چكيده به لاتين
Atmospheric turbulence is one of the major constraints in astronomical imaging that results from the changes in the atmospheric refractive index. It is important to know the main parameters of the atmospheric turbulence in imaging, astronomy, adaptive optics, free space communications, indoor turbulent environments, and laser propagation in the atmosphere. In this thesis, the parameters of Fried r0, averaged wind speed v_bar , and atmospheric coherence time t0 were estimated using a four aperture differential motion monitor (four aperture DIMM) through a unique analytical theory. The theory presented here showed that the velocity of the defocus aberration is statistically related to the atmospheric turbulence parameters as measured by the angle of arrival (AA) fluctuations and by using the variance of the 4-spot defocus velocity and the extracted analytic relation we could estimate the atmospheric coherence time. The estimation of the turbulence parameters was performed in two simulation and experimental steps. In the first step, sequences of a star image with a frequency of 700 Hz were considered, and the atmospheric defocus and its variations were simulated by a four aperture DIMM instrument for 10 km near the ground with two single-layer and three-layer atmospheric models. In the second step, the experimental data collection was implemented using a four aperture DIMM instrument at the Iranian National Observatory. The empirical approach is based on the four aperture DIMM defocus velocity theory, which uses the angle of arrival measurement of the Capella star light in a 4-spot configuration. Here, we measured the defocus variance and then the variance of the defocus velocity to estimate the turbulence parameters using the theory presented. The data were collected at the top of the Gargash Mountain at the Iranian National Observatory site at 3,600 meters above sea level using a 12-inch Cassegrain Meade telescope which provided us with short exposure images of 480 to 620 frames-per-second imaging rate for the Capella star. The results of the data analysis showed that in the four aperture defocus velocity method, the averaged wind speed was definitely estimated, and other turbulence parameters were estimated more precisely due to the symmetry in the four aperture DIMM configuration. The experimental data from four aperture DIMM instrument measurement and using the 4-aperture defocus velocity theory led to estimations of r0=8 to 16 cm, v_bar= 10 to 25 m/s and t0=1.7 to 4 ms in which were in good agreement with simulation results and other experimental methods with error less than 20 %.
