Atmospheric Turbulence Correction Research Articles

Semi-supervised correction model for turbulence-distorted images

Significant progress has been made in addressing turbulence distortion in recent years, but persistent challenges remain. Firstly, existing methods heavily rely on fully supervised optimization strategies and synthetic datasets, posing difficulties in effectively utilizing unlabeled real data for training. Secondly, most approaches construct networks in a straightforward manner, overlooking the representation model of phase distortion and point spread function (PSF) in spatial and channel dimensions. This oversight restricts the potential for distortion correction. To address these challenges, this paper proposes a semi-supervised atmospheric turbulence correction method based on the mean-teacher framework. Our approach imposes constraints on the unlabeled data of student networks using pseudo-labels generated by teacher networks, thereby enhancing the generalization ability by leveraging information from unlabeled data. Furthermore, we introduce to use no-reference image quality assessment criterion to select the most reliable pseudo-label for each unlabeled sample by predicting physical parameters that indicating the level of degradation. Additionally, we propose to combine sliding window-based self-attention with channel attention to facilitate local-global context interaction. This design is inspired by the representation of phase distortion and PSF, which can be characterized by coefficients and basis functions corresponding to the channel-wise representation of convolutional neural network features. Moreover, the base functions exhibit spatial correlation, akin to Zenike and Airy disks. Experimental results show that the proposed method surpasses state-of-the-art models.

Entanglement assisted communication over the free-space optical link with azimuthal phase correction for atmospheric turbulence by adaptive optics.

To improve the reliability of entanglement-assisted (EA) communication over turbulent FSO channels we propose to perform the optical phase-conjugation on idler photons rather than turbulence affected signal photons and use adaptive optics. We experimentally demonstrate that reliable EA communication over outdoor 1.45 km FSO link, established at University of Arizona campus, is possible in strong turbulence regime.

PredictionNet: a long short-term memory-based attention network for atmospheric turbulence prediction in adaptive optics.

Adaptive optics (AO) has great applications in many fields and has attracted wide attention from researchers. However, both traditional and deep learning-based AO methods have inherent time delay caused by wavefront sensors and controllers, leading to the inability to truly achieve real-time atmospheric turbulence correction. Hence, future turbulent wavefront prediction plays a particularly important role in AO. Facing the challenge of accurately predicting stochastic turbulence, we combine the convolutional neural network with a turbulence correction time series model and propose a long short-term memory attention-based network, named PredictionNet, to achieve real-time AO correction. Especially, PredictionNet takes the spatiotemporal coupling characteristics of turbulence wavefront into consideration and can improve the accuracy of prediction effectively. The combination of the numerical simulation by a professional software package and the real turbulence experiment by digital holography demonstrates in detail that PredictionNet is more accurate and more stable than traditional methods. Furthermore, the result compared with AO without prediction confirms that predictive AO with PredictionNet is useful.

Single-mode interferometric field of view in partial turbulence correction

Context. The GRAVITY instrument on the ESO VLTI is setting a new mark in the landscape of optical interferometers. Long exposures are possible for the first time in this wavelength domain, delivering a dramatic improvement for astrophysics. In particular, faint objects can be observed at the angular resolution of the VLTI, with exposures exceeding by many orders of magnitude the coherence time of atmospheric turbulence. Aims. Modern interferometers, especially those that combine light collected by large telescopes such as the Unit Telescopes of the VLT, benefit from partial correction of atmospheric turbulence. We investigate in this paper the influence of atmospheric turbulence on the maximum field of view of interferometers such as GRAVITY, where wavefronts are filtered with single-mode fibres. The basic question is whether the maximum field of view is restricted to the diffraction limit of single apertures or if it is wider in practice. We discuss in particular the case of the field around Sgr A* , with an emphasis on the detectability of the massive main-sequence star S2. Methods. We theoretically investigated the interferometric and photometric lobes of the interferometer and took atmospheric turbulence into account. We computed the lobe functions with and without partial correction for atmospheric turbulence. Results. The main result of the paper is that the field of view of the interferometer is widened by tip-tilt residues if higher orders of atmospheric turbulence are corrected for. As a particular case, the S2 star can be detected in interferometric frames centred on Sgr A* even when the distance between the two objects is up to about twice the diffraction limit of a single aperture. We also show that the visibilities are not biased in this case if the pointing accuracies of the fibres are of the order of 10 mas.

