Opening a new chapter in computational astrophysical imaging, a Tsinghua team de

In the early 17th century, humanity began to point observational instruments towards the distant universe, hoping to capture photons that have traveled across millennia and receive messages from distant galaxies.

However, atmospheric turbulence, like a transparent ghost floating in the air, interferes with the progress of photons, concealing the secrets of the early universe. In 1964, American physicist Richard Feynman pointed out that "turbulence is one of the most important unsolved problems in classical physics."

Atmospheric turbulence, a highly chaotic system, is one of the most elusive phenomena in turbulence, with its motion patterns exhibiting strong randomness, making it difficult to model, detect, and predict accurately.

A cross-disciplinary team of imaging and intelligent technology at Tsinghua University has developed a wide-field wavefront computational sensing chip (WISE, Wide-field Wavefront Sensor), achieving real-time detection and prediction of atmospheric turbulence over a range of more than 1100 arcseconds (diagonal).

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This imaging technology has the advantages of a large field of view, high resolution, and strong robustness, with a sensing range that is nearly a thousand times greater than that of the widely used Shack-Hartmann wavefront sensor.The detection capability of the WISE chip is equivalent to the sum of hundreds or even thousands of wavefront sensors, and can be widely applied to existing optical systems to enable large-area detection and prediction of atmospheric turbulence, correct atmospheric turbulence disturbances, and achieve efficient collection and precise reconstruction of a wide range of optical signals.

Recently, the related paper was published in Nature Photonics[1] with the title "Direct Observation of Atmospheric Turbulence with a Video-rate Wide-field Wavefront Sensor."

Professor Fang Lu, Academician Dai Qionghai, and Associate Professor Wu Jiamin from Tsinghua University are the corresponding authors. Graduate student Guo Yudou, undergraduate student Hao Yuhan, and research assistant Wan Sen from Tsinghua University are the co-first authors. Postdoctoral fellow Zhang Hao and research assistant Zhu Laiyu participated in this research.

Looking far into the distance, the eyes can see thousands of miles: Real-time observation of atmospheric turbulence at an angle of one thousand arcseconds.For the human eye, the pupil size is relatively small, and atmospheric turbulence causes stars to appear "twinkling brightly"; in ground-based deep space exploration, large-aperture optical telescopes are constrained by atmospheric turbulence, resulting in a significant decrease in observation resolution and signal-to-noise ratio.

For example, under conditions of poor seeing, if no turbulence correction is made, the performance of an 8-meter aperture telescope is no different from that of a 30-centimeter aperture telescope. The existence of atmospheric turbulence, this photonic ghost, severely disturbs the propagation of light signals and has become the bottleneck of ground-based deep space exploration.

Over the past century, people have tried to accurately model the motion process of turbulence using mathematics. For instance, the Navier-Stokes equations provide an effective turbulence simulation method.

However, the large scale and high complexity of atmospheric turbulence make numerical methods difficult to apply. Therefore, experimental measurements based on physics have become the current mainstream approach.

Adaptive optics technology uses Shack-Hartmann wavefront sensors, combined with deformable mirrors and negative feedback control systems, to achieve the detection and correction of transient, local wavefronts for the first time.However, its observation and correction diameter in the visible light band is only 5-10 arcseconds. To achieve turbulence space non-uniform (anisoplanatism) detection with a larger field of view, multiple wavefront sensors corresponding to different fields of view need to be introduced for separate detection. This not only increases the system complexity but also makes it difficult to apply on a large scale.

The research team delved deeply into the physical essence of atmospheric turbulence, which manipulates photons through the refraction angle deflection brought by non-uniform refraction.

Therefore, high-precision acquisition and reconstruction of the four-dimensional light field in space and angle can reveal the hidden turbulence information in the high-dimensional angle domain, thereby breaking through the difficulty of atmospheric turbulence space non-uniform observation.

Compared with the Shack-Hartmann wavefront sensor used in traditional adaptive optics, WISE can capture spatial non-uniform turbulence information in a larger field of view. This advantage is determined by the system architecture.

