云台40m射电望远镜的指向误差校正

首先综述了射电望远镜指向误差校正的原理并对软件校准的两种方法进行了比较。其次阐述了利用软件校准的方法对云南天文台40m射电望远镜进行指向校正的过程,并给出其指向校正结果。结果是经过指向校正后该射电望远镜的指向精度达到观测要求。     

一种赤道式射电望远镜极轴的精密校准方法

使用一具附有测微器的光学望远镜配合赤道式射电望远镜进行极轴精密调整,不需用精确的度盘和记时设备。极轴测量和校准精度可达6角秒左右。本文详细叙述了这种方法的测量原理和操作步骤。     

考虑轨道不平度的射电望远镜指向修正方法

目前国内外射电望远镜指向修正大多采用线性修正模型,这种方法未考虑非线性偏差对指向的影响,其修正精度有一定局限,为此提出一种用于大型轮轨式射电望远镜考虑方位轴非线性倾斜偏差的指向修正方法。针对国家天文台密云50 m射电望远镜,对其进行全轨水平度测量,通过有限元建模并仿真计算天线方位轴的非线性倾斜偏差,同时通过实验测量了由于轨道不平度引起的俯仰轴角测量误差并进行了有限元仿真验证。应用天线指向试验数据进行考虑方位轴非线性倾斜偏差指向误差修正,得到新的指向辨识参数及指向残差分布。该修正方法与线性修正模型相比,总的指向精度提高了约0.578 3″。     

采用光学指向系统建立天马13m望远镜指向模型

描述了采用光学望远镜辅助天马13m射电望远镜进行指向测量以及建立指向误差修正模型的方法. 对于小口径望远镜, 指向校准目标源比较少, 用射电法建立指向模型难以覆盖全天区. 利用上海天文台天马13m射 电望远镜进行光学望远镜辅助射电望远镜指向测量研究, 在13m天线背架上安装一套光学指向系统, 获得了优 于3''的重复测量误差. 此外, 通过对影响天线指向因素的分析, 建立了包含8个误差项的指向误差修正模型以及 光轴和电轴偏差模型. 将指向模型代入天线伺服控制系统, 对校准目标射电源进行十字扫描, 得到指向样本残差约 为5''. 该研究可以为实现高精度指向建模提供一种参考方法.     

南极5 m太赫兹望远镜指向与调焦校准精度分析

指向校准与副面调焦是天线测量的重要组成部分,对射电望远镜实际的观测性能有重要影响.根据南极5 m太赫兹望远镜(DATE5)指向精度和离焦增益损失的指标要求,计算得到了指向校准与副面调焦观测过程中对信噪比的要求,并以此为依据选取了若干可行的候选校准源,包括太阳系行星和超致密电离氢区.分析了大气吸收和校准源角直径对测量精度的影响.仿真分析结果表明:望远镜在南极工作时,这些校准源在一定的高度角范围内可以提供足够强的流量密度,用来验证预先建立的太赫兹和光学望远镜两光轴指向偏差模型以及副面调焦模型.     

一种测定望远镜极轴偏差的方法

利用望远镜的恒动跟踪，对两颗恒星进行无斗星曝光０．５小时，扣除蒙气差影响，用球面三角方法计算出望远镜极轴指向的偏差在赤经和赤纬方向上的分量，据此对望远镜极轴进行调整，就可以方便地把望远镜极轴指向调整到较高精度。     

赤道式天文望远镜极轴调整分析及观测结果

天文望远镜的指向精度和跟踪精度是评判一架天文望远镜的重要指标。赤道式天文望远镜的指向精度和跟踪精度受到极轴位置准确性的影响。正确地安装好极轴位置显得非常重要。极轴校正是一项很重要的工作。介绍了用观测恒星的方法来校正极轴位置,分析了合理地选择目标恒星,得出结论:在调整极轴东西方向时选择子午圈上的恒星作为观测目标恒星,在调整极轴俯仰方向时选择90°时角圈上的恒星作为观测目标恒星。经过多次调整极轴东西方向和极轴俯仰方向并对两个方向交替进行调整,望远镜的极轴将处于相当准确的位置。     

