Annotation of the results obtained in 2020

1. Review of the results characterizing the problem of applying methods of adaptive optics on the BTA is carried out. The properties of the BTA complex (telescope + automatic control system) as an oscillatory system are considered. Information about the astroclimatic characteristics of the BTA installation site and the characteristics of the telescope itself in the tower is analyzed. The possibility of AO systems for solving low-frequency problems of BTA optics (mechanical and thermal deformations of the main mirror, focus drift) is estimated. The possibilities of using AO tools in high-resolution spectroscopy methods, primarily on the NES BTA spectrograph, are considered. The gain provided by the AO is estimated both in the flux luminosity and in the number of simultaneously transmitted spectrum elements. Based on the results of the work, a paper was published: V. G. Klochkova, Yu. V. Sheldakova, V. V. Vlasyuk, and A. V. Kudryashov, "Improving the efficiency of high-resolution spectroscopy on the 6-m telescope using adaptive optics techniques". Astrophys. Bull., v.75, No.4, 360-373, 2020 https://www.sao.ru/Doc-k8/Science/Public/Bulletin/Vol75/N4/ASPB528.pdf .
2. A model of radiation propagation in a turbulent medium has been developed, which makes it possible to estimate the possible degree of aberration correction. The appearance of points with zero intensity in a plane wave was considered. The dependence of the results on the number of grid nodes is shown. To complement the model of radiation propagation through a turbulent medium, a numerical study of the propagation of laser radiation through a randomly inhomogeneous medium was carried out. The results are described in a paper: I. Galaktionov, J. Sheldakova, A. Nikitin, V. Samarkin, V. Parfenov, A. Kudryashov, "Laser beam focusing through a moderately scattering medium using a bimorph mirror", Opt. Express 28(25), pp.38061-38075, 2020 https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-28-25-38061 .
3. The spectral characteristics of atmospheric turbulence simulated in the laboratory are investigated. Based on the results, A.N. Nikitin, A. Rukosuev, J.V. Sheldakova, V.M. Belousov, I. Galaktionov, V. Samarkin, A. Kudryashov, "FPGA-based 2 kHz closed-loop adaptive optical system with stacked actuator deformable mirror", Proc. SPIE 11508, Unconventional Imaging and Adaptive Optics 2020, 115080K, 2020 and A. Kudryashov, A. Rukosuev, A. Nikitin, I. Galaktionov, J. Sheldakova, "Real-time 1.5 kHz adaptive optical system to correct for atmospheric turbulence", Opt. Express 28(25), pp.37546-37552, 2020 https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-28-25-37546 .
4. A system for stabilizing the position of the light beam has been developed. The FPGA architecture has been developed, and a data exchange protocol between the FPGA and the upper-level software has been created.

Annotation of the results obtained in 2021:1.

1. Laboratory tests of the FPGA-based radiation stabilization system have been carried out. The setup for stabilizing the position of the light beam consisted of an FPGA, two mirrors with control units, two-quadrant photodiodes, and a computer. The beam position in space was stabilized using two mirrors, which, depending on the applied voltage, made it possible to change their tilt both along the X and Y axes. The tilts of the mirror were provided using piezoceramic packages with a capacity of 1 μF, which are structurally aligned with the adjusting screws. After reflection from the mirrors, the laser beam was split into two beams, the position of each part was fixed using a square photodiode. The control was carried out using an FPGA, which included calculating the coordinates of two beams and the generation of voltages for feeding to the correction devices. The control unit for the correcting mirror was a direct current amplifier that provided amplification of the control signal generated by the FPGA to a value determined by the parameters of the piezoelectric package of the control mirror (in the range from -30 to + 190 volts). A digital processing module on the FPGA platform, which digitizes voltages from photodiodes, performs an algorithmic procedure for calculating voltage codes for a correcting mirror and makes a digital-to-analog conversion for a correcting mirror. Experiments carried out on the prototype of the facility showed that the use of a system for stabilizing the beam's position in space made it possible to reduce the amplitude of the yaw of the beam by approximately 15 dB. The residual angular error was +/- 2 μrad. A working correction system corrects the constant beam displacement and its dynamic component. The beam deviations from the center of one of the quadrant sensors were recorded before and after the correction.
2. A slicer for the star image generated by the adaptive system has been developed. The adaptive system partially eliminates the distortion of the star image caused by atmospheric turbulence. However, the resulting decrease in the angular dimensions of the turbulent disk of the stellar image is not always sufficient to match the spectrograph slit width with the resolution element of the receiver that records the spectrum. Two types of stellar image slicers are considered that are optimal for working with an echelle spectrograph equipped with a set of cross-dispersion diffraction gratings. One of the designs has been tested. As the adaptive optics is installed before the image slicer, matching the slicer and the adaptive optics layout is needed. The used designs of the slicer significantly minimize the loss of light at the input of the matched spectrograph.
3. Studies have been carried out to obtain information on distortions along the path of radiation propagation from data characterizing the distribution of optical vortices in the observation plane. The study of the features of the propagation of vortex radiation was carried out based on numerical experiments. Phase modulation was performed in the plane of the source aperture using one of the Zernike polynomials or a phase screen, the spectral density of refractive index fluctuations, on which the von Karman spectrum was set. A detailed description of the used algorithms is given in the publication [Kanev F.Yu., Aksenov V.P., Veretekhin I.D. Analysis of the accuracy of algorithms for registering optical vortices // Optics of the atmosphere and ocean. 2021.Vol. 34.No. 1.P. 5-16. DOI: 15372 / AOO 202110101]. It is shown that the presence of vortices in the wavefront means that polynomials describe the phase of the radiation with a number higher than 5 (coma, trefoil). Specifying the phase by tilt, defocus, astigmatism does not lead to the appearance of dislocations in areas with a diameter less than 1.4 of the initial beam diameter.

Publications

1. Toporovsky V.V., Kudryashov A.V., Samarkin V.V., Rukosuev A.L., Nikitin A.N., Sheldakova Yu.V., Otrubyannikova O.V. Исследование охлаждаемого деформируемого зеркала на пьезоактюаторах для коррекции атмосферных флуктуаций фазы Оптика атмосферы и океана, 33, №9, сс.677-684 (year - 2020).

2. Sheldakova Yu.V., Rukosuev A.L., Belousov V.N, Galaktionov I.V., Nikitin A.N., Samarkin V.V., Kudryashov A.V. FPGA-based 2 kHz closed-loop adaptive optical system with stacked actuator deformable mirror Proceesings of SPIE, 11508, сс.115080K (year - 2020).

3. Klochkova V.G., Sheldakova Yu.V., Vlasyuk V.V., Kudryashov A.V. О повышении эффективности спектроскопии высокого разрешения на БТА методами адаптивной оптики Астрофизический бюллетень, том 75, № 4, сс.528–542 (year - 2020).

4. I.Galaktionov, J.Sheldakova, A.Nikitin, V.Samarkin, V.Parfenov, A.Kudryashov Laser beam focusing through a moderately scattering medium using a bimorph mirror Optics Express, Vol.28, No.25/7, pp.38061-38075 (year - 2020).

5. A.V. KUDRYASHOV, A.L. RUKOSUEV, A.N. NIKITIN, I.V. GALAKTIONOV, J.V. SHELDAKOVA Real-time 1.5 kHz adaptive optical system to correct for atmospheric turbulence Optics Express, Vol. 28, Issue 25, pp. 37546-37552 (year - 2020).