Preamble

Astronomical Techniques and Instruments, Vol. 2, November 2025, 358–365 Article Open Access Performance analysis and strategy optimization of mechanical defrosting for an Antarctic near-infrared telescope using aper- ture photometry Jiali Chen 1,2,3 , Zhengyang Li 1,2,4* , Zhixu Wu , Jia’nan Cong 1,2,3 , Zichong Zhang Kaiwen Zheng 1,2,3 1 Nanjing Institute of Astronomical Optics Technology Chinese Academy of Sciences Nanjing 210042, China 2 CAS Key Laboratory of Astronomical Optics Technology Nanjing Institute of Astronomical Optics Technology Nanjing 210042, China 4 Polar Research Institute of China Shanghai 200136, China

5 Nanchang University

, Nanchang 330031, China

*Correspondences:

INTRODUCTION

Infrared observations are capable of penetrating inter- stellar dust, and are extensively employed in astronomi- cal research to probe protostellar evolution within star-form- ing regions, analyze the atmospheric composition of exo- planets, trace accretion processes in active galactic nuclei, and detect radiation signatures from high-redshift objects in the early universe. At typical astronomical sites, the per- formance of infrared telescopes in terms of survey depth,

limiting magnitude, and exposure time of astronomical imaging systems is limited by near-infrared sky back- ground radiation. Dome A in Antarctica possesses unique geographical advantages, with thin, dry air containing extremely low precipitable water vapor, high atmospheric transparency, and exceptional seeing conditions. Optical telescopes deployed at this site can achieve uninterrupted observations for 3–4 consecutive months, significantly improving observational efficiency [ 1 ] . With winter tempera- tures below −80°C at Dome A, thermal infrared back-

3 University of Chinese Academy of Sciences , Beijing 100049, China

6 School of Astronomy and Space Science , Nanjing University , Nanjing 210093, China

7 Key Laboratory of Modern Astronomy and Astrophysics Nanjing University Nanjing 210093, China © 2025 Editorial Office of Astronomical Techniques and Instruments, Yunnan Observatories, Chinese Academy of Sciences. This is an open access article under the CC BY 4.0 license ( Citation: Chen, J. L., Li, Z. Y., Wu, Z. X., et al. 2025. Performance analysis and strategy optimization of mechanical defrosting for an Antarctic near-infrared telescope using aperture photometry.

Astronomical Techniques and Instruments (6): 358−365.

Abstract

Dome A, in Antarctica, offers an exceptional site for ground-based infrared astronomy, with its extremely low atmospheric infrared background noise and excellent seeing conditions. However, deploying near-infrared telescopes in the harsh environment of Antarctica faces the critical challenge of frost accumulation on optical mirrors.

While indium tin oxide heating films effectively defrost visible-band Antarctic astronomical telescopes, their thermal radiation at infrared wavelengths introduces significant stray light, severely degrading the signal-to-noise ratio for infrared observations. To address this limitation, we have designed a mechanical snow-removal system capable of efficiently clearing frost from sealing window surfaces at temperatures as low as –80°C. Aperture photometry of target sources, Canopus and HD 2151, revealed that after six days without intervention, floating snow extinction reduced target brightness by up to 3 magnitudes. Following mechanical defrosting, the source flux recovered to stable levels, with measured magnitudes showing rapid initial improvement followed by stabilization. Data analysis indicates that a frost removal strategy operating every 48 h, with each operation consisting of 4–6 cycles, enables efficient removal of frost and snow without introducing additional thermal noise. Future work will focus on optimizing the adaptive control algorithm and exploring novel low-temperature defrosting materials to extend the periods during which Antarctic infrared telescopes can operate unattended.

Keywords

Dome A; Antarctic infrared thermal radiation; Mechanical defrosting; Aperture photometry

ground radiation shifts longward of 4 µm, with signifi- cant corresponding suppression of near-infrared sky back- ground noise. A collaboration between a research team led by Dr. Jian Wang, from the University of Science and Technology of China, and the Astronomy Research Labora- tory of the Polar Research Institute of China, has devel- oped a near-infrared sky background measurement instru- ment for Antarctic observations. The instrument mea- sures the intensity of the sky background at Dome A in Antarctica across the J, H, and Ks bands , with values ranging from 600–1 100 µJy arcsec 600 µJy arc- , and 200–900 µJy arcsec . This demonstrates that Dome A in Antarctica, with its optimal visibility and geo- graphical location, is regarded as the foremost site on Earth for far-infrared to submillimeter wave observations.

