In connection with the wide use of optoelectronic systems, we review the development of adaptive optics as an effective tool that allows using controllable optical elements to eliminate irregular distortions that occur as light propagates in an inhomogeneous medium. The subject matter of this rapidly developing field of science and technology is described. Of the ideas under development in recent years, many have been around for quite a long time, but it is only now, with the development of an up-to-date optoelectronic element base, that they have started being widely incorporated into science and engineering practice. We discuss the development of adaptive optics from mere ideas to their application in astronomy, high-power laser physics, and medicine. The current state of adaptive optics in stellar and solar astronomy is reviewed, and some results of its use in distortion correction systems of high-power laser systems and facilities are presented.

https://iopscience.iop.org/article/10.3367/UFNe.0184.201406b.0599/pdf

Adaptive optics in the formation of optical beams and images

With the recent increased usage of video services, the focus has recently shifted from the traditional quality of service-based video delivery to quality of experience (QoE)-based video delivery. Over the past 15 years, many video quality assessment metrics have been proposed with the goal to predict the video quality as perceived by the end user. HTTP adaptive streaming (HAS) has recently gained much attention and is currently used by the majority of video streaming services, such as Netflix and YouTube. HAS, using reliable transport protocols, such as TCP, does not suffer from image artifacts due to packet losses, which are common in traditional streaming technologies. Hence, the QoE models developed for other streaming technologies alone are not sufficient. Recently, many works have focused on developing QoE models targeting HAS-based applications. Also, the recently published ITU-T Recommendation series P.1203 proposes a parametric bitstream-based model for the quality assessment of progressive download and adaptive audiovisual streaming services over a reliable transport. The main contribution of this paper is to present a comprehensive overview of recent and currently undergoing works in the field of QoE modeling for HAS. The HAS QoE models, influence factors, and subjective test methodologies are discussed, as well as existing challenges and shortcomings. The survey can serve as a guideline for researchers interested in QoE modeling for HAS and also discusses possible future work.

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QoE Modeling for HTTP Adaptive Video Streaming–A Survey and Open Challenges

A study of atmospheric adaptive optics. It covers: adaptive phase correction of optical wave distortions in random inhomogeneous media; enhancement of images transmitted through the atmosphere by methods of adaptive optics; and equations for the moment of the corrected field.

Atmospheric Adaptive Optics

A method to expand the dynamic range for a Shack–Hartmann wavefront sensor (SHWFS) is proposed. An SHWFS consists of a microlens array and an image sensor, and it has been widely used to measure the wavefront aberration of a lightwave in various fields. However, a very large aberrated wave cannot be correctly measured due to the finite dynamic range that depends on the diameter of each microlens. The proposed method enables an SHWFS to measure wavefronts with larger aberrations by applying holography and pattern matching technologies. For measurement of a spherical wave, the proposed method is compared with a conventional one by numerical simulations and optical experiments. Their results confirm the performance of the proposed method.

Holographic Shack–Hartmann wavefront sensor based on the correlation peak displacement detection method for wavefront sensing with large dynamic range

In ultrasonic equipment (anemometers and thermometers), for the measurement of parameters of atmospheric turbulence, a standard algorithm that calculates parameters from temporary structural functions constructed on the registered data is usually used. The algorithm is based on the Kolmogorov–Obukhov law. The experience of using ultrasonic meters shows that such an approach can lead to significant errors. Therefore, an improved algorithm for calculating the parameters is developed, which allows more accurate estimation of the structural characteristics of turbulent fluctuations, with an error that is not more than 10%. The algorithm was used in the development of a new ultrasonic hardware-software complex, autonomous meteorological complex AMK-03-4, which differs from similar measuring instruments of turbulent atmosphere parameters by the presence of four identical ultrasonic anemometers. The design of the complex allows not only registration of the characteristics of turbulence, but also measurement of the statistical characteristics of the spatial derivatives of turbulent temperature fluctuations and orthogonal components of wind speed along each of the axes of the Cartesian coordinate system. This makes it possible to investigate the space–time structure of turbulent meteorological fields of the surface layer of the atmosphere for subsequent applications in the Monin–Obukhov similarity theory and to study turbulent coherent structures. The new measurement data of the spatial derivatives of temperature at stable stratification (at positive Monin–Obukhov parameters) were obtained, at which the behavior of the derivatives was been investigated earlier. In the most part of the interval of positive Monin–Obukhov parameters, the vertical derivative of the temperature is close to a constant value. This fact can be considered as a new significant result in similarity theory.

