The Shack-Hartmann wavefront sensor is widely used because of high light energy utilization and the simultaneous measurement of the optical wavefront phase distribution and intensity distribution. The accuracy of the centroid computation has a great influence on the detection accuracy of the Shack-Hartmann wavefront sensor. In this paper, a new method is proposed to improve the accuracy of centroid computation. This method include three steps. First of all, we use a new sliding template method to locate the spot automatically and obtain the approximate center of the spot. Next, we take an adaptive threshold method. After the processing of subtracting the threshold , we use the center of gravity(CoG) method to calculate the spot centroid. A series of simulations are conducted to verify the effectiveness and accuracy of this new method. Compared with the widely-used optimum threshold algorithm and the CoG method, the new algorithm not only enhances the accuracy of centroid computation but also has strong stability.
Centroid computation of Gaussian spot is often conducted to get the exact position of a target or to measure wave-front
slopes in the fields of target tracking and wave-front sensing. Center of Gravity (CoG) is the most traditional method of
centroid computation, known as its low algorithmic complexity. However both electronic noise from the detector and
photonic noise from the environment reduces its accuracy. In order to improve the accuracy, thresholding is unavoidable
before centroid computation, and optimum threshold need to be selected. In this paper, the model of Gaussian spot is
established to analyze the performance of optimum threshold under different Signal-to-Noise Ratio (SNR) conditions.
Besides, two optimum threshold selection methods are introduced: T<sub>m</sub>CoG (using m % of the maximum intensity of spot
as threshold), and T<sub>k</sub>CoG ( usingμ<sub>n</sub> +κσ n as the threshold), μ<sub>n</sub> and σ<sub>n</sub> are the mean value and deviation of back noise.
Firstly, their impact on the detection error under various SNR conditions is simulated respectively to find the way to
decide the value of k or m. Then, a comparison between them is made. According to the simulation result, T<sub>m</sub>CoG is
superior over T<sub>k</sub>CoG for the accuracy of selected threshold, and detection error is also lower.
Proc. SPIE. 7658, 5th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Optoelectronic Materials and Devices for Detector, Imager, Display, and Energy Conversion Technology
KEYWORDS: Mirrors, Digital signal processing, Sensors, Data storage, Wavefronts, Adaptive optics, Field programmable gate arrays, Control systems, Deformable mirrors, Charge-coupled devices
A non-conventional adaptive optics system based on direct system performance metric optimization is illustrated. The
system does not require wave-front sensor which is difficult to work under the poor condition such as beam cleanup for
the anomalous light beam. The system comprises a high speed wavefront controller based on Stochastic Parallel Gradient
Descent (SPGD) Algorithm, a deformable mirror, a tip/tilt mirror and a far-field system performance metric sensor. The
architecture of the wave-front controller is based on a combination of Field Programmable Gate Array (FPGA) and
floating-point Digital Signal Processor (DSP). The Zernike coefficient information is applied to improve the iteration
speed. The experimental results show that the beam cleanup system based on SPGD keep a high iteration speed. The
controller can compensate the wavefront aberration and tilt excursion effectively.
The 61-element upgraded adaptive optical system for the 1.2m telescope of Yannan Observatory for astronomical observation had been in operation since May 2004. In this paper, the 61-element upgraded adaptive optical system for 1.2m telescope of Yunnan Observatory will be briefly described. The performance on the 61-element upgraded adaptive optical system is analyzed. Furthermore, the observational results for the stars will be presented.
The capability of real time wave-front reconstruction is important for an adaptive optics (AO) system. The bandwidth of system and the real-time processing ability of the wave-front processor is mainly affected by the speed of calculation. The system requires enough number of subapertures and high sampling frequency to compensate atmospheric turbulence. The number of reconstruction operation is increased accordingly. Since the performance of AO system improves with the decrease of calculation latency, it is necessary to study how to increase the speed of wavefront reconstruction. There are
two methods to improve the real time of the reconstruction. One is to convert the wavefront reconstruction matrix, such as by wavelet or FFT. The other is enhancing the performance of the processing element. Analysis shows that the latency cutting is performed with the cost of reconstruction precision by the former method. In this article, the latter method is adopted. From the characteristic of the wavefront reconstruction algorithm, a systolic array by FPGA is properly designed to implement real-time wavefront reconstruction. The system delay is reduced greatly by the utilization of pipeline and parallel processing. The minimum latency of reconstruction is the reconstruction calculation of one subaperture.
The 61-element adaptive optical system built for the 1.2m telescope of Yannan Observatory for astronomical observation is being upgraded. The Hartmann-Shack wavefront sensor, the tracking system, and the imaging system have been manufactured newly. The wavelengths for the Hartmann-Shack wavefront sensor and the imaging observation range from 400-700nm and 700-1000nm respectively. The arrangement of subapertures is hexagon matched with triangle arrangement of actuators. The detector of Hartmann-Shack sensor is a high-quantum-efficiency CCD with variable frame rate. The tracking system consists of two cascade control loops in order to improve the low-frequency compensation performance. In this paper, the upgrade on 61-element adaptive optical system for 1.2m telescope of Yunnan Observatory will be shown. The preliminary results of the upgraded 61-element adaptive optical system will be presented.