一种测定望远镜极轴偏差的方法

利用望远镜的恒动跟踪，对两颗恒星进行无导星曝光０．５小时，扣除蒙气差影响，用球面三角方法计算出望远镜极轴指向的偏差在赤经和赤纬方向上的分量，据此对望远镜极轴进行调整，就可以方便地把望远镜极轴指向调整到较高精度。     

天马13m射电望远镜高精度指向模型的建立

天马13 m射电望远镜是专为空间大地测量的新一代甚长基线干涉测量(Very Long Baseline Interferometry, VLBI)天线,即VGOS(VLBI Global Observing System)系统。VGOS观测将从调度、相关、观测策略到分析各方面改变甚长基线干涉测量。与传统测地观测相比,VGOS观测将数据精度提高1～2个数量级。天马13 m射电望远镜安装了3～15 GHz宽频制冷接收机,一般要求天线指向偏差小于最高频率波束宽度的1/10。为满足高精度指向要求,详细介绍了建立指向的方法和天线控制扫描策略,给出了系统误差修正模型的完全表达式,明确了指向修正模型中的参数意义。基于该天线指向扫描的实测数据,实测评估了望远镜的指向精度。采用最小二乘法对覆盖全天区的数据样本进行拟合,得到天马13 m射电望远镜指向模型,并加载到天线伺服控制系统进行验证,得到了优于10″的盲指误差。     

南山26米射电望远镜轨道高差测量及其对指向精度的影响

轮轨式天线的轨道作为承载整个天线重量的基础,其精度直接影响天线在方位方向运转的平稳性,引起天线轴系偏差从而影响天线的指向精度。介绍改造后的乌鲁木齐南山26 m射电望远镜(Nanshan Radio Telescope, NSRT)轨道结构以及轨道高差测量,并建立天线在不同方位、俯仰角下轨道高差引起天线指向偏差的数学模型。利用"十字扫描"法实测多颗标准源在相应位置的指向数据,并通过高斯拟合得到指向偏差。通过分析可知,轨道高差引起的指向偏差经过修正可以提高天线的指向精度。     

A new calibration model for pointing a radio telescope that considers nonlinear errors in the azimuth axis

A new calibration model of a radio telescope that includes pointing error is presented, which considers nonlinear errors in the azimuth axis. For a large radio telescope, in particular for a telescope with a turntable, it is difficult to correct pointing errors using a traditional linear calibration model, because errors produced by the wheel-on-rail or center bearing structures are generally nonlinear. Fourier expansion is made for the oblique error and parameters describing the inclination direction along the azimuth axis based on the linear calibration model, and a new calibration model for pointing is derived. The new pointing model is applied to the 40 m radio telescope administered by Yunnan Observatories, which is a telescope that uses a turntable. The results show that this model can significantly reduce the residual systematic errors due to nonlinearity in the azimuth axis compared with the linear model.     

An improved astrometric calibration technique for space debris observation

An optical survey is the main technique for detecting space debris. Due to the specific characteristics of observation, the pointing errors and tracking errors of the telescope as well as image degradation may be significant, which make it difficult for astrometric calibration. Here we present an improved method that corrects the pointing and tracking errors, and measures the image position precisely. The pipeline is tested on a number of CCD images obtained from a 1-m telescope administered by Xinjiang Astronomical Observatory while observing a GPS satellite. The results show that the position measurement error of the background stars is around 0.1 pixel, while the time cost for a single frame is about 7.5 s; hence the reliability and accuracy of our method are demonstrated. In addition, our method shows a versatile and feasible way to perform space debris observation utilizing non-dedicated telescopes, which means more sensors could be involved and the ability to perform surveys could be improved.     

