新疆天文台25m南山射电望远镜日照温度场研究

中国科学院新疆天文台25 m南山射电望远镜(25 m NSRT)受日照导致天线结构温度不均匀,其指向精度和效率均有一定程度的损失.为研究25 m NSRT日照下的热力学特性,构建了晴空下射电望远镜热环境参数和热力学有限元模型,考虑了射电望远镜背架、面板等结构件对光线的反射和遮挡作用,通过比较25 m NSRT典型观测工况下各构件的平均温度、均方根温差、单位温差特征距离等特性参数,发现各部件热容量的差异是天线部件间存在大尺度温差现象的主要原因;分析了各类俯仰角和太阳照射角下天线背架结构的温度分布特征,表明日照区域的背架结构温度呈近似线性分布,平均梯度可达0.25℃·m^-1.     

26米天线座架温度分布规律及变形影响分析

为满足大口径地面射电望远镜的高精度要求,开展射电望远镜热影响研究,保持结构的热稳定性,对提高望远镜指向精度有重要意义。以改造后南山26 m天线为研究对象,利用FEMAP软件建立座架结构模型,设定边界条件,得到某年6月4日全天各时刻天线座架上的温度场分布,将温度场结果与结构耦合,得到热变形,建立了温度测量系统,与仿真结果相比,当天局部最高温度可达31.33℃,最大温差可达10℃,最大位移量可达1.8 mm,与实测结果变化规律基本一致。分析座架温度分布规律及其变形影响,为后期天线的温度补偿和结构优化提供参考依据。     

65 m射电望远镜背架结构日照温度效应实验研究

借助精密工程测量手段,对65 m天线背架的日照温度效应进行了实验研究.在天线背架设计坐标系下分析实验数据,建立了日照温度变形模型,得出背架结构沿X方向变形系数为Kx=1.30×10-5,沿Z方向变形系数为Kz=-0.44×10-5,沿Y方向变形偏向太阳照射一侧.根据实验数据建立的背架变形模型,模拟分析了65 m天线背架日照温度变形对面形精度的影响,得出环境温度变化1℃带来的面形精度(均方根值rms)影响分量约为0.09 mm.实验结果为65 m天线主动面调节提供了很有价值的参考数据.     

基于插值方法的天线背架结构热分析

大型射电望远镜的天线背架具有结构复杂和节点多等特点,若能够较为精确地重构天线背架的温度场,将对背架热变形等分析有很大帮助。通过在天线结构上安装一定数目的传感器,测量指定位置的温度值,利用距离反比加权插值方法与这些离散的温度值来重构天线背架的温度场,并对不同姿态下的温度场进行分析。而距离反比加权插值效果与距离计算方式、距离方次以及搜索半径相关,以交叉验证的方式对距离反比加权插值方法进行了优化,使精度优于0.5℃。另外,通过插值方法可得出任意时刻的背架温度场,并对不同状态下的背架温度场进行分析,得出影响温度场分布的因素,以及不同方向温差对天线造成的焦距误差与指向误差。     

超大口径射电望远镜温度场分析

随着天线口径增大、频率升高,日照热对其性能的影响愈发严重.针对待建的新疆110 m大口径全可动射电望远镜,建立其热分析有限元模型,研究天线在夏至日不同太阳时刻的温度场分布特性.结果如下:一天中,主反射面的最高温度可达42.86℃,出现在14时,同时刻撑腿的温度也达到峰值,为41.74℃.背架水平分区温差在5时、18时、19.5时均超过1℃,会对天线指向性能产生较大影响.夜间天线结构温度分布亦不均匀,俯仰结构的温差明显高于其他构件,最大温差为6.42℃,通过数值模拟与试验相结合的方法,证明了构件的壁厚差异是导致结构夜间温差较大的主要原因.     

德令哈13.7m望远镜重力变形研究

借助于数字摄影测量结果调整天线面板,使德令哈13.7 m望远镜在仰角52°时获得最佳反射面面形,从而使天线效率在观测仰角范围内得到整体优化.与之前基于经纬仪测量的面板调整结果相比,天线口径效率提高约1倍.依据不同俯仰姿态下的测量结果,得到了天线的重力变形模型,包括副面偏移和倾斜、主面焦距和面形偏差随仰角变化的规律.根据不同仰角的面形偏差测量数据反演反射面重力变形模型时,采用了数据拟合方法,这样可以减小测量误差对模型精度的影响.     

德令哈13.7m望远镜热变形研究

热变形是影响大型毫米波天线性能的关键因素之一.这种变形会改变天线反射面的轴线和焦距、恶化反射面面形,导致天线效率和指向精度恶化,同时也使天线效率和指向的长时间稳定性降低.由于太阳的周日运动,造成望远镜4个主支撑腿(简称支腿)之间存在温差,且该温差是变化的,从而引起方位轴的倾斜周期性变化.采用数字摄影测量、倾斜仪测量及天文实测等多种仪器和方法,研究了德令哈13.7m望远镜天线的热变形规律,并根据实时测量的主面温度分布,实现了副面自适应调焦补偿,使得天线效率提高近1倍.测量发现常规指向模型修正后的残差与天线主反射面(包括反射面板和背架,简称主面)直径方向温差存在线性相关性,故在指向解析模型中加入了温差修正项,并通过采用隔热材料包裹支腿的办法显著减小了支腿之间的温差,从而改善了望远镜的指向精度.     