Calculations of efficiency of adaptive optics system for atmospheric turbulence correction

The random refractive-index fluctuations associated withatmospheric turbulence is induced wavefront distortions of optical wave propagating trough atmosphere.The adaptive optical systems are designedto achievementsdiffractive-limited image by compensating phase aberrations in real time. The efficiency ofadaptive optics correction is limited by time delay and with taking into account the spatial characteristics are defined by isoplanatic patch size. In this paper we investigate theoretically the efficiency of adaptive optics system for atmospheric turbulence correction. We study isoplanatic patch for X and Y tilt wavefront aberrations in Zernike terms depends on characteristics of turbulent atmosphere for the estimations of the spatio-temporal characteristics of adaptive optical system applications.

Practical considerations for real-time turbulence mitigation in long-range imagery

Atmospheric turbulence degrades imagery by imparting scintillation and warping effects that blur the collected pictures and reduce the effective level of detail. While this reduction in image quality can occur in a wide range of scenarios, it is particularly noticeable when capturing over long distances, when close to the ground, or in hot and humid environments. For decades, researchers have attempted to correct these problems through device and signal processing solutions. While fully digital approaches have the advantage of not requiring specialized hardware, they have been difficult to realize in real-time scenarios due to a variety of practical considerations, including computational performance, the need to integrate with cameras, and the ability to handle complex scenes. We address these challenges and our experience overcoming them. We enumerate the considerations for developing an image processing approach to atmospheric turbulence correction and describe how we approached them to develop software capable of real-time enhancement of long-range imagery.

Pupil-transformation multiconjugate adaptive optics for solar high-resolution imaging

We propose a multiconjugate adaptive optics (MCAO) system called pupil-transformation MCAO (PT-MCAO) for solar high-angular resolution imaging over a large field of view. The PT-MCAO, consisting of two deformable mirrors (DMs), uses a Shack–Hartmann wavefront sensor located on the telescope pupil to measure the wavefront slopes from several guide stars. The average slopes are used to control the first DM conjugated on the telescope aperture by a solar ground-layer adaptive optics (AO) approach while the remaining slopes are used to control the second DM conjugated on a high altitude by a conventional solar AO via a geometric PT. The PT-MCAO uses a similar hardware configuration as the conventional star-oriented MCAO. However, a distinctive feature of our PT-MCAO is that it avoids the construction of tomography wavefront, which is a time-consuming and complex process for the solar real-time atmospheric turbulence correction. For the PT-MCAO, current widely used and fully understood conventional solar AO closed-loop control algorithms can be directly used to control the two DMs, which greatly reduces the real-time calculation power requirement and makes the PT-MCAO easy to implement. In this publication, we discuss the PT-MCAO methodology, its unique features, and compare its performance with that of the conventional solar star-oriented MCAO systems, which demonstrate that the PT-MCAO can be immediately used for solar high-resolution imaging.

Experimental analysis of the fitting error of diffractive liquid crystal wavefront correctors for atmospheric turbulence corrections

An experimental analysis was conducted to investigate the fitting error of diffractive liquid crystal wavefront correctors (LCWFCs). First, an experiment was performed to validate the theoretical equations presented in our previous work Cao et al., 2009 [9]. The results showed an apparent discrepancy between the theoretical and measured results for the fitting error. This difference was examined and the influence of nonlinearities and rounding errors generated by the LCWFC was analyzed and discussed. Finally, the fitting error formula of the LCWFC was modified to obtain a more effective tool for the design of LCWFCs for atmospheric turbulence correction. These results will be useful for researchers who design liquid crystal adaptive optics systems for large-aperture ground-based telescopes.

LINC-NIRVANA for the large binocular telescope: setting up the world’s largest near infrared binoculars for astronomy

LINC-NIRVANA (LN) is the near-infrared, Fizeau-type imaging interferometer for the large binocular telescope (LBT) on Mt. Graham, Arizona (elevation of 3267 m). The instrument is currently being built by a consortium of German and Italian institutes under the leadership of the Max Planck Institute for Astronomy in Heidelberg, Germany. It will combine the radiation from both 8.4 m primary mirrors of LBT in such a way that the sensitivity of a 11.9 m telescope and the spatial resolution of a 22.8 m telescope will be obtained within a 10.5×10.5 arcsec 2 scientific field of view. Interferometric fringes of the combined beams are tracked in an oval field with diameters of 1 and 1.5 arcmin. In addition, both incoming beams are individually corrected by LN’s multiconjugate adaptive optics system to reduce atmospheric image distortion over a circular field of up to 6 arcmin in diameter. A comprehensive technical overview of the instrument is presented, comprising the detailed design of LN’s four major systems for interferometric imaging and fringe tracking, both in the near infrared range of 1 to 2.4 μm, as well as atmospheric turbulence correction at two altitudes, both in the visible range of 0.6 to 0.9 μm. The resulting performance capabilities and a short outlook of some of the major science goals will be presented. In addition, the roadmap for the related assembly, integration, and verification process are discussed. To avoid late interface-related risks, strategies for early hardware as well as software interactions with the telescope have been elaborated. The goal is to ship LN to the LBT in 2014.