射电天线指向改正模型中轴系误差的完全表达式

为了提高射电天线的目标跟踪精度,采用直接法推导了射电天线轴系误差对指向的影响,并给出轴系误差指向改正模型的完全表达式,明确了指向改正模型中各轴系参数的定义,传统分项以及球谐函数所推导的轴系误差项为该模型的简化形式.基于此,评估得出基本参数改正模型中,因忽略轴系误差高次谐项而引起的指向精度损失可能达到1′′量级,具体需结合轴系误差大小而定;同时明确了基本参数改正模型(如22项指向模型)中与轴系误差有关的部分高次谐项系数的物理意义.为高精度轴系误差指向改正模型的建立提供了理论依据.     

Analysis and Correction of the Influence of Track Irregularity on the Antenna Pointing

According to the influence mechanism of the antenna track irregularity on the telescope pointing accuracy, the distribution of the track errors and their influence on the pointing of the Urumqi Nanshan 26 m telescope are reanalyzed after the antenna track was reformed by using the whole-body welding technology, and hereby the pointing error model is correspondingly revised. By using the moving least-squares method, the measured height errors of the antenna track plane are fitted with a closed curve, and the tilt of the antenna azimuth axis caused by the track irregularity can be determined accordingly. Comparing it with the measured deviation of the antenna azimuth axis caused by the deformation of antenna pedestal, it can be found that both deviations are strongly correlated. A new pointing error model is established in view of the gravity deformation of antenna pedestal, which includes the north-south and east-west components, as well as the antenna track irregularity. Finally, by scanning a known calibration radio source at different positions in the sky, the measured pointing errors are fitted with the new pointing error model. The result shows that the sinusoidal component of the model error can be well constrained by the new pointing correction model, indicating that the new model can reflect very well the antenna pointing error, and can amend it to a certain extend.     

Precise Determination of the Orientation of the Solar Image

Accurate heliographic coordinates of objects on the Sun have to be known in several fields of solar physics. One of the factors that affect the accuracy of the measurements of the heliographic coordinates is the accuracy of the orientation of a solar image. In this paper the well-known drift method for determining the orientation of the solar image is applied to data taken with a solar telescope equipped with a CCD camera. The factors that influence the accuracy of the method are systematically discussed, and the necessary corrections are determined. These factors are as follows: the trajectory of the center of the solar disk on the CCD with the telescope drive turned off, the astronomical refraction, the change of the declination of the Sun, and the optical distortion of the telescope. The method can be used on any solar telescope that is equipped with a CCD camera and is capable of taking solar full-disk images. As an example to illustrate the method and its application, the orientation of solar images taken with the Gyula heliograph is determined. As a byproduct, a new method to determine the optical distortion of a solar telescope is proposed.     

低纬子午环配备科学CCD的初步方案

根据低纬子午环配备科学ＣＣＤ后仍然能对天体位置作绝对测定的要求，提出了该仪器配备科学ＣＣＤ的初步方案，包括在子午方向和卯酉方向观测时，ＣＣＤ芯片如何跟踪星像，如何将芯片致冷，提高信噪比，既能保持镜筒的平衡，又不破坏观测室内空气的稳定性，如何更准确地测定光轴指向的变化。文中还对一次观测的天区面积作了估计。     

基于光学辅助指向测量的太赫兹望远镜指向误差模型研究

针对太赫兹波段天文点源目标较少, 指向测量相对困难的特点, 研究了利用与太赫兹天线共轴的小型光学望远镜来辅助太赫兹望远镜指向测量以及建立指向误差修正模型的方法. 依托紫金山天文台1.2 m斜轴式太赫兹天线开展了光学辅助指向测量的实验研究, 利用一台安装在天线背架上的100mm口径折射式光学望远镜获得了优于2$''$的指向测量精度. 此外, 通过对斜轴天线的结构分析以及大气折射和本地恒星时(Local Sidereal Time, LST)偏差等误差来源的分析, 建立了包含23个误差项的斜轴式光学指向修正模型, 实现了约3$''$的拟合精度. 最后, 借助高精度数字摄影测量对光电轴一致性进行了标定, 并针对其对指向模型精度的影响进行了讨论. 研究成果将为南极5 m太赫兹望远镜(The 5m Dome A Terahertz Explorer, DATE5)及其他太赫兹望远镜提供指向测量和指向修正模型方面的技术参考.