亚毫米波天线有限元模型参数修正复合优化方法

天线有限元模型与实际天线往往存在结构参数误差和材料参数误差等,使有限元模型不能真实反映实际天线结构在各种载荷下的变形特性.针对亚毫米波天线,提出了亚毫米波天线有限元模型参数修正复合优化方法.该方法包括3步,第1步建立天线参数化有限元模型,并进行参数灵敏度分析;第2步,以第1步中得到的高灵敏度参数为优化变量建立优化函数,以初始模型与假想实际天线变形特性的差值最小为目标进行优化;第3步,基于零阶优化法和一阶优化法对天线有限元模型进行复合优化,使天线有限元模型逐渐逼近实际天线.为验证所提出参数修正优化方法的可行性,针对一台1.2 m亚毫米波天线,将对其进行假想实验获得的变形特性作为优化目标,分别对多个材料参数在不同的载荷工况下进行优化,结果表明天线有限元模型经过参数优化后与假想实际天线一致,证明了所提出的亚毫米波天线有限元模型参数优化修正方法可行,并在重力载荷下对修正优化后的天线模型的变形规律进行分析.这种优化方法可以推广应用到大口径亚毫米波天线的有限元模型修正中.     

轨道不平度对天线指向的影响分析与修正

基于天线轨道不平度对望远镜指向的影响机理,测量并分析采用整体焊接轨道技术后的乌鲁木齐南山26 m射电望远镜(NanShan Radio Telescope,NSRT)天线轨道误差的分布情况及对指向的影响,从而对指向模型进行修正.采用移动最小二乘法(Moving Least-Squares,MLS)将测得的天线轨道高度差拟合成一个闭合的轨道高度分布曲线,确定因轨道不平度影响的天线方位轴偏差,对比实测天线座架变形的方位轴偏差,发现二者具有较强的相关性.考虑到天线座架重力变形的偏差包含南北向和东西向偏差,建立新的指向偏差模型进行校正.最后通过对已知校准射电源进行扫描测量,将指向偏差数据进行拟合.结果表明:新建的指向修正模型能够抑制已知指向模型偏差中的正弦分量,从而说明所建模型很好地反映了天线的指向偏差并对偏差具有一定修正作用.     

LAMOST焦面板的结构分析

大天区面积多目标光纤光谱望远镜（LAMOST）是我国跨世纪的一项重大科学工程,由于其独特的设计思想,它同时兼备了大口径与大视场的能力并能同时观测约4000个天体。LAMOST的焦面板是一块直径1．75m、曲率半径20m并有4000余个小孔的球冠状板,4000套光纤定位系统被安装在焦面板上。由于光纤光的位置受板的变形的影响从而将影响最终的成像质量,焦面板的变形必须被控制在较小的范围内。利用有限元分析软件ANSYS,本创建了焦面板的有限元模型并讨论了在不同重力和温度场中焦面板的变形情况。最后针对其设计要求给出了适当的结构和材料。     

On the theory of the oblateness of the Sun

In this paper we first emphasize why it is important to know the successive zonal harmonics of the Sun's figure with high accuracy: mainly fundamental astrometry, helioseismology, planetary motions and relativistic effects. Then we briefly comment why the Sun appears oblate, going back to primitive definitions in order to underline some discrepancies in theories and to emphasize again the relevant hypotheses. We propose a new theoretical approach entirely based on an expansion in terms of Legendre's functions, including the differential rotation of the Sun at the surface. This permits linking the two first spherical harmonic coefficients (J ₂ and J ₄) with the geometric parameters that can be measured on the Sun (equatorial and polar radii). We emphasize the difficulties in inferring gravitational oblateness from visual measurements of the geometric oblateness, and more generally a dynamical flattening. Results are given for different observed rotational laws. It is shown that the surface oblateness is surely upper bounded by 11 milliarcsecond. As a consequence of the observed surface and sub-surface differential rotation laws, we deduce a measure of the two first gravitational harmonics, the quadrupole and the octopole moment of the Sun: J ₂=−(6.13±2.52)×10⁻⁷ if all observed data are taken into account, and respectively, J ₂=−(6.84±3.75)×10⁻⁷ if only sunspot data are considered, and J ₂=−(3.49±1.86)×10⁻⁷ in the case of helioseismic data alone. The value deduced from all available data for the octopole is: J ₄=(2.8±2.1)×10⁻¹². These values are compared to some others found in the literature. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005238718479     

Accounting for the viscosity of the fluid core in the problem of the physical libration of the moon

A two-component theoretical model of the physical libration of the Moon in longitude is constructed with account taken of the viscosity of the core. In the new version, a hydrodynamic problem of motion of a fluid filling a solid rotating shell is solved. It is found that surfaces of equal angular velocity are spherical, and a velocity field of the fluid core of the Moon is described by elementary functions. A distribution of the internal pressure in the core is found. An angular momentum exchange between the fluid core and solid mantle is described by a third-order differential equation with a right-hand side. The roots of a characteristic equation are studied and the stability of rotation is proved. A libration angle as a function of time is found using the derived solution of the differential equation. Limiting cases of infinitely large and infinitely small viscosity are considered and an effect of lag of a libration phase from a phase of action of an external moment of forces is ascertained. This makes it possible to estimate the viscosity and sizes of the lunar fluid core from data of observations.     

