The temporal error of the adaptive optical system leads to a significant degradations in the characteristics of the system operating in the atmosphere. One of the methods for solving this problem is the use of prediction algorithms based on the analysis of the evolution of phase fluctuations. In the paper the wavefront sensor as the key element of atmospheric adaptive optical system with predictions algorithms is considered. The results of the development and testing of the Shack-Hartmann wavefront sensor providing measurements of phase fluctuations, determination of the Fried parameter and wind speed using original design solutions and software are presented. The practical and theoretical aspects of using the Shack-Harmann wavefront sensor are discussed. For it dynamic range, sensitivity and accuracy of the sensor are estimated. The influences of parameters of microlens array on range of measurements of the Shack-Harmann wavefront sensor are studied. The tests of the S-H WFS were carry out with acoustic measurements of wind speed and the structural constant of the refractive index of the atmosphere, as well as in adaptive optics system in laboratory test bench.
The results of experimental studies of the effect of atmospheric turbulence in the adaptive optical image correction system at the Baikal Large Solar Vacuum Telescope (LSVT) are presented. To eliminate the jitter and stabilize the image on the receiver, the appropriate hardware and software system that corrects the tilts of incoming wave front with frequencies up to 1 kHz has been developed in the Laboratory of the Coherent Adaptive Optics (LCAO). To obtain digital images of high resolution, the tip-tilt adaptive system is combined with a post-detection computer processing of frames using fast 2D parallel real time algorithms. Experimental data confirm the high efficiency of the dual adaptive system for stabilizing and forming images on the LSVT.
Fast adaptive optical system can be used, for example, for correction of laser beam passed through a strong turbulent atmosphere. The frequency that such a system should operate with to achieve an acceptable level of wavefront correction is about 1 - 1.5 kHz. There are two most popular methods to develop this system: by using a standard PC computer and by using FPGA. This paper presents the advantages and disadvantages of each of these approaches. The results obtained with the use of these systems are presented. Recommendation for achieving higher performance are given.
The optical turbulence characteristics statistics including "seeing", Fried radius, wind speed height profiles are discussed. Distribution of the mean Fried radius obtained from the data of image motion measurements by the Brandt sensor is given. Also, the Fried radius values calculated from the Shack-Hartmann data are given. Using the height profile of the structure characteristic of air refractive index fluctuations obtained from spectral multiscale turbulence model the results of the Fried radius simulations from micrometeorological mast measurement data given.
The studies were carried out preliminary within the framework of the RSF project 15-19-20013 "Creation of an adaptive system ensuring operation of a large-sized solar telescope under conditions of strong atmospheric turbulence." It is known that the use of adaptive optics (AO) systems is the most radical way for reducing the effect of atmospheric distortions. AO systems correct distortions introduced by the Earth's atmosphere into the image of a space object in real time. The creation of an AO system for large-aperture solar telescopes still is a scientific and engineering challenge. Currently, for the largest Russian solar telescopes, in particular, the Large Solar Vacuum Telescope (LSVT), the Automated Solar Telescope (AST) and the 3-meter Large Solar Telescope (LST) under development, promising new- generation systems are being developed and implemented based on the principles of multi-mirror adaptive correction.
The history of the development of works on the creation of the elemental base of adaptive optics for a solar Russian telescope is briefly described. We consider separate issues of the development of the wavefront sensor (WFS) based on the Shack–Hartmann correlation sensor to provide measurements under conditions of strong turbulence. The parameters of the WFS are calculated on the basis of long-term observations of meteorological characteristics and optical measurements of the Fried parameter. A special adjusting device with a block of fast rasters changing is developed. The use of rasters of various dimensions is provided in the WFS. The sensor uses the original algorithm for determining the maximum of the correlation function. The effect on the accuracy of measuring the phase distortions of turbulence occurring in the rooms of the telescope itself is analyzed.
The possibilities of forming optical images on horizontal extended atmospheric paths are explored. Two different methods for improving image quality are analyzed. It is known that at the present time the solution of the problem of long-range vision with super-high resolution is conducted along several independent lines, firstly, on the development of methods based on the classical technique of adaptive optics, i.e., by correcting the distorted wavefront itself, and, secondly, on the use of digital post-detection techniques, as well as, on the way to attract purely engineering solutions. These methods are applied, both for terrestrial systems, and for astronomical instruments. They can be instrumental (for example, adaptive correction, the use of polarization filters, receiver gating, etc.), mixed (adaptive correction and subsequent processing of images on computers or special processors) or program-algorithmic only. The analysis is carried out for systems operating on horizontal paths. This, first of all, is due to the fact that any horizontal path by the strength of turbulence far exceeds any astronomical one. On extended horizontal paths, in addition to phase distortions that cause the effects of jitter and blurring of the image, there are fluctuations in the intensity of the received radiation, which leads to the appearance of flickering effects of the image, as well as to the manifestation of ambiguity in describing the phase distortions of the optical wave. Numerical and analytical calculations are performed. Experiments were carried out on the atmospheric paths from 160 m to 3.2 km long in city conditions.