Antarctic optical and infrared telescopes typically have one or more exposed optical surfaces required to with- stand the extreme environment. During Antarctic polar night, temperatures fall to −80°C, while the relative humid- ity level approaches 100%. When supersaturated air flows across optical surfaces below the frost point, direct deposi- tion of water vapor occurs, forming frost layers on mir- . Frost accumulation significantly reduces mirror trans- mittance, severely diminishing telescope efficiency.

Uneven frost deposition induces random phase fluctua- tions, amplifies scattered light, and introduces image distor- tion, collectively degrading optical resolution and image quality. Critically, frost contamination compromises data integrity by impairing high-precision photometric and spec- troscopic measurements, which is particularly detrimental for frontier investigations of dark matter and exoplanets that rely on high-sensitivity measurements. To address this issue, the Nanjing Institute of Astronomy Optics & Technology (NIAOT), Chinese Academy of Sciences, devel- oped anti-frost technology for Indium Tin Oxide (ITO) film coatings on astronomical telescope mirrors. This tech- nology was first successfully implemented on the Chi- nese Small Telescope Array (CSTAR), at Kunlun Station in Antarctica (located approximately 7.3 km southwest of Dome A, at an altitude of 4 087 m), in January 2008 and has since been adopted for use on the Antarctic Sur- vey Telescope (AST3-1, AST3-2, and AST3-3) and space debris telescopes.

However, for Antarctic infrared telescopes, ITO-based mirror defrosting introduces additional infrared thermal radi- ation at the optical pupil plane. This thermal radiation gen- erates isoplanatic stray light across the entire field of view, which cannot be mitigated through conventional baf- fling or absorption techniques, ultimately degrading observa- tional contrast and sensitivity. Furthermore, conditions at Kunlun Station are currently not equipped for personnel to overwinter, preventing manual intervention to remove frost and snow from the mirror surface. To enable the removal or reduction of frost and snow accumulation on the mirror surface of Antarctic telescopes and mitigate loss of light transmission or reflectivity, and to overcome the challenge of thermal noise from conventional heating-

based defrosting methods, we present a mechanical snow- removal device and a fully automated mechanical de-icing method specifically designed for the first near-infrared tele- scope in Antarctica. These systems facilitate the efficient clearance of loose surface snow from the sealing window at temperatures as low as −80°C while maintaining observa- tion quality. In this paper, we first establish a thermal radia- tion model for ITO heating and de-icing, in which the seal- ing window has a circular temperature distribution. This model is combined with the Stefan-Boltzmann law to quan- titatively analyze the infrared thermal radiation emitted by mirror heating. Subsequently, we analyze photometric data acquired by the Antarctic 15 cm Near Infrared Tele- scope (15-NIRT) operating under mechanical de-icing.

We conduct continuous observations of the target sources Canopus and HD 2151 using aperture photometry and ana- lyze the extinction ratios of these targets at different snow removal and de-icing frequencies, along with trends in their apparent magnitudes. Finally, based on the analysis results, we optimize the mechanical de-icing strategy to minimize any potential impact on telescope operation.

ANALYSIS OF THERMAL RADIATI ON FOR HEATED SEALING WINDOW DEFROSTING ITO Heating Defrosting Principle ITO is a functional thin-film material composed of 90% indium oxide (In ) and 10% tin oxide (SnO ). In optoelectronics, ITO functions as a thin-film material with a wide bandgap, ranging from 3.5 to 4.3 eV. Because of its high carrier concentration and electron mobility, ITO exhibits low resistivity, reaching levels as low as Ωcm, and demonstrates excellent electrical conductiv- . Consequently, de-icing approaches using ITO- coated mirrors, coupled with electrode heating, have been investigated shows the frosting issue encoun- tered during the operation of AST3-2 in the extreme envi- ronment of Antarctica. The results confirm that a combina- tion of ITO coating and electrode heating can effectively clear frost and snow from the window surface.

Early implementations of ITO-based de-icing emplo- yed direct current (DC) heating with point electrodes.

These electrodes were significantly smaller than the mir- ror surface area, and de-icing was achieved by applying a fixed voltage difference between the positive and nega- tive terminals. However, in this method, the current is local- ized near the electrodes, causing reduced current density in regions distant from the electrodes and consequently resulting in heterogeneous temperatures across the mirror surface. This consequent heterogeneous temperature distribu- tion causes non-uniform thermal expansion across the mir- ror surface, inducing turbulence effects near the window.

