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$J$ and $H$ band sky brightness measurements from polar day to polar night at Dome A, Antarctica

This paper presents the first continuous $J$ and $H$ -band sky brightness measurements at Dome A, Antarctica, from polar day to polar night, revealing distinct twilight boundaries, darker nighttime backgrounds during the polar night compared to regular cycles, and a correlation between $H$ -band brightness and solar activity.

Original authors: Jinji Li, Bin Ma, Haonan Yang, Pu Lin, Zhongnan Dong, Michael C. B. Ashley, Lu Feng, Yi Hu, Zhaohui Shang, Yun Shi, Shijie Sun, Xu Yang

Published 2026-07-03

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Jinji Li, Bin Ma, Haonan Yang, Pu Lin, Zhongnan Dong, Michael C. B. Ashley, Lu Feng, Yi Hu, Zhaohui Shang, Yun Shi, Shijie Sun, Xu Yang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the night sky not as a perfect, black canvas, but as a slightly glowing, hazy window. For astronomers trying to see faint, distant stars, this "haze" (called sky brightness) is like trying to spot a firefly while standing next to a streetlamp. The dimmer the streetlamp, the better the view.

This paper is a report card on the "streetlamp" levels at Dome A, the highest point on the Antarctic ice sheet. It's considered one of the best places on Earth for looking at the universe, but until now, nobody had taken continuous measurements of its near-infrared "glow" from the bright summer days all the way through the dark winter nights.

Here is the story of what they found, using simple analogies:

1. The Mission: Watching the Glow Change

The team used a special telescope called AIRBT (Antarctic Infrared Binocular Telescope) that acts like a pair of high-tech eyes. They set it to stare at one fixed spot in the sky for months, from late summer (when the sun is still up) through the twilight, and into the deep, dark polar night.

They measured the sky in two specific "colors" of light that human eyes can't see but cameras can: the J-band and the H-band. Think of these as two different shades of infrared light.

2. The Results: How Dark is it?

The team found that the sky at Dome A is incredibly dark, but it changes depending on the time of year and the sun's position.

The Daytime "Streetlamp": When the sun is high, the sky is very bright (like a bright overcast day).
The Twilight Transition: As the sun dips below the horizon, the sky doesn't just snap to black. It fades gradually. The team found the exact moment the sky becomes "nighttime" dark:
- In the J-band, the sky turns fully dark when the sun is about 9.3 degrees below the horizon.
- In the H-band, it takes a bit longer, turning dark when the sun is 7.4 degrees below.
- Analogy: Imagine a dimmer switch. At mid-latitude places (like Hawaii or Chile), you have to turn the switch down very far (sun 12 degrees down) to get the room truly dark. At Dome A, the switch turns off much sooner (sun only 7–9 degrees down), giving astronomers a longer "night" to work with.
The Polar Night: Once the sun is gone for good (polar night), the sky gets even darker.
- The J-band sky is about 15.3 on the brightness scale (where higher numbers mean darker).
- The H-band sky is about 13.4.
- Comparison: This is comparable to the famous Mauna Kea observatory in Hawaii for J and H bands, but Dome A is significantly darker in the K-band (a different infrared color), making it a top-tier location.

3. The "Sunspot" Connection

The paper noticed something interesting about the sun itself. The observations happened during a time when the sun was very active (near "solar maximum"), meaning it had many sunspots.

The Metaphor: Think of the sun as a noisy neighbor. When the neighbor is quiet (solar minimum), the sky is very still. When the neighbor is loud and throwing parties (solar maximum), the sky gets a bit brighter.
The Finding: During the polar night, the team saw that when the sun had more sunspots, the H-band sky got slightly brighter. It's like the sun's "noise" is traveling all the way to Antarctica and making the sky glow a tiny bit more. This suggests that to get the absolute darkest skies, you might want to observe when the sun is quiet, not when it's throwing a party.

4. The "Frost" Problem

The telescope sat outside for months without anyone touching it. A challenge they faced was frost forming on the telescope's window.

Analogy: Imagine taking a photo through a window that is slowly getting covered in ice. If you don't account for the ice, your photo looks darker than it really is.
The Fix: The team developed a clever mathematical trick to figure out how much "ice" was on the window and corrected for it. They found that while the J-band window stayed mostly clear, the H-band window got frosty, which would have made the sky look artificially bright if they hadn't fixed it.

5. Why This Matters

This paper is the first time we have a continuous, season-long map of how the sky brightness changes at Dome A.

More Nighttime: Because the sky gets dark at a higher sun angle than at other observatories, Dome A offers 25–35% more usable nighttime for infrared observations compared to mid-latitude sites.
Stability: Once the sun is down, the sky at Dome A stays remarkably stable, unlike other places where the sky brightness might fluctuate wildly.