Rotation of the elastic Earth: the role of the angular-velocity-dependence of the elasticity-caused perturbation

We calculate the so-called convective term, which shows up in the expression for the angular velocity of the elastic Earth, within the Andoyer formalism. The term emerges due to the fact that the elasticity-caused perturbation depends not only on the instantaneous orientation of the Earth but also on its instantaneous angular velocity. We demonstrate that this term makes a considerable contribution into the overall angular velocity. At the same time the convective term turns out to be automatically included into the correction to the nutation series due to the elasticity, if the series is defined by the perturbation of the figure axis (and not of the rotational axis) in accordance with the current IAU resolution. Hence it is not necessary to take the effect of the convective term into consideration in the perturbation of the elastic Earth as far as the nutation is related to the motion of the figure axis.     

Impact of the Quadrupole Moment of the Sun on the Dynamics of the Earth-Moon System

Range of values of the Sun's mass quadrupole moment of coefficient J₂ arising both from experimental and theoretical determinations enlarge across literature on two orders of magnitude, from around 10^-7 until to 10^-5. The accurate knowledge of the Moon's physical librations, for which the Lunar Laser Ranging data reach an outstanding precision level, prove to be appropriate to reduce the interval of J₂ values by giving an upper bound of J₂. A solar quadrupole moment as high as 1.1 10^-5 given either from the upper bounds of the error bars of the observations, or from the Roche's theory, is not compatible with the knowledge of the lunar librations accurately modeled and observed with the LLR experiment. The suitable values of J₂ have to be smaller than 3.0 10^-6. As a consequence, this upper bound of 3.0 10^-6 is accepted to study the impact of the Sun's quadrupole moment of mass on the dynamics of the Earth-Moon system. Such as effect (with J₂ = 5.5±1.3 × 10^-6) has been already tested in 1983 by Campbell & Moffat using analytical approximate equations, and thus for the orbits of Mercury, Venus, the Earth and Icarus. The approximate equations are no longer sufficient compared with present observational data and exact equations are required. As if to compute the effect on the lunar librations, we have used our BJV relativistic model of solar system integration including the spin-orbit coupled motion of the Moon. The model is solved by numerical integration. The BJV model stems from general relativity by using the DSX formalism for purposes of celestial mechanics when it is about to deal with a system of n extended, weakly self-gravitating, rotating and deformable bodies in mutual interactions. The resulting effects on the orbital elements of the Earth have been computed and plotted over 160 and 1600 years. The impact of the quadrupole moment of the Sun on the Earth's orbital motion is mainly characterized by variations of , , and . As a consequence, the Sun's quadrupole moment of mass could play a sensible role over long time periods of integration of solar system models. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the role of the motion of the photospheric footpoints of the magnetic field lines

By means of a simple relation between the velocity v of the fluid particle and the velocity v_f of the photospheric footpoint of the magnetic field line v_z and B_z being respectively the components of v and the magnetic field B normal to the photospheric surface, it is shown formally that through the phtospheric surface the transport of all the quantities attributed to the magnetic field, such as the magnetic flux, the magnetic energy and the helicity, is independent of v_z, and v_f is the only kinematical quantity on which the transport depends. In addition, in the neighborhood of the neutral line the velocity v_l of the moving curve of constant B_z is found to be equal approximately to the component of v or v_f in the direction of v_l. Since v_l can be measured or extimated, so can the components of v and v_f near the neutral line.     

Determination of the temperature of the solar corona from the spectrum of the electron-scattering continuum

When the K-corona is formed by the scattering of photospheric radiation from free electrons, the Fraunhofer lines are greatly broadened by the thermal motions of the hot electrons. This paper discusses the possibility of measuring the coronal electron temperature from the residual depressions in the K-coronal spectrum. If the ratio of the intensities at 4100 Å and 3900 Å can be measured to an accuracy of ±1%, the coronal temperature can be inferred to an accuracy of ±0.2 MK. The temperature of a coronal inhomogeneity may also be measured by this method, provided the position angle is known.Now at Fraunhofer Institute, Freiburg, Germany.     

Calculation of the global system of the field-aligned currents on the dayside of the magnetosphere

Using the well-known equation for the normal component of the current which exist near the tangential discontinuity in the plasma in the case of the frozen-in magnetic field, and supposing that the current closes in the ionosphere in the auroral oval in the region 1, one calculates and compares with the data of observations the dependence of the density of the field-aligned current at the level of the ionosphere on the local time.     

On the systematic errors of the determinations of the absolute orientation of the meridian marks

The classical method of determination of the absolute azimuth (or Bessel's parameter n) can secure sufficiently precision for RA from observations of stars at high geographical latitudes during polar night only.