Changhui Rao

Spatial and temporal characterization of phase fluctuations in non-Kolmogorov atmospheric turbulence

Abstract Atmospheric turbulence severely limits the performance of ground-based imaging and laser propagation systems. Some observational results, showing atmospheric turbulence which does not obey Kolmogorovs theory, have prompted the study of optical propagation through non-Kolmogorov turbulence. This paper presents a theoretical approach to analyse the spatial and temporal characterizations of phase fluctuations in non-Kolmogorov turbulence. The spatial structure function, the temporal structure function and the temporal power spectrum of phase fluctuations are derived. The generalized coherence length ρ0, the characteristic frequency fR and the characteristic time TR are expressed as functions of the index structure constant along the propagation path and the wind velocity. The long exposure MTF, the short exposure MTF and the imaging Strehl ratio are computed.

New vacuum solar telescope and observations with high resolution

The New Vacuum Solar Telescope (NVST) is a one meter vacuum solar telescope that aims to observe fine structures on the Sun. The main goals of NVST are high resolution imaging and spectral observations, including measurements of the solar magnetic field. NVST is the primary ground-based facility used by the Chinese solar research community in this solar cycle. It is located by Fuxian Lake in southwest China, where the seeing is good enough to perform high resolution observations. We first introduce the general conditions at the Fuxian Solar Observatory and the primary science cases of NVST. Then, the basic structures of this telescope and instruments are described in detail. Finally, some typical high resolution data of the solar photosphere and chromosphere are also shown.

Error analysis of CCD-based point source centroid computation under the background light.

The CCD-based point source centroid computation (PSCC) error under the background light is analyzed integrally in theory, numerical simulation and experiment. Furthermore, a comprehensive formula of the PSCC error caused by the diversified error sources is put forward. The optimum threshold to reduce the effects of all the error sources to a minimum is selected. The best threshold level is N(B) +3sigma(B), where N(B) is the average value of the error sources and sigma(B) is the mean-square value of the fluctuation of the error sources. The simulation and experiment results are in great accordance with the theoretical analysis.

Performance of the 37-element solar adaptive optics for the 26 cm solar fine structure telescope at Yunnan Astronomical Observatory

A 37-element solar adaptive optics system, which consists of a fine tracking loop with a tip/tilt mirror and a correlation tracker, and a high-order correction loop with a 37-element deformable mirror, a correlating Shack-Hartmann wavefront sensor, and a real-time controller, was built and installed at the 26 cm solar fine structure telescope of the Yunnan Astronomical Observatory in 2009. In this system, the absolute difference algorithm is used. A new architecture with field-programmable gate array (FPGA) and digital signal processor (DSP) for the real-time controller based on systolic array and pipeline was developed. The computational latencies of the fine tracking loop and high-order correction loop are about 35 and 100 mu s, respectively. The tracking residual root-mean-square error is less than 0.1 arcsec, and the wavefront residual root-mean-square error is about 0.05 wavelengths (lambda = 550 nm) after correction. The observational results show that the contrast and resolution of the solar images are improved after the correction by this adaptive optics system

Measuring the power-law exponent of an atmospheric turbulence phase power spectrum with a Shack Hartmann wave-front sensor.

For non-Kolmogorov turbulence we develop a differential angle-of-arrival fluctuation coefficient, which is the ratio between the transverse and longitudinal differential angle-of-arrival variances, and a slope structure-correlation coefficient, which is the ratio between the transverse and longitudinal differences of the slope correlation function and the slope structure function, to measure the power-law exponent of a phase power spectrum with a Shack-Hartmann wave-front sensor: The differential arrival-of-angle fluctuation coefficient and the slope structure-correlation coefficient are both related to power-law exponent beta and are independent of strength parameter rho(0) of the turbulence. We compare the methods developed and use them to evaluate beta in recently completed horizontal atmospheric experiments for 1000-m laser beam propagation.

Wavefront sensorless adaptive optics: a general model-based approach

Wavefront sensorless adaptive optics (AO) systems have been widely studied in recent years. To reach optimum results, such systems require an efficient correction method. In this paper, a general model-based correction method for a wavefront sensorless AO system is presented. The general model-based approach is set up based on a relationship wherein the second moments (SM) of the wavefront gradients are approximately proportionate to the FWHM of the far-field intensity distribution. The general model-based method is capable of taking various common sets of functions as predetermined bias functions and correcting the aberrations by using fewer photodetector measurements. Numerical simulations of AO corrections of various random aberrations are performed. The results show that the Strehl ratio is improved from 0.07 to about 0.90, with only N + 1 photodetector measurement for the AO correction system using N aberration modes as the predetermined bias functions.

37-element solar adaptive optics for 26-cm solar fine structure telescope at Yunnan Astronomical Observatory

A 37-element solar adaptive optics (AO) system was built and installed at the 26-cm solar fine structure telescope of Yunnan Astronomical Observatory. The AO system is composed of a fine tracking loop with a tip/tilt mirror and a correlation tracker, a high-order correction loop with a 37-element deformable mirror, a correlating Shack-Hartmann wavefront sensor based on the absolute difference algorithm, and a real time controller. The system was completed on Sep. 28, 2009 and was used to obtain AO-corrected highresolution solar images. The contrast and resolution of the images are clearly improved after wavefront correction by AO. To the best of out knowledge, this system is the first solar AO system in China.

Atmospheric parameter measurements for non-Kolmogorov turbulence with Shack-Hartmann wavefront sensor

For non-Kolmogorov turbulence, the slope structure function and the slope correlation function are used to characterize atmospheric turbulence parameters--(beta) , which is the power-law exponent of the phase power spectrum, and (rho) 0, which shows the strength of atmospheric turbulence. The slope structure-correlation function, which is related to (rho) 0 and (beta) , and the slope structure-correlation coefficient, which is only the function of (beta) and independent of (rho) 0, are developed considering the subaperture size and additive noise. The Shack-Hartmann wavefront sensor data in the recently completed horizontal atmospheric experiments for 1000 m laser beam propagation are utilized to evaluate (beta) and (rho) 0 according to the developed theory.

A Tilt-correction Adaptive Optical System for the Solar Telescope of Nanjing University

A tilt-correction adaptive optical system installed on the 430 mm Solar Telescope of Nanjing University has been put in operation. It consists of a tip-tilt mirror, a correlation tracker and an imaging CCD camera. An absolute difference algorithm is used for detecting image motion in the correlation tracker. The sampling frequency of the system is 419 Hz. We give a description of the systems configuration, an analysis of its performance and a report of our observational results. A residual jitter of 0.14 arcsec has been achieved. The error rejection bandwidth of the system can be adjusted in the range 5–28 Hz according to the beacon size and the strength of atmospheric turbulence.

Phase-shifts n pi/2 calibration method for phase-stepping interferometry.

A novel phase-shifts n pi/2 calibration method for phase-stepping interferometry, in which the sum intensities and the squared sum of a series of phase-shifted interferograms are firstly calculated, and then by the minimization of the variances of these sums perform the n pi/2 phase-shifts calibration, is proposed in this paper. The proposed method can overcome effectively the effect of the variations of background and modulation intensities in interferograms with any phase structure, and it is also insensitive to the nonlinearity of phase shifter. Numerical simulation and experiments are implemented to verify the effectiveness of this method.