Annotation

The project is aimed at solving the problem of improving the efficiency of Eurasia's largest 6-meter astronomical telescopes located at low altitude relative to sea level (BTA SAO RAS) and operating, as the other telescopes in Russia, in conditions of strong atmospheric turbulence and aerosol scattering. The low efficiency of this telescope is due to its location and, as a result, the influence of atmospheric structural inhomogeneities — turbulence, refraction, aerosol particles. Random inhomogeneities of the refractive index lead to significant decrease in the angular resolution of the telescope and to decrease in the light flux to the resolution element. This, in turn, reduces the possibility of observing point-like astronomical objects (stars, quasars), both in photometric and spectroscopic studies. The problem of increasing the efficiency of the BTA is solved by developing the effective spectral equipment, using promising optical schemes, reducing losses on the optical surfaces, and increasing the quantum efficiency of radiation receivers. However, the BTA remains the only large-diameter telescope that does not use the methods of drastic increasing the efficiency of telescopes - the methods of active and adaptive optics.

   One of the ways of solving these problems — the use of adaptive optical elements which are able to compensate for distortions of optical radiation in turbulent media. At the same time, the incorporation of adaptive optical systems in large astronomical telescopes is quite a complex scientific and technological task and requires an individual approach. For correct choice of elements and methods of compensation of phase fluctuations caused by atmospheric turbulence it is necessary:

   1) to carry out fundamental researches of variations of the variability of different parameters of an astroclimate (daily and seasonal);

   2) to develop individual methods of phase measurement in a context of low signal-to-noise ratio.

The latter task is especially actual while the development of wavefront sensors for the telescopes operating without an artificial reference star.

   The aim of this project is to solve a topical scientific and technical problem related to the application of methods and adaptive optics system for the 6-meter telescope of Special astrophysical observatory RAS to increase its resolution in turbulent and scattering atmosphere. For the first time the problem of compensation of phase fluctuations in wide-aperture (6 and more meters) ground telescopes located in rather unfavorable astroclimatic conditions will be set and, most importantly, solved.

   One of the key features of the developed adaptive system for the BTA SAO RAS telescope will be the use of four correction loops with modern (including extremely fast) deformable mirrors and a fundamentally new algorithm for the adaptive loop, as well as programmable special units based on FPGA technology. The first loop will include compensation for quasi-static aberrations of the main mirror of the telescope, including the aberrations caused by its displacements inside the dome. The second loop will be designed to correct for the general tilts of the radiation coming from the reference star – it should be rather slow stabilizing system. And, the other two loops will include compensation for low- and high-order aberrations. An important problem (but still unsolved) is the need to obtain information about the residual phase fluctuations for each correction loop. A local model of turbulent atmosphere will be used to develop adaptive system control algorithms. This model will be obtained as a result of studies using measurements of the current state of the level of turbulence of the atmosphere near the telescope. When radiation propagates over a long track with strong turbulence, the frequency of phase fluctuations can exceed 200 Hz. An adaptive system operating at a frequency of more than 2000 Hz is needed to correct these distortions. Shack-Hartmann sensor, used in most modern adaptive systems, is able to provide a data processing speed of several kHz even in the case of a large number of subapertures of microlens array (about 400). However, with the increase in the frequency of image acquisition, the exposure time is reduced, and, the amount of light energy entering the wavefront sensor is reduced accordingly. This leads to the need for an ultra-sensitive cooled camera, which has its own problems. Therefore, the use of Shack-Hartmann sensor and the phase conjugation algorithm in such a system is associated with some difficulties, and sometimes it is fundamentally impossible, also due to the non-uniqueness of the solution of the wavefront approximation problem by the least squares method as it was shown in the works of J. Sheldakova. The problem is not only the development of algorithms for controlling adaptive mirrors, but also the electronic system itself - modern computers and their interfaces do not allow to control the output stage of amplifiers with frequencies over than 700 - 800 Hz. Therefore, in the project it is proposed to use FPGA. Moreover, a fast adaptive optical multi-dither system under FPGA control consisting of a stacked-actuators deformable mirror, a mirror control unit, and a high-speed optical radiation receiver with a phase detector will be developed. To achieve the required speed, the system will use multi-dither algorithm with frequency modulation of adaptive corrector channels. Our preliminary estimates have shown that the frequency of one loop of correction of such an adaptive system might reach 2500-3000 Hz, which will allow to compensate for the phase fluctuations on low-aperture ground-based telescopes with a frequency of 250-300 Hz.

   Based on the results of laboratory and field tests of the model, the schemes of existing high-resolution BTA spectrographs will be finalized, and proposals for the development of ultra-high spectral resolution systems will be set