These temperature variations not only degrade the optical imaging quality but also reduce the imaging resolution.

To address this, Zheng et al. proposed a method of de-

icing using multi-phase alternating current (AC), com- bined with linear electrodes. Their experiments demon- strated an 81.61% reduction in the standard deviation of the mirror temperature and a 76.31% reduction in the peak-to-valley value, significantly improving heating unifor- mity and de-icing efficiency [ 6 ] .

Thermal Radiation Effects on Near infrared Telescope Sealing Window

“Stray light” refers to unwanted light in an optical sys- tem, comprising both non-target radiation scattered onto the detector or imaging surface and target radiation reach- ing the detector via paths not intended in the design of the instrument. Stray light degrades image plane contrast and the modulation transfer function (MTF), leading to reduced tonal range, degraded sharpness, and perturbed energy distribution. It can also generate spurious spots on the image plane or saturate the target signal entirely with stray radiation-induced noise. For infrared systems operat- ing during polar night, the dominant sources of stray light are moonlight, sky background radiation, and intrinsic ther- mal emission from the system itself. As the impact of moonlight can be mitigated by reorienting the telescope, this analysis focuses on sky background radiation and sys- tem thermal emission under moonless conditions. The sky background illuminance on the telescope image plane is given by

E sky = I ( c λ 1 − c λ 2

where = 2.997 is the speed of light, are the endpoints of the band range, is the back- ground brightness of the sky, = 7.903 × 10 arcsec the field of view of the telescope, and = 0.075 m is the radius of the pupil. The efficiency of the telescope sys- tem is = 78.73%, and the camera sensor area is 3.277 × 10 During the Antarctic winter, the sky background flux in the J, H, and Ks bands is approximately 600–1 100 µJy arcsec 600 µJy arcsec and 200–900 µJy arc- , respectively. For the corresponding bands, we take the sky background intensities to be = 600 µJy arcsec 100 µJy arcsec , and = 200 µJy arc- . The resulting image plane illuminances, calculated using Equation (1) for each band, are given in Waveband Wavelength/µm Illumination, /(W m Emissivity is one of the main parameters in analyz- ing thermal radiation from infrared optics. According to the Stefan-Boltzmann law, the intensity of thermal radia- tion is proportional to the fourth power of temperature, where the radiance, in W m , is given by

j ∗ = εσ T 4 , (2)

where is the blackbody radiance coefficient, which is equal to 1 for an absolute blackbody, and is the abso- lute temperature in Kelvins. The scaling factor, , is the Ste- fan-Boltzmann constant, given by

σ = 2 π 5 k 4

where k is Boltzmann’s constant (1.380 649 × 10 −23 J K −1 ), and h is Planck’s constant (6.626 07 × 10 −34 J s). Accord- ing to Planck’s law of blackbody radiation, the radiative power per unit wavelength per unit volume can be expressed as

Using Planck’s law, we calculate the spectral radi- ance curves for an ideal blackbody at a range of tempera- tures. These curves represent the total electromagnetic power radiated per unit area, per unit solid angle, per unit wavelength. shows the spectral distribution of black- body radiance. At −60°C, the J-band spectral radiance remains below 0.1 W m µm. However, when the tempera- ture rises to −40°C, this radiance increases to approxi- mately 10 W m µm, representing an enhancement of nearly two orders of magnitude. Thermal radiation effects consequently become non-negligible. At longer wave- lengths in the near-infrared H and K bands, blackbody radi- ance intensifies significantly. Particularly in the K band, spectral radiance exceeds 10 µm even at −80°C, posing considerable challenges for proposed Antarctic K- band infrared optics.

For infrared systems, the energy of infrared radiation can be calculated using Planck’s formula,

M = ε ∫ λ 2

Black body power density K-band Spectral radiance/(W m H-band

193 K (−80°C)

J-band where correspond to the respective wavelength bands, and is the emissivity. The radiance of each sur- face in the optical system can therefore be calculated at dif- ferent wavelengths.

