Study On Measuring Method Of Greenwood Frequency Of Atmospheric Turbulence

Characteristics of atmospheric turbulence is the fundamental basis for the sitingof large-diameter ground-based telescope and the design of adaptive optical system.The atmospheric coherence length and Greenwood frequency is the two criticalparameters for describing the characteristics of atmospheric turbulence, so theirmeasurement methods need to be studied. Currently, the method of measuringatmospheric coherence length is relatively mature, but it is not good for themeasurement of Greenwood frequency. Therefore, this thesis is researched on theaccurate measurement of Greenwood frequency to provide a direct basis for thesiting of large-diameter telescope and the design of adaptive optical system.As the problems of the phase difference method, such as that the measurementaccuracy of Greenwood frequency is very low, and it doesn’t consider themeasurement parameters and so on. Some works are conducted in this paper to solvethe problems.Firstly it proposed an high accuracy and speed reconstruction method forarbitrary shape wave-front. By coupling the information of wave-front shape to thegeometry matrix, it greatly improves the reconstruction accuracy. Compared totraditional wave-front reconstruction method and iterative method, the reconstructionaccuracy increased30times and the calculation speed increased3times respectively.Secondly the impact of effective sampling points and the sampling density on wave-front reconstruction accuracy have been studied, the results show that for theannular wave-front with5λ magnitude, when the effective sampling points andsampling density increases600and65respectively, it had no effect on thereconstruction accuracy. Which provides necessary conditions for the measurementof atmospheric turbulence Greenwood frequency accurately.Thirdly it studied the removing method of noise in the Hartmann detector. Forthe turbulence of62Hz, the measurement deviation decreased from30%to0.6%byremoving the noise, which greatly improved the measurement accuracy. And theeffects of measurement parameters on the measurement accuracy of Greenwoodfrequency is analyzed, when the spot size is3×3pixels, sampling points and thestatistical number are400, the stable and accurate measurements can be achieved.Then it studied the relationship between the Greenwood frequency and its maximumsampling time, and get the empirical formula，which provides an basis for theselection for the frame rate of Hartmann detector.Finally, the turbulence of8Hz-108Hz is measured, the results show that themeasured value is almost the same as theoretical value without consideration of thedisturbance of the air in the Lab. As the disturbance would not exist when itmeasures the Greenwood frequency outside, it is considered to achieve an accuratemeasurement of Greenwood frequency.In summary, this thesis ultimately achieve the accurate measurements ofGreenwood frequency of atmospheric turbulence with the high precision wave-frontreconstruction method, the Hartmann detector noise cancellation method and theappropriate selection of various measurement parameters, which provide theoreticaland practical significance for the measurement of Greenwood frequency, and providethe fundamental basis for site selection of telescope and design of adaptive opticssystems.