https://www.mdpi.com/2073-4433/10/8/460/pdf

Measurement of Atmospheric Turbulence Characteristics by the Ultrasonic Anemometers and the Calibration Processes

New results obtained within Russian Science Foundation Project no. 15-19-20013 are presented. The project was started in the middle of 2015 and is aimed at the solution of actual problems of the development of the optoelectronic industry using adaptive optics for astronomy, laser, and other applications. The progress in the development of the up-to-date adaptive optics system for the largest Russian solar telescope, i.e., the 1-m Big Solar Vacuum Telescope of the Baikal Astrophysical Observatory, is described.

Adaptive optics system for solar telescope operating under strong atmospheric turbulence

A Shack-Hartmann type wavefront sensor designed on the basis of a low-aperture off-axis diffraction lens array is described. Tests experiments showed that the sensor is capable of measuring wavefront slopes at array subapertures of size 640×640 µm with an error not exceeding 4.80 arcseconds (0.15 pixel), which corresponds to the standard deviation (SD) (SD = 0.0170λ) at the reconstructed wavefront. Advantages of the array used in the sensor and the technology of its manufacture are discussed. The sensor tested in the experiments can be used to measure atmospheric turbulence parameters and as an element of adaptive optical imaging systems.

Shack-Hartmann sensor based on a low-aperture off-axis diffraction lens array

Image quality analyzer (IQA) which used as device for efficiency analysis of adaptive optics application is described. In
analyzer marketed possibility estimations quality of images on three different criterions of quality images: contrast,
sharpnesses and the spectral criterion. At present given analyzer is introduced on Big Solar Vacuum Telescope in stale
work that allows at observations to conduct the choice of the most contrasting images of Sun. Is it hereinafter planned use the analyzer in composition of the ANGARA adaptive correction system.

Image quality analyzer

A high precision Shack-Hartmann wavefront (WF) sensor has been developed on the basis of a low-aperture off-axis
diffraction lens array. The device is capable of measuring WF slopes at array sub-apertures of size 640x640 μm with an
error not exceeding 4.80 arcsec (0.15 pixel), which corresponds to the standard deviation equal to 0.017λ at the
reconstructed WF with wavelength λ . Also the modification of this sensor for adaptive system of solar telescope using
extended scenes as tracking objects, such as sunspot, pores, solar granulation and limb, is presented.
The software package developed for the proposed WF sensors includes three algorithms of local WF slopes estimation
(modified centroids, normalized cross-correlation and fast Fourier-demodulation), as well as three methods of WF
reconstruction (modal Zernike polynomials expansion, deformable mirror response functions expansion and phase
unwrapping), that can be selected during operation with accordance to the application.

/pdf/wavefront-sensors-for-adaptive-optical-systems-2b8vl87zkd.pdf

Wavefront sensors for adaptive optical systems

Wavefront aberrations at the entrance aperture of the Large Solar Vacuum Telescope were measured with a wavefront sensor of the adaptive optics system by a sunspot. To calculate the image shifts, a correlation algorithm with quadratic interpolation of the correlation function maximum position is used. The quality of astronomical vision, characterized by the Fried length, was estimated from the same experimental data as the statistical characteristics of the fluctuations of the coefficients of expansion of wavefront aberrations in Zernicke polynomials. The results were obtained at a Fried length of 51.6 mm in a sample 43 s long with a sampling frequency of 70 Hz. The means and standard deviations of the expansion coefficients are calculated. The analysis of the given spectra shows that the wavefront aberrations should be compensated in the frequency band 0–20 Hz for the effective correction of the images formed.

O. N. Emaleev

Papers

Adaptive optics system for solar telescope operating under strong atmospheric turbulence

Shack-Hartmann sensor based on a low-aperture off-axis diffraction lens array

Image quality analyzer

Wavefront sensors for adaptive optical systems

Development of Components for Adaptive Optics Systems for Solar Telescopes