Thermal radiation analysis was performed on the actual optomechanical system using 15-NIRT. With the ambient temperature set to −60°C (213.15 K) and the sen- sor detection range spanning 0.4–1.7 µm, we analyzed ther- mal radiation across four spectral bands (0.4–1.1 µm, 1.1–1.4 µm, 1.4–1.7 µm, and 1.7–2.5 µm) to inform the K-band optical design. Given that the filter coating func- tions as an anti-reflection layer in the 1.1–1.4 µm band, while acting as a reflector in the other three bands, the

Surface Absorptance Specular reflectance Specular transmittance

At the ambient temperature of –60 °C, the maximum irradiance on the image plane from optomechanical ther- mal radiation within the 0.4–1.7 µm band is 4.186 8 × 10 −12 W m −2 , which is substantially lower than sky back-

resulting radiant exitance values for each optical surface configuration are given in surfaces Surface type Temperature/°C Emissivity Wavelength/µm Mirror Black paint The sealing window temperature decreases gradually from −40°C externally to −58°C internally, simulating the resulting uneven temperature distribution caused by heat- ing. We conducted a radiant emittance analysis with Trace- Pro ray tracing software, using the surface optical proper- ties detailed in shows the ray tracing of opti- cal-mechanical thermal radiation, and shows the irradiance of thermal radiation on the imaging plane at dif- ferent wavelengths.

ground contributions. Consequently, thermal radiation negli- gibly affects imaging quality in the J and H bands. How- ever, within the 1.7–2.5 µm band, thermal irradiance reaches 4.209 6 × 10 −7 W m −2 , comparable to K-band sky background levels, which compromises imaging quality.

This thermal influence is projected to intensify further at longer mid-infrared wavelengths, meaning that ITO mir- ror heating becomes unsuitable for normal system opera- tion in the K-band and longer wavelengths regimes.

Other Effects of ITO Heating Defrosting

Beyond the imaging effects of thermal radiation gener- ated by ITO electrode heating, the near-infrared transmit- tance of ITO coatings also impacts optical performance.

While ITO has 85% transmittance at visible wavelengths, it shows significant reflection in near-infrared bands, caused by carrier plasmon oscillations [ 7 ] , and increased film thickness further exacerbates scattering and absorp- tion losses. These properties are characterized by the Drude model for ITO materials [ 8 ] .

Integrated BRDF A BRDF B BRDF g Integrated BTDF A BTDF B BTDF g Mirror 3 × 10 9.5 × 10 1 × 10 7 × 10 1 × 10 Anti reflect 1 × 10 1.5 × 10 1.324 × 10 1 × 10 1.5 × 10 7.53 × 10 1 × 10 Black paint 9.5 × 10 1 × 10 5 × 10 1.9 × 10 BRDF: Bidirectional Reflectance Distribution Function; BTDF: Bidirectional Transmittance Distribution Function.

7 × 10 7 × 10 8 × 10 6 × 10 The material permittivity, ), is given by where is the high frequency permittivity, is the angu- lar frequency, is the plasma frequency, and is the damping rate. The real part ( ) and imaginary part ( of the permittivity are given by

ε 1 = ε ∞ − ω 2 p ω 2 + γ, (7)

ε 2 = γω 2 p ω ( ω 2 + γ 2 ) . (8)

The complex refractive index, , is given by

˜ n 2 = ( n + i k ) 2 , (9)

where is the ordinary refractive index, and is the extinc- tion coefficient, which are given, respectively, by According to Fabry-Perot interference theory, the total field amplitude is given by

r = r 01 + r 10 e i2 ϕ

t = t 01 t 10 e i2 ϕ

where is the total reflection amplitude coefficient, the total transmission amplitude coefficient, is the ampli- tude reflection coefficient, is the amplitude transmis- sion coefficient, e is the natural exponential, and is the single-pass phase accumulation during light propagation through the thin film. These coefficients satisfy the Stokes relation . The coefficients are calculated using the relationships

t ij t ji + r 2 ij = 1

ϕ = 2 π λ ˜ nd , (16)

where is the corresponding wavelength and is the film thickness. Derivations of the formula for the calculation of the ITO film curve parameters can be obtained from Equation (14) and Equation (15), as

R = | r | 2 , T = | t | 2 , (17)

A = 1 − R − T , (18)

where is the reflectance of the ITO thin film correspond- ing to the thickness as a function of wavelength, is the transmittance, and is the absorptance. For a thickness of 0.2 µm , we used Python to calculate spectra for across the 0.4–3 µm range (shown in ). For wavelengths further into the infrared, ITO transmittance decreases progressively, maintaining 70%–75% in the J- band, dropping to a minimum of 40% in the H-band, and falling to only 20%–30% in the K-band. This severe attenu- ation fundamentally compromises infrared light collection efficiency. Consequently, ITO-coated electrode heating is unsuitable for de-icing sealing windows on near-infrared telescopes.