Adaptive correction of vortex laser beam in a closed-loop system with phase only liquid crystal spatial light modulator

We propose and demonstrate the wave front correction of a vortex laser beam by using dual phase only liquid crystal spatial light modulators (LC-SLMs) and a stochastic parallel gradient descent (SPGD) algorithm. One phase only LC-SLM is used to generate vortex laser beam by loading spiral phase screen onto the wave front of input quasi-Gaussian beam. The other phase only LC-SLM under SPGD controller based on the subzone control method adaptively compensates the wave front of vortex laser beam. Numerical simulation and experimental results show that after correction, vortex doughnut like beam is focused into a beam with airy disk pattern distribution in the far field. The adaptive corrections of vortex laser beam with different optical topological charges are studied. The results show that the optical topological charge has little influence on adaptive correction. The powers in the main lobe of far field intensity distributions of vortex laser beams with different optical topological charges are all greatly improved by adaptive correction. The technique proposed in this paper can be used in optical communication, relay mirror and atmospheric turbulence correction.

Stress polishing of thin shells for adaptive secondary mirrors

Context. Adaptive secondary mirrors (ASM) are, or will be, key components on all modern telescopes, providing improved seeing conditions or diffraction limited images, thanks to the high-order atmospheric turbulence correction obtained by controlling the shape of a thin mirror. Their development is a key milestone towards future extremely large telescopes (ELT) where this technology is mandatory for successful observations.

Open-loop control of liquid-crystal spatial light modulators for vertical atmospheric turbulence wavefront correction

We present an open-loop adaptive optics (AO) system based on two liquid-crystal spatial light modulators (LCSLMs) that profit from high precision wavefront generation and good repeatability. A wide optical bandwidth of 300 nm is designed for the system, and a new open-loop optical layout is invented to conveniently switch between the open and closed loop. The corresponding control algorithm is introduced with a loop frequency (the reciprocal of the total time delay of a correction loop) of 103 Hz. The system was mounted onto a 2.16 m telescope for vertical atmospheric turbulence correction. The full width at half-maximum of the image of the star α Boo reached 0.636 arc sec after the open-loop correction, while it was 2.12 arc sec before the correction. The result indicates that the open-loop AO system based on LCSLMs potentially has the ability to be used for general astronomical applications.

Extreme adaptive optics in the mid-IR: The METIS AO system

METIS, the Mid-infrared ELT Imager and Spectrograph, is currently in its phase A study as one of the candidate first-light instruments for the European Extremely Large Telescope. METIS will feature several observational modes, ranging from diffraction limited imaging in L, M and N-bands to high-resolution Integral Field spectroscopy for the L and M-bands. METIS in its current design gives sensitivities similar to Spitzer in imaging and low-resolution spectroscopy and with its high-resolution spectrograph will provide unprecedented line sensitivity. The design of METIS is optimized for both galactic science cases (e.g. conditions in the early solar system, formation and evolution of proto-planetary disks and properties of exoplanets) and extragalactic science cases (e.g. the growth of Supermassive Black Holes). METIS will require a high-order adaptive optics (AO) system to meet its scientific goals, both to provide correction for atmospheric turbulence as well as reduce the impact of wind shake, leading to a residual image motion of 3 - 5 mas rms. METIS is expected to feature both an internal Single Conjugate AO system as well as an external Laser Tomography AO system. The challenges for the METIS AO system are mainly in the broad correction range, an excellent image stability required for coronagraphy and in providing a high sky coverage to be available for as many science targets as possible. An additional challenge for METIS is the need to compensate for composition turbulence, mainly in the form of fast fluctuations in water vapor concentration. Water vapor fluctuations impact the performance of METIS in several ways: Atmospheric dispersion causes a broadening of the point-spread function, both in the science channel and the wavefront channel, but can be corrected using a Atmospheric Dispersion Corrector. Variations in the water vapor composition cannot be corrected this way and are currently estimated to give a residual image motion of ≤10 mas rms. This effect can, especially for coronagraphy, not be neglected. Chromatic optical path difference errors, caused by changes in the index of refraction along the path through the atmosphere were found to be negligible in the case of METIS due to attenuation by the outer scale (at typical values of 25 m). Chromatic anisoplanatism is the effect that the light at different wavelengths travels through a slightly different light path through the atmosphere and can be–at least partly–corrected. The last effect is composition turbulence, mainly caused by fast (> 1 Hz) fluctuations in the water vapor content. Based on data for ALMA and radiometer probes, this leads to a maximum loss in Strehl ratio between 5 and 10%. This mainly has an impact on coronagraphy and the METIS AO team is actively investigating ways to compensate also water vapor turbulence. The main challenge is currently obtaining reliable data on the distribution and magnitude of precipitable water vapor fluctuations.