In Summary:
Dome A is a "super-dark" location for infrared astronomy. It offers longer nights and a very stable background, but the team warns that the specific measurements they took were during a "loud" time for the sun. If we want to see the absolute faintest signals, we might need to wait for a quieter time in the sun's 11-year cycle. But for now, we know exactly how dark the Antarctic night really is.

Technical Summary: J and H band sky brightness measurements from polar day to polar night at Dome A, Antarctica

Problem and Context
The near-infrared (NIR) sky brightness is a critical parameter for evaluating the performance of ground-based infrared observatories, as it directly limits sensitivity and photometric precision. While the Antarctic plateau is established as the darkest ground-based site for infrared astronomy, continuous monitoring of the NIR sky background at Dome A has been lacking. Previous studies at Dome A were limited to short observing windows, daytime-only measurements, or specific periods (e.g., solar minimum), failing to characterize the continuous evolution of J- and H-band sky backgrounds across the transition from polar day to polar night. Furthermore, the influence of the solar cycle on the NIR background at this specific site remained unquantified.

Methodology
This study utilizes data from the Antarctic Infrared Binocular Telescope (AIRBT), installed at Dome A in 2023. The telescope, equipped with two InGaAs detectors, conducted continuous fixed-pointing observations in the J and H bands from February to May 2024. This period covered a broad range of solar elevations, transitioning from late polar day (solar elevation $\sim$ 19.6 $^\circ$ ) to early polar night (solar elevation $\sim$ −25.1 $^\circ$ ).

Key methodological steps included:

Data Acquisition: Automated image acquisition with exposure times typically set to 2 seconds to balance efficiency against stellar trailing, ensuring unsaturated images across varying illumination conditions.
Dark Current Correction: Due to the lack of on-site dark frames, laboratory dark-current measurements were rescaled using the "warm-pixel scaling method" to match the detector temperature variations (ranging from −45.4 $^\circ$ C to −62.9 $^\circ$ C) observed during the campaign.
Photometric Calibration: Zeropoints were derived via cross-matching with 2MASS stars. To mitigate throughput losses caused by frosting on the entrance window (which affected the H-band more severely than the J-band), the authors adopted a "clear zeropoint" based on the median values from the first few clear nights, rather than real-time zeropoints.
Analysis: Sky brightness was calculated as a function of solar elevation. The study defined twilight–nighttime boundaries based on the dawn branch of the data to avoid asymmetries found in the dusk branch.

Key Results

Sky Brightness Levels: The median sky brightness was measured at 5.2 mag arcsec $^{-2}$ (J band) and 2.9 mag arcsec $^{-2}$ (H band) during daytime. During the polar night, the median nighttime sky brightness was 15.3 mag arcsec $^{-2}$ in J and 13.4 mag arcsec $^{-2}$ in H.
Twilight Boundaries: The transition from twilight to stable nighttime occurs at solar elevations of −9.3 $^\circ$ $\pm$ 0.3 $^\circ$ in the J band and −7.4 $^\circ$ $\pm$ 0.3 $^\circ$ in the H band. These boundaries are lower (darker) than those derived from previous studies at Dome A, likely due to differences in definition and the solar cycle phase.
Polar Night vs. Regular Cycle: At the same solar elevation, the NIR sky background during the polar night is approximately 0.1 mag arcsec $^{-2}$ (J band) and 0.4 mag arcsec $^{-2}$ (H band) darker than during periods of regular day–night alternation.
Stability: Once the site enters nighttime, the NIR sky background at Dome A stabilizes much more rapidly and remains more stable than at mid-latitude sites (e.g., Mauna Kea), where the background continues to decline significantly for hours after sunset.
Solar Cycle Correlation: During the polar night, the H-band sky brightness shows a notable correlation with the sunspot number (Spearman rank coefficient $r_s = -0.795$ ), suggesting that the observed brightness is influenced by solar activity. The J-band correlation is weaker ( $r_s = -0.337$ ). No significant dependence on geomagnetic activity (Kp index) was found.
Observing Time: Dome A offers approximately 3219 hours of J-band and 3493 hours of H-band nighttime annually, representing a 25–35% increase over optical (i-band) observing time.

Significance and Claims
The authors claim these results provide the first season-long characterization of NIR sky brightness at Dome A, revealing systematic differences between polar and non-polar environments. A primary conclusion is that the measured sky brightness values are likely elevated because the observations were conducted near solar maximum, whereas previous Antarctic measurements were taken near solar minimum. This highlights the necessity of long-term monitoring across the solar cycle to establish accurate baselines. The study confirms that Dome A offers exceptionally stable and dark NIR conditions, particularly during the polar night, making it a superior location for deep NIR imaging and high-precision time-domain astronomy compared to mid-latitude sites. The data also serves as a reference for future comparative measurements at different phases of the solar cycle.

1. The Mission: Watching the Glow Change

2. The Results: How Dark is it?

3. The "Sunspot" Connection

4. The "Frost" Problem

5. Why This Matters

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