The work discusses the spatial scales of atmospheric optical distortions including the outer scale of turbulence and the Fried radius. It is assumed that the energy spectrum of atmospheric turbulence is not limited strictly  and the outer scale is considered in application to astronomical telescopes. In the case when the telescope diameter is larger or comparable with the outer scale the optical distortions substantially differ from the results of the Kolmogorov model. For a given diameter it is possible to introduce a certain spatial scale at distances larger than the size of which the refractive index fluctuations no longer have a significant effect on the quality of astronomical images. Estimates of the outer scale of turbulence are reported for both a atmospheric layer from 0 up to 20 km and surface layer.
According to the work plan for the RSF project, during 2016 measurements were taken in all seasons of the year: February, April, May, August and October. With the use of the whole set of equipment of the stand on the BSVT, the task was set to work out methods for recording and correcting the distortions of the phase of optical radiation passing through a layer of turbulent atmosphere. Complex on-site meteorological observations were organized and conducted at the site of the BSVT. Observations were carried out with the aim of developing and improving the local computational model of turbulent characteristics in the entire thickness of the active atmosphere in the "optical turbulence" range, including the surface layer. As the initial meteorological information for calculations, the model uses two-level data of pulsating observations of temperature and wind speed at the BSVT site, as well as current NCEP/NCAR archival data for the period from 1948 to 2015.
The results of optical measurements of the quality of astronomical seeing on the Large solar vacuum telescope (LSVT) in spring and summer are shown. It is noticed that in the summer measurements, the quality of vision is higher on average 2.5 times than in the spring. Information on the seasonal variability of the astronomical quality of vision can be useful in the planning of scientific experiments for the LSVT, as well as to improve the performance of existing adaptive system
In this article, we describe the development of the newest adaptive optics system for the Big Solar Vacuum Telescope of the Baikal Astrophysical Observatory. This system is a result of collaboration between VE Zuev Institute of Atmospheric Optics SB RAS, Tomsk, and Institute of Solar-Terrestrial Physics SB RAS, Irkutsk. The system includes two active mirrors for the correction: domestic tip-tilt and bimorph deformable (Active Optics NightN Ltd.), and separate wavefront sensors (WFS). A correlation S-H wave-front sensor is based on a Allies Prosilica GX-1050 GigE camera with speed of 309 Hz and frame size of 1248x1248 pixels. A personal computer is used for bimorph deformable mirror image processing. The mirror was successfully used during the 2010–2014 observing seasons. The system developed is capable of correcting up to 35 modes, thus providing diffraction limited images at visible wavelengths.
The estimations of the Fried parameter according to micrometeorological and optical measurements in the atmospheric surface layer in the area of l. Baikal, Baikal astrophysical Observatory (BAO). According to the archive of NCEP/NCAR Reanalysis data obtained vertical distribution of temperature pulsations, and revealed the most pronounced atmospheric layers with high turbulence. It is established that the values of the fried parameter at the location of the BAO are in the range from 1.5 to 5.5 cm in inter, the atmospheric coherence radius is characterized by low values of the Fried parameter. Turbulyzed atmospheric layers of the atmosphere located at an altitude of about 2.5 km and 11.5 km above the observatory, respectively. The average values of the fried radius is 4.6 cm.
The criteria image qualities based on wave front aberration caused by atmospheric turbulence using in adaptive optics are summarized. Atmospheric turbulence profile for Big Solar Vacuum Telescope (BSVT) observatory is obtained based on satellite date. On this based the development of adaptive optics systems of BSVT are discussed.
The estimations of the fried parameter according to micrometeorological and optical measurements in the atmospheric surface layer in the area of lake Baikal, Baikal astrophysical Observatory. According to the archive of NCEP/NCAR Reanalysis data obtained vertical distribution of temperature pulsations, and revealed the most pronounced atmospheric layers with high turbulence. A comparison of astronomical conditions vision in winter and in summer. By the registration of optical radiation of the Sun with telescopes, ground-based there is a need to compensate for the effects of atmospheric turbulence. Atmospheric turbulence reduces the angular resolution of the observed objects and distorts the structure of the obtained images. To improve image quality, and ideally closer to angular resolution, limited only by diffraction, it is necessary to implement and use adaptive optics system. The specificity of image correction using adaptive optics is that it is necessary not only to compensate for the random jitter of the image as a whole, but also adjust the geometry of the individual parts of the image. Evaluation of atmospheric radius of coherence (Fried parameter) are of interest not only for site-testing research space, but also are the basis for the efficient operation of adaptive optical systems 1 .
Experimental data of our turbulence measurements (by ultrasonic digital sensors) in the various areas and meteosituations show that large areas are often observed in open atmosphere, in which one large coherent structure has the main influence. Turbulence in such areas is called as coherent. Incoherent Kolmogorov turbulence is detecting, as a rule, on sites with a flat surface. Coherent turbulence differs from Kolmogorov turbulence by the faster decrease of a time spectrum in an inertial interval (f – 8/ 3 instead of f – 5/ 3) and the smaller contribution of the high-frequency components.
We are considering of new devices for solar astronomical telescope as a tools for adaptive optics correction. One of
them is a high precision Shack-Hartmann wave-front sensor has been developed on the basis of a low-aperture offaxis
diffraction lens array. The second device is image quality analyzer. Efficiency of the adaptive optical in system
of the imaging is valued by quality of the updated image.