a larger array is obtained by splicing. The largest MCT chip used for astronomical observation in China is 640×512. Its dark current can reach less than 5 electrons and the readout noise is less than 80 electrons(see Figure 10).
Fig.
10
Measured data for domestic mercury cadmium telluride chips: (a) dark current; (b) readout noise
2.4 Telescope control system
In order to adapt to the extreme low temperature, ice and snow invasion, and unattended operation environment of Kunlun Station in Antarctica, the telescope control system needs to be specially designed. The Nanjing Institute of Astronomical Optics and Technology has carried out a series of studies on fault diagnosis and reliability analysis of the control system [23-26].
A design of an axial disc permanent magnet synchronous motor combined with a labyrinth seal structure [27] has been applied to the Antarctic telescope. The axial disc permanent magnet synchronous motor has a flat disc structure: the outer shell is generally pressed with steel plate; the permanent magnet can be made of ferrite or neodymium iron boron, arranged alternately with N and S poles, and fixed on one or both end caps(see Figure 11). After the winding is wound, it is generally pressed into a disc structure with epoxy resin and fixed in the armature. The axial disc permanent magnet synchronous motor has the advantages of both disc motors and permanent magnet motors, with high efficiency and high power density, and a compact structure that occupies less space, making it more suitable for use in complex environments.
In order to cope with the unmanned operation mode in Antarctica, the telescope software system suite realizes the functions of fully automatic observation, remote control operation and maintenance, and local data processing of the telescope(see Figure 12). The observation control software system(OCS) has established a fully automatic workflow [28] and pipeline data processing suite [29], which can complete fully automatic observation and Antarctic astronomical data processing locally. The telescope control software system(TCS) realizes the functions of remote control, remote calibration, real-time status monitoring and fault diagnosis of the telescope [18].
2 Optimized design of 5 Dewars/cold box
A Dewar/cold box is a component that provides a vacuum and cryogenic environment for detectors and optical elements. It is designed to reduce noise from detector components and suppress the effects of thermal radiation and stray light from optical elements and their supporting structures on detection performance. A Dewar/ cold box is typically a vacuum-insulated container, achieving insulation through a vacuum structure and a high-reflectivity coating(such as gold plating) to prevent external heat from entering.
The dewar/cold box mainly comprises a vacuum chamber, light-transmitting window, aperture, cryogenic optical tube, mirror, filter, and detector chip, as well as a refrigerator and heat dissipation device. The dewar/cold optical box assembly integrates optical, thermodynamic, and electronic interfaces, representing a typical example of multidisciplinary integrated design. Figure 13 shows a cross-sectional view of the dewar structure of the AST3-3 telescope. Its key technologies include: selecting appropriate processes for the light-transmitting window, mechanical refrigerator, cold box/dewar shell coating, and external electrical components to maintain vacuum levels, considering the Antarctic's extreme temperatures(-80 ̊C), large temperature fluctuations, and ice and snow cover; heat dissipation of the mechanical refrigerator and-80 ̊C cryogenic cold start technology; and overall power distribution and control under the low-power requirements of Antarctica.
Fig.
13
Cross-sectional view of the dewar structure of the AST3-3 telescope
3 Conclusion
Developing near-infrared survey telescope technology in Antarctica and conducting K-band sky surveys in Antarctica are of paramount importance to the field of contemporary astronomy. This article analyzes and summarizes some key technologies and existing technological accumulations, providing a reference for the future development of Antarctic near-infrared telescopes. However, potential technological innovations and breakthroughs extend far beyond what is presented in this article. For example, very long baseline interferometry(VLBI) technology can be used as a reference to explore cross-station array linkage and construct a distributed observation link between Zhongshan Station, Taishan Station, and Kunlun Station. Regarding improving detection limits and survey efficiency, the development of large-area infrared detectors still needs to overcome bottlenecks in materials and processes, readout and noise, cryogenics and thermal control, splicing and packaging, and cost control. In terms of adapting to extreme environments, new low-temperature anti-frost materials need to be developed to solve the problem of mirror frost formation in extremely cold environments of-80 ̊C. Regarding the optimization of observation and data application, it is also necessary to improve the data transmission and processing system, establish a collaborative platform for near-station preprocessing and domestic remote in-depth analysis, and promote the rapid sharing and transformation of observational data. Furthermore, a multi-band collaborative observation system should be constructed to promote the coordinated observation of near-infrared telescopes, visible light survey telescopes, and terahertz telescopes, achieving multi-band data complementarity for sudden astronomical events and enhancing the scientific value of observational results.