液晶自适应光学在天文学研究中的应用展望

液晶波前校正器作为一种高单元密度的新型波前校正器件,通过相息图的衍射可以轻松实现十微米的波前位相校正量。因此,基于液晶波前校正器的自适应光学(LCAO)系统是21世纪天文观测领域非常有希望普及的系统。但是液晶波前校正器存在响应速度慢(10ms)、能量利用率低的双重问题,国际上一直处于探索研究中。本课题组不但解决了能量问题,而且在速度方面不断取得进步,所研制的LCAO系统与1.23m口径望远镜连接,清晰观测到土星及其环绕的光环带,分辨出4.8和5.5视星等的α-Com双星,成像分辨率达到1.8倍衍射极限分辨率;目前系统延迟时间只有2ms,可以说已达到工程应用水平,在装备8~10m级大口径天文学望远镜方面极具应用潜力。     

An optimal error estimate for upwind Finite Volume methods for nonlinear hyperbolic conservation laws

The purpose of this paper is to show that the cell-centered upwind Finite Volume scheme applied to general hyperbolic systems of m conservation laws approximates smooth solutions to the continuous problem at order one in space and time. As it is now well understood, there is a lack of consistency for order one upwind Finite Volume schemes: the truncation error does not tend to zero as the time step and the grid size tend to zero. Here, following our previous papers on scalar equations, we construct a corrector that allows us to prove the expected error estimate for nonlinear systems of equations in one dimension.     

Chaos in a new fractional-order system without equilibrium points

Chaotic systems without equilibrium points represent an almost unexplored field of research, since they can have neither homoclinic nor heteroclinic orbits and the Shilnikov method cannot be used to demonstrate the presence of chaos. In this paper a new fractional-order chaotic system with no equilibrium points is presented. The proposed system can be considered “elegant” in the sense given by Sprott, since the corresponding system equations contain very few terms and the system parameters have a minimum of digits. When the system order is as low as 2.94, the dynamic behavior is analyzed using the predictor–corrector algorithm and the presence of chaos in the absence of equilibria is validated by applying three different methods. Finally, an example of observer-based synchronization applied to the proposed chaotic fractional-order system is illustrated.     

Configuration optimization of laser guide stars and wavefront correctors for multi-conjugation adaptive optics

Multi-conjugation adaptive optics(MCAOs) have been investigated and used in the large aperture optical telescopes for high-resolution imaging with large field of view(FOV).The atmospheric tomographic phase reconstruction and projection of three-dimensional turbulence volume onto wavefront correctors,such as deformable mirrors(DMs) or liquid crystal wavefront correctors(LCWCs),is a very important step in the data processing of an MCAO's controller.In this paper,a method according to the wavefront reconstruction performance of MCAO is presented to evaluate the optimized configuration of multi laser guide stars(LGSs) and the reasonable conjugation heights of LCWCs.Analytical formulations are derived for the different configurations and are used to generate optimized parameters for MCAO.Several examples are given to demonstrate our LGSs configuration optimization method.Compared with traditional methods,our method has minimum wavefront tomographic error,which will be helpful to get higher imaging resolution at large FOV in MCAO.     

A high precision phase reconstruction algorithm for multi-laser guide stars adaptive optics

Adaptive optics(AO) systems are widespread and considered as an essential part of any large aperture telescope for obtaining a high resolution imaging at present.To enlarge the imaging field of view(FOV),multi-laser guide stars(LGSs) are currently being investigated and used for the large aperture optical telescopes.LGS measurement is necessary and pivotal to obtain the cumulative phase distortion along a target in the multi-LGSs AO system.We propose a high precision phase reconstruction algorithm to estimate the phase for a target with an uncertain turbulence profile based on the interpolation.By comparing with the conventional average method,the proposed method reduces the root mean square(RMS) error from 130 nm to 85 nm with a 30% reduction for narrow FOV.We confirm that such phase reconstruction algorithm is validated for both narrow field AO and wide field AO.     

基于DVB-S标准的低信噪比数字解调器

提出了一种基于DVB-S标准的全数字QPSK解调模块,阐述了数字解调模块的频率恢复与相位跟踪两个主要环路的算法与实现,并针对抗噪声要求提出了自主设计的频率矫正器,实验结论表明整个设计可正常工作在满足DVB-S标准要求的噪声环境中.     

Fourth‐order compact scheme for the one‐dimensional sine‐Gordon equation

Finite difference scheme to the generalized one‐dimensional sine‐Gordon equation is considered in this paper. After approximating the second order derivative in the space variable by the compact finite difference, we transform the sine‐Gordon equation into an initial‐value problem of a second‐order ordinary differential equation. Then Padé approximant is used to approximate the time derivatives. The resulting fully discrete nonlinear finite‐difference equation is solved by a predictor‐corrector scheme. Both Dirichlet and Neumann boundary conditions are considered in our proposed algorithm. Stability analysis and error estimate are given for homogeneous Dirichlet boundary value problems using energy method. Numerical results are given to verify the condition for stability and convergence and to examine the accuracy and efficiency of the proposed algorithm. © 2008 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2009     

高精度二极管激光温度控制器设计

二极管激光器(LD)的输出激光频率(波长)对温度变化极其敏感,要实现数MHz的窄线宽,激光器的工作温度必须控制在m℃的精度内。本文通过高精度LD温度控制器的设计,介绍了其PID控制参数的具体整定方法。实验中,采用了2种参数整定方法得到了近似一致的结果。实测表明,参数调准后的温度控制器控温精度为±2m℃。     

无光焦度校正板在主镜后的牛顿光学系统的设计

描述了无光焦度校正板放在主反射镜后的无光焦度校正板-牛顿光学系统的设计.光栏放在主镜上,改变相对孔径A,选择校正板光焦度φ21、φ22和遮拦比α,进行光学系统的光线追迹,求解出光学系统的最佳选择,并给出各种设计曲线和设计结果,对光学系统进行合理的评价.     

A stabilized finite element predictor–corrector scheme for the incompressible Navier–Stokes equations using a nodal‐based implementation

A finite element model to solve the incompressible Navier–Stokes equations based on the stabilization with orthogonal subscales, a predictor–corrector scheme to segregate the pressure and a nodal based implementation is presented in this paper. The stabilization consists of adding a least‐squares form of the component orthogonal to the finite element space of the convective and pressure gradient terms, which allows to deal with convection‐dominated flows and to use equal velocity–pressure interpolation. The pressure segregation is inspired in fractional step schemes, although the converged solution corresponds to that of a monolithic time integration. Finally, the nodal‐based implementation is based on an a priori calculation of the integrals appearing in the formulation and then the construction of the matrix and right‐hand side vector of the final algebraic system to be solved. After appropriate approximations, this matrix and this vector can be constructed directly for each nodal point, without the need to loop over the elements and thus making the calculations much faster. Some issues related to this implementation for fractional step and our predictor–corrector scheme, which is the main contribution of this paper, are discussed. Copyright © 2004 John Wiley & Sons, Ltd.