A simple method for evaluating the wavefront compensation error of diffractive liquid-crystal wavefront correctors

A simple method for evaluating the wavefront compensation error of diffractive liquid-crystal wavefront correctors (DLCWFCs) for atmospheric turbulence correction is reported. A simple formula which describes the relationship between pixel number, DLCWFC aperture, quantization level, and atmospheric coherence length was derived based on the calculated atmospheric turbulence wavefronts using Kolmogorov atmospheric turbulence theory. It was found that the pixel number across the DLCWFC aperture is a linear function of the telescope aperture and the quantization level, and it is an exponential function of the atmosphere coherence length. These results are useful for people using DLCWFCs in atmospheric turbulence correction for large-aperture telescopes.

The Telescope: Its History, Technology, and Future (review)

Reviewed by: The Telescope: Its History, Technology, and Future W. Patrick McCray (bio) The Telescope: Its History, Technology, and Future. By Geoff Andersen. Princeton, N.J.: Princeton University Press, 2007. Pp. 248. $29.95. Historians and astronomers have something important in common. Both endeavor to interpret past events on the basis of evidence that has survived to the present. But rather than uncovering the past in archives and via oral history, astronomers typically understand the development of planets, stars, and galaxies by collecting and analyzing light that left distant sources a long time ago. Bigger and better telescopes, as physicist Geoff Andersen explains, enable scientists to gather more light and produce sharper images. And while scientists and engineers study the universe using all sorts of once-exotic wavelengths of light, the ground-based optical telescope is still their primary research tool. [End Page 789] Andersen’s book introduces the reader to the technology and, to a lesser degree, the history behind the astronomer’s workhorse. Historians—who presumably are not the book’s primary audience—will not be surprised to find that the author does not develop a specific historical argument. However, they may be somewhat disappointed that Andersen’s research neglects major works pertaining to the history of modern astronomical instrumentation such as Robert Smith’s essential history of the Hubble Space Telescope (even though Eric Chaisson’s problematic “personal” history of that project is referenced.) First published in Australia and New Zealand in 2006 before appearing in the United States, Andersen’s book is strongest when explaining the technical details of how a modern telescope works and addressing topics such as aberration, the correction of atmospheric turbulence via adaptive optics, and interferometry. In my view, the best chapter is the one titled “So You Want to Build an Observatory?” which walks the reader through the technical challenges scientists must deal with when building a new telescope. The book would have been strengthened and enlivened by more discussion of how astronomers actually use these ever-more-expensive facilities (new observatories on the drawing board are estimated to cost some $1 billion) and deal with the torrents of data they produce. Other than an overview early in the book of the telescope’s history before the twentieth century, the attention to historical context is cursory and not terribly illuminating. To be fair, Andersen’s intent was (contrary to his title) to cover technical workings first and history second. Still, it would have been useful for him to draw on the rich array of books published recently about such fascinating topics as space surveillance or the sociological machinations that the astronomy community resorts to when seeking funding. The book would also have been improved by some basic checking of facts and a few words of clarification in many places. Just an example: Lyman Spitzer Jr. was not exactly “working for RAND” in 1946 (p. 110)—he was a young professor at Yale who later played a key role in championing the space telescope. To continue with this example, by choosing not to reveal more about the embarrassing debacle that initially hobbled Hubble’s mirror, but opting instead for “rumor has it” and “whatever the cause” (p. 120), Andersen leaves the reader less than fully informed and wanting more. Finally, perhaps most distracting is the book’s tendency to wander from topic to topic with little to guide the reader. A chapter on non-traditional observatories covers everything from liquid-mirror telescopes to gravitational wave detectors which are not telescopes at all (and which, to date, have yet to unambiguously observe anything). The focus on the technology of the post-1945 period makes The Telescope a complement to Fred Watson’s 2005 book Stargazer, which treats the telescope’s early history in greater depth. Written in a clear and accessible style, most of Andersen’s story will be familiar to historians and amateur astronomers, [End Page 790] . but perhaps it will prove useful for a general reader who is looking for a gentle introduction to more detailed and comprehensive treatments. W. Patrick McCray Dr. McCray is a professor in the history department at the University of California, Santa Barbara. On a clear...