J-band H-band K-band ITO thin film optical properties Transmittance (T) Reflectance (R) Absorptance (A) Optical response ITO in the 0.4–3 µm wavelength range.

PERFORMANCE ANALYSIS OF ME CHANICAL SNOW REMOVAL AND DEFROSTING

3.1 . Design of a Device for Mechanical Snow Removal

The performance of the snow removal system was

tested on 15-NIRT. The telescope uses a modified hyper- bolic Newtonian optical design, and compensates for spheri- cal aberration and coma using a hyperbolic primary mir- ror and a corrector lens group, achieving excellent image quality and a wide field of view. For a fully enclosed mechanical structure, protecting the instrument interior from wind-blown snow and ice (which could degrade imag- ing performance and shorten equipment lifespan), a seal- ing window is incorporated into the optical system. Made from fused silica, the sealing window incorporates a J- band filter coating on its surface, defining the operational wavelength range as 1.1–1.4 µm The snow-removal device was upgraded and installed on the near-infrared telescope by Dr. Chao Chen (NIAOT) during China’s 41st Antarctic Research Expedi- tion, in 2024. As shown in , the device is mounted on the right side of the sealing window. It can be remotely controlled to initiate and terminate operation.

The brush bristles possess moderate hardness, enabling effective snow removal while preventing damage to the fil- ter coating of the sealing window.

Antarctica. (B) The snow removal device.

3.2 . Mechanical Defrosting Performance Evaluation

Method

Currently, no suitable sensor is available to detect frost formation on optical mirror surfaces without compro- mising astronomical observations. Consequently, we have no way to visually monitor the effectiveness of the mechani- cal de-icing. Here, we propose quantifying the effective- ness of mechanical de-icing through aperture photometry.

Aperture photometry is a fundamental technique in astro- nomical image analysis for measuring the brightness of celestial objects, such as stars or galaxies. It involves mea- suring the flux within a small aperture region defined by the focal plane diaphragm or image-processing software.

The source intensity is calculated by summing the counts within the aperture containing the source and subtracting the estimated sky background contribution derived from nearby image regions.

Aperture photometry employs spatial segmentation to isolate the light signal from the target celestial object from any surrounding background contribution (e.g., atmo-

spheric glow). The inner aperture defines a region encom- passing the target, collecting the total flux (source and local background). The aperture size critically affects the measurement: An excessively large aperture increases back- ground noise, while an overly small aperture leads to source signal loss. The outer aperture measures the local background intensity, typically chosen as an annulus sur- rounding the inner aperture or as discrete regions nearby.

This selection aims to minimize distance to the target while avoiding contamination from the its halo or any neighboring celestial objects. When the stellar Point Spread Function (PSF) extends beyond the aperture, the resulting flux loss necessitates aperture correction of the instrumental magnitude. The correction formula, calculat- ing the aperture correction value, Δm′ , is

∆ m ′ = m B − m S , (19)

where are the large and small apertures of bright stars, respectively. The large aperture is usually sev- eral times larger than the small aperture, covering almost the entire target source flux, while the small aperture is con- sistent with the dark star photometric aperture. When a star is well-isolated within the stellar field, its photomet- ric value can be measured using aperture photometry (as illustrated in ). The fundamental steps are as fol- is the photometric aperture. The circular ring, with a thickness of starting at the radius , is used to calculate the sky background. (1) Pre-processing (subtracting background and dark current, removing flat field); (2) Searching for the target source; (3) Determine the center of the star image in the given aperture, pixels; (4) Confirm the background value of the sky light near the aperture; (5) The total intensity, , obtained by adding up all the pixel values that fall within the aperture;

(6) Calculate the size of celestial objects, m ′ = −2.5 log [ F a − N a × B ]; (7) Performing aperture correction Δm′ ; (8) Obtain the magnitude of the instrument m = m′ + Δm′ .

3.3 . Quantitative Analysis of Mechanical Defrosting Effectiveness

Observations of the target star Canopus were obtained at UTC 2025−03−05 T13:38:27 and UTC 2025−03−11 T13:33:18, each with a 0.1 s exposure. We Prior to each observing session, a reference image was acquired for each target. Following each snow removal operation, the target was exposed identically.

