Antarctic Infrared Binocular Telescope. I. System Overview, Laboratory Testing, and On-Sky Performance Evaluation

TL;DR

The paper introduces the Antarctic Infrared Binocular Telescope (AIRBT), a dual 15 cm infrared time-domain facility designed for Dome A to probe the dynamic infrared sky in the 1.0–1.7 μm band. It details comprehensive laboratory characterization of InGaAs detectors, including bias, readout noise, dark current, nonlinearity, and the photon transfer curve, highlighting time-dependent extra noise and recommending fixed-exposure PTCs for unbiased gain estimation. On-sky tests at Zhuhai validate photometric and astrometric performance, deriving J and H zero-points, throughput, and color terms while establishing 5σ limits of 11.2 mag in J and 9.7 mag in H for a 3 s exposure, with photometric precision approaching 20 mmag for bright sources and ~8 mmag when stacked. The results indicate AIRBT’s viability as a pathfinder for Dome A, where reduced sky background and scintillation are expected to enable deeper, sub-percent precision infrared time-domain surveys, informing future large-aperture Antarctic instruments.

Abstract

Infrared time-domain surveys remain significantly underdeveloped compared with their optical counterparts. We have developed the Antarctic Infrared Binocular Telescope (AIRBT) to study the dynamic infrared sky at Dome A, Antarctica, taking advantage of the superb infrared observational conditions at this site. AIRBT consists of two identical 15 cm f/3 optical tube assemblies and two cost-effective indium gallium arsenide (InGaAs) cameras equipped with J and H filters, respectively. The cameras have 640 x 512 pixels with a size of 15 micrometers, providing a scale of 6.9 arcseconds per pixel and a field of view of 1.22 x 0.97 square degrees. We characterize the performance of the InGaAs cameras, including bias, readout noise, dark current, nonlinearity, and photon transfer curve. Our analysis highlights the distinct behaviors of InGaAs cameras compared with charge-coupled devices (CCDs). The bias and readout noise show temperature dependence, and the noise measured from the photon transfer curves has additional components that increase with exposure time. On-sky tests were conducted in October 2022 including system calibration, limiting depth, and photometric precision. For a single 3-second exposure, we achieved 5-sigma limiting magnitudes of 11.2 mag (Vega system) in J band and 9.7 mag in H band. The best photometric precision reached 20 millimagnitudes at the bright end, which could be further improved to sub-percent levels through image stacking. AIRBT was installed at Dome A in January 2023, and scientific observations began as soon as darkness set in.

Figures (11)

• Figure 1: Optical design of the telescope, which is an RC design with lens correctors. The entrance pupil has a diameter of 15 cm and the overall focal ratio is $f/$3.
• Figure 2: Counts (orange) and noise (purple) versus time from dark frames. Solid curves represent results with the FPA temperature around 20 ℃, while dashed curves represent FPA temperature around 55 ℃. We show that both the bias and readout noise take time to stabilize and decrease with lower FPA temperature.
• Figure 3: Bias and dark current. Upper left: Bias frame with temporally stable stripe patterns. Upper right: Dark current frame with a 5 s integration at 55 ℃ in middle gain mode. Lower left: Dark current versus temperature in high gain (black) and middle gain (red) modes. The dark current decreases exponentially with temperature until 40 ℃ . The dark current in middle gain mode is higher than that in high gain mode. Lower right: Distribution and cumulative curve of middle gain dark current at 55 ℃, following approximate normal statistics. All data are acquired in middle gain mode.
• Figure 4: Left: Overall linearity. Middle: Nonlinearity, which stays below 1% between 1000 and 12,500 ADU. Right: Single-pixel linearity, where 117 pixels show no response or nonlinear behavior. All data were acquired in middle gain mode.
• Figure 5: The PTCs and gain estimation. (a) PTCs obtained by increasing exposure time with a stable light source. Different curves represent varying intensity levels. Variance for identical counts increases with lower light intensity, requiring longer integration time and resulting in higher variance. (b) PTCs for fixed exposure time with increasing light intensity. Different curves correspond to different exposure times. All PTC slopes remain consistent, indicating a gain of 2.6 e$^-$/ADU. Longer integration times lead to greater interference, suggesting that additional noise accumulates over time. (c) Extra noise obtained by subtracting photon shot noise from the PTCs in (a). (d) Noise from the PTCs in (b) with extra noise subtracted. All data points fall on the same line.