The Shack-Hartmann wavefront sensor in adaptive optics realizes direct aperture segmentation on the conjugate pupil plane, and its spatial sampling is limited, only able to detect the average wavefront within a certain field of view.WISE employs an indirect pupil segmentation scheme, equipped with a distributed microlens array, where each microlens records the angle information of incoming photons from different viewing directions, thereby effectively minimizing crosstalk and capturing spatially non-uniform turbulence information over a larger field of view.

In the Earth-to-Moon observation experiment, WISE achieved real-time detection of about 500 spatially coherent turbulence wavefronts within a 1100 arcsecond (diameter) field of view at a rate of 30Hz. The observation performance of a single WISE chip is equivalent to nearly 1000 traditional wavefront sensors.

In addition to lateral distribution, the detection results of the WISE chip can also be used to reconstruct the high-precision vertical distribution of atmospheric turbulence at different altitudes, with a resolution and stability that is several times better than traditional adaptive optics.

The WISE chip has broken the barriers of wide-area atmospheric turbulence observation, restoring the spatially non-uniform distribution of atmospheric turbulence and revealing the dynamic laws of atmospheric turbulence.To the Extensive and the Exquisite: WISE Enhances High-Precision Turbulence Prediction

In the unidirectional transmission of light signals, precise turbulence detection is sufficient to eliminate errors. However, in bidirectional interactions, the rapid evolution of turbulence presents new challenges.

Typical interactive processes (such as space optical communication) consist of a downward detection link and an upward compensation link. Due to the time difference between the two links, direct compensation based on detection results is not feasible. Instead, it is necessary to predict the future distribution of turbulence for compensation, known as pre-compensation. At this point, the accuracy of turbulence prediction is crucial.

As the saying "To the Extensive and the Exquisite" describes the dialectical relationship between broad depth and fine subtlety, the WISE chip's ability to detect "extensive" atmospheric turbulence can significantly enhance the "exquisite" accuracy of turbulence prediction, achieving a transformation from "extensive" to "exquisite."

The following video demonstrates the temporal evolution of turbulence distribution, changing from spatially consistent turbulence within a small field of view to spatially inconsistent turbulence over a wide area.When we only observe a small field of view of turbulence, it is difficult to find its temporal evolution rules, which is the difficulty in predicting turbulence based on traditional adaptive optics technology.

When the field of view is expanded, the evolution rules of turbulence become traceable. As described by the Taylor frozen flow hypothesis, in the observation data of a large range, the overall flow of atmospheric turbulence can be clearly observed, which will provide strong support for the precise prediction of turbulence.

Based on the WISE chip and the time-space neural network model, the research group has achieved high-precision turbulence prediction in a large field of view, and the predicted wavefront error has been reduced from 224nm to 109nm, which is a significant improvement compared to traditional adaptive optics.

The WISE chip has explored a new path for the study of the spatiotemporal dynamic evolution laws of atmospheric turbulence.

Researchers carried out a series of experiments at the Xinglong Observatory of the National Astronomical Observatory of China. The WISE chip, through an 80-cm aperture telescope, achieved high-speed correction of the full field dynamic turbulence within a 1100 arcsecond field of view in the 400,000-kilometer Earth-Moon observation, significantly improving the imaging resolution and signal-to-noise ratio.From scanning light field element imaging [2] to the WISE chip, the photon ghost is no longer mysterious, and the telescope's field of view can penetrate the atmosphere.

The interdisciplinary team of imaging and intelligent technology at Tsinghua University continues to innovate in the field of computational imaging, empowering astronomy with computation, and opening a new chapter in computational astronomical imaging. When the field of view is infinite, the horizon of vision will also be boundless.

In the future, the laboratory will further leverage the advantages of the meta-imaging wide-area wavefront sensing to assist the new generation of wide-field high-resolution ground-based optical surveys, looking far into the clouds, and seeing thousands of miles away.