Approximately 15 images were obtained per target, with the entire sequence completed within 30 minutes, to mini- mize atmospheric turbulence effects on photometry. The targets selected were Canopus and HD 2151. To maxi- mize snow accumulation effects, no snow removal was per- formed for 3 days preceding the Canopus observation.

HD 2151 was observed one day after Canopus to evalu- ate 24-hour snow accumulation changes. Both targets were exposed for 0.1 s. Data were processed and ana- lyzed using Python. Initial steps included error correction by subtracting bias and dark current, and applying flat- field correction. Flux measurements were then performed on the corrected images. Magnitude analysis used the derived flux data. The first image served as the reference.

We calculated the magnitude difference between each post-snow-removal measurement pre-snow- removal reference magnitude, and the resulting flux and magnitude variations are shown in shows the variation in target flux. As snow removal operations increase, the target flux initially rises and then stabilizes, providing preliminary evidence for the effectiveness of the device in mitigating the effects of accu- mulated snow and frost. shows the relative magni- tude change of the target before and after snow removal, calculated by subtracting the magnitude measured with- out snow removal from that measured after snow removal. Under identical exposure times, we observed an increase in target brightness, which aligns with the expected performance of the snow removal device.

Further analysis reveals that after 3 days without snow removal, Canopus showed a change of approxi- performed data analysis using the MaxIm DL 6 software package. After six days without snow removal, accumu- lated frost/snow attenuates the target flux by approxi- mately 3 magnitudes, severely degrading the observa- tional data quality (see mately 1 magnitude. By contrast, HD 2151, observed only 1 day after its previous snow removal, showed a much smaller relative magnitude change of ~0.2 magni- tudes. This indicates that the impact of snow accumula- tion on observations increases with time since the last removal. The data also show that magnitude changes satu- rate after approximately 5 snow removal cycles. Based on Flux/(net counts) Before-de-icing Magnitude difference Before-de-icing

(A) shows the variation in target flux, while (B) displays the relative magnitude variation.

these results, we suggest optimizing the snow removal fre- quency to around 48 h, with each session comprising 4–6 cycles, for operational efficiency.

CONCLUSION

We have evaluated the effectiveness of a mechanical snow removal device using 15-NIRT at Dome A, in Antarc- tica. We first analyzed the performance of heating using ITO coatings for defrosting, then examined its limitations for near-infrared applications, including thermal radiation from sealing window heating and reduced transmittance of ITO films in the NIR band. We used changes in tar- get flux and magnitude before and after snow removal to perform a quantitative analysis. Approximately 15 expo- sures were acquired for each target (Canopus and HD 2151), with the total sequence completed within 30 min to minimize atmospheric turbulence effects. Data were processed using Python. The results align with the expected performance of the device. Target flux increased and stabilized with successive snow removal operations, and the relative magnitude change peaked and then plateaued, consistently reaching its maximum during the to 6 cycle. Considering snow removal intervals of 1–3 days, we recommend an optimized regimen of snow removal at 48-hour intervals, with each session compris- ing 4–6 cycles. This approach provides a straightforward but effective solution for removing accumulated snow and frost from the sealing window, significantly improving observational efficiency.

ACKNOWLEDGEMENTS This work was supported by the Space Debris Research Project, China (KJSP2020010102), the National Key R&D Program of China (2022YFC2807300), and the National Natural Science Foundation of China (12573081).

AI DISCLOSURE STATEMENT Deepseek was employed for code error checking dur- ing the calculations in this paper and for language and grammar checking of the article. The authors carefully reviewed, edited, and revised the Deepseek-generated texts to their own preferences, assuming ultimate responsi- bility for the content of the publication.

AUTHOR CONTRIBUTION

Zhengyang Li and Zhixu Wu supervised the project, contributed to conceptualization and oversaw the techni-

cal workflow. Jiali Chen and Kaiwen Zheng acquired the observational data. Jia’nan Cong supervised and guided the thermal radiation analysis. Zichong Zhang performed data analysis and processing. Jiali Chen and Zhixu Wu drafted the manuscript and edited it for language. All authors read and approved the final manuscript.

DECLARATION OF INTERESTS

Zhengyang Li is an executive editor-in-chief for Astro- nomical Techniques and Instruments and he was not involved in the editorial review or the decision to publish this article. The authors declare no competing interests.

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