Ground‐Layer Wave Front Reconstruction from Multiple Natural Guide Stars

Observational tests of ground layer wavefront recovery have been made in open loop using a constellation of four natural guide stars at the 1.55 m Kuiper telescope in Arizona. Such tests explore the effectiveness of wide-field seeing improvement by correction of low-lying atmospheric turbulence with ground-layer adaptive optics (GLAO). The wavefronts from the four stars were measured simultaneously on a Shack-Hartmann wavefront sensor (WFS). The WFS placed a 5 x 5 array of square subapertures across the pupil of the telescope, allowing for wavefront reconstruction up to the fifth radial Zernike order. We find that the wavefront aberration in each star can be roughly halved by subtracting the average of the wavefronts from the other three stars. Wavefront correction on this basis leads to a reduction in width of the seeing-limited stellar image by up to a factor of 3, with image sharpening effective from the visible to near infrared wavelengths over a field of at least 2 arc minutes. We conclude that GLAO correction will be a valuable tool that can increase resolution and spectrographic throughput across a broad range of seeing-limited observations.

Atmospheric dispersion compensation for extremely large telescopes

Achieving diffraction limited imaging with future ground-based optical telescopes will require adaptive optics for correction of atmospheric turbulence and also efficient techniques for atmospheric dispersion compensation. We study the benefit of using a linear atmospheric dispersion corrector (ADC) coupled with a deformable mirror on a 42-m Extremely Large Telescope (ELT) operating in the VIRJ spectral bands. The ADC design consists of two identical thin wedges made of F5 glass. The amount of dispersion introduced by the ADC is adjusted by translating one of the wedges along the optical axis so that it always cancels atmospheric dispersion as it varies with telescope elevation. We show that the ADC working in conjunction with a deformable mirror provides diffraction-limited image quality over a 1-arcmin field.

High-speed horizontal-path atmospheric turbulence correction with a large-actuator-number microelectromechanical system spatial light modulator in an interferometric phase-conjugation engine.

Results of atmospheric propagation for a high-speed, large-actuator-number adaptive optics system are presented. The system uses a microelectromechanical system- (MEMS-) based spatial light modulator correction device with 1024 actuators. Tests over a 1.35-km path achieved correction speeds in excess of 800 Hz and Strehl ratios close to 0.5. The wave-front sensor was based on a quadrature interferometer that directly measures phase. This technique does not require global wave-front reconstruction, making it relatively insensitive to scintillation and phase residues. The results demonstrate the potential of large-actuator-number MEMS-based spatial light modulators to replace conventional deformable mirrors.

Spatially filtered wave-front sensor for high-order adaptive optics.

Adaptive optics (AO) systems take sampled measurements of the wave-front phase. Because in the general case the spatial-frequency content of the phase aberration is not band limited, aliasing will occur. This aliasing will cause increased residual error and increased scattered light in the point-spread function (PSF). The spatially filtered wave-front sensor (SFWFS) mitigates this phenomenon by using a field stop at a focal plane before the wave-front sensor. This stop acts as a low-pass filter on the phase, significantly reducing the high-spatial-frequency content phase seen by the wave-front sensor at moderate to high Strehl ratios. We study the properties and performance of the SFWFS for open- and closed-loop correction of atmospheric turbulence, segmented-primary-mirror errors, and sensing with broadband light. In closed loop the filter reduces high-spatial-frequency phase power by a factor of 10(3) to 10(8). In a full AO-system simulation, this translates to a reduction by up to 625 times in the residual error power due to aliasing over a specific spatial frequency range. The final PSF (generated with apodization of the pupil) has up to a 100 times reduction in intensity out to lambda/2d.

Single-mode versus multimode interferometry: A performance study

We compare performances of ground-based single-mode and multimode (speckle) interferometers in the presence of partial Adaptive Optics correction of atmospheric turbulence. It is first shown that for compact sources (i.e. sources smaller than the Airy disk of a single telescope) not entirely resolved by the interferometer, the remarkable property of spatial filtering of single-mode waveguides coupled with AO correction significantly reduces the speckle noise which arises from residual wavefront corrugations. Focusing on those sources, and in the light of the AMBER experiment (the near infrared instrument of the VLTI), we show that single-mode interferometry produces a better Signal-to-Noise Ratio on the visibility than speckle interferometry. This is true for bright sources (K < 5), and in any case as soon as Strehl ratio of 0.2 is achieved. Finally, the fiber estimator is much more robust -- by two orders of magnitude -- than the speckle estimator with respect to Strehl ratio variations during the calibration procedure. The present analysis theoretically explains why interferometry with fibers can produce visibility measurements with a very high precision, 1% or less