大型射电望远镜主反射面调整方法研究

随着毫米波天文学和空间通信的重要性日益提高, 对天线性能提出了越来越高的要求, 而天线性能往往受到其反射器表面精度的限制. 微波全息技术是一种快速有效的检测反射面天线表面轮廓的测量技术. 通过微波全息测量得到天线口径场, 计算天马65m射电望远镜反射面与理想抛物面的偏差. 天马65m射电望远镜的主反射面板是放射状的, 有14圈. 面板的每个角都固定在面板下方促动器的螺栓上进行上下移动, 且相邻面板交点处的拐角共用一个促动器. 采用平面拟合的方法可以计算各块面板拐角处的调整值, 但是同一个促动器会得到4个不同的调整量. 通过平面拟合, 同时以天线照明函数为权重的平差计算方法得到相邻面板拐角的一个平差值, 即天马65m射电望远镜1104个促动器的最佳调整值. 通过多次调整和新算法的应用, 天马65m射电望远镜反射面的面形精度逐渐提高到了0.24mm.     

偏焦全息测量技术在上海65米射电望远镜中的应用研究

上海65米射电望远镜是亚洲最大、全方位可转动的大型射电望远镜,在国内首次配备主动面系统。对主动面系统应用的关键技术——偏焦全息测量技术在上海65米射电望远镜中的初步应用情况进行介绍。首先,介绍常用的天线主反射面面型测量方法。接着,介绍偏焦全息测量技术的应用方法。采用飞行扫描,利用X波段致冷接收机和连续谱终端,将射电源3C84作为信号源,通过移动副反射面测量得到一幅聚焦天线方向图和两幅偏焦天线方向图。将三幅天线方向图作为偏焦全息测量算法的输入,获得天线口径面相位分布的Zernike系数。依据Zernike系数计算得到天线主反射面型面误差的均方根值(root mean square,RMS),该均方根值与前期照相测量的结果基本相符。最后,对今后的改进工作进行展望。     

FAST工程建设进展

"十一五"国家重大科技基础设施建设项目—500 m口径球面射电望远镜(FAST)工程,是利用贵州天然喀斯特洼地作为望远镜台址,建造世界第一大单口径射电望远镜—500 m口径主动反射球面射电望远镜,以实现大天区面积、高精度的天文观测.FAST工程由中国科学院和贵州省人民政府联合共建,于2016年9月25日竣工.     

FAST照明口径分析

500 m口径球面射电望远镜(Five-hundred-meter Aperture Spherical radio Telescope, FAST)在观测时,将球形反射面内部照明区域的形状变为300 m口径抛物面,实现望远镜的主焦天线功能。照明口径(球面变位到抛物面的口径)对望远镜的观测性能起决定作用。为了望远镜潜在的性能提升及后续发展,在500 m口径球面射电望远镜照明口径300 m及大于300 m的情况下,通过不同焦距抛物面与基准球面变位距离的计算,进行球面到不同口径抛物面变位分析,对望远镜增大照明口径的可行性进行探讨。给出了在500 m口径球面射电望远镜反射面变位驱动促动器最大运动行程范围内,增大口径抛物面变位时抛物面焦距及焦径比的选取。在驱动行程方面初步说明了变位的理论可行性,相关分析同样适用于其他口径的抛物面。     

射电天文中焦面阵或多波束馈源的应用

焦面阵技术或者多波束馈源系统已经日益广泛地应用于现代射电望远镜，因为它可以充分地利用同一射电望远镜反射面所能提供的信息，在观测比射电望远镜方向瓣大得多的展源时数倍乃至数十倍地提高观测的速度；当存在大气层或电离层的起伏或不均匀影响观测成像质量时，可消除这种影响，提高观测质量；利用焦面阵各单个馈源接收到的信息的互相关，则可以实时监控射电望远镜的反射面、二次反射面、指向精度，从而降低地面上大射电望远镜或空间射电望远镜的精度要求和造价。目前焦面阵已经愈来愈广泛地配置在毫米波射电望远镜和大型射电望远镜的主要波段。对此作了一个较新和全面的评述，对焦面阵应用中的限制，包括相位误差的限制和性能价格比的考虑和可能的前景作了简要的介绍。还分析了在计划中的大型主动球反射面射电望远镜（即FAST）上，配置焦面阵的相应限制、问题和难点，提出了初步的建议，并给出经中英双方讨论后初步拟定的FAST频段、波束及低噪声放大器的配置。     

从KARST到FAST---中国天眼的概念起源与决策过程

在大科学时代,大科学工程的建设日益复杂,设计者需综合考虑设备性能、技术储备、经费、风险、环境等因素才能进行合理决策.在国内外天文学发展的背景中,梳理了500 m口径球面射电望远镜(Five-hundredmeter Aperture Spherical radio Telescope, FAST)概念形成的过程以及FAST团队在其中所做的决策,包括大望远镜中国方案的设想、先导单元的提出、主动反射面技术的采用等,并对决策过程进行了分析探讨. FAST诞生于我国与国际天文学发展的互动与融合进程,实现了从跟进到占据先机的转变,可为在相关基础薄弱的领域建设大科学工程提供参考.     

大口径望远镜反射面日照温度场效应实验研究

针对大口径射电望远镜主反射面在日照作用下产生温度梯度及形变问题,建立了完整的温度测量及精度评估的系统。在特定的测量条件下,利用红外热像仪测量了望远镜主反射面的温度,获得了不同时刻的温度分布情况,并得到最大温差为7.8℃。采用全息法分析主反射面的精度变化情况,得到由于温度变化导致的最大误差为0.33 mm。结果表明:温度对望远镜主反射面的精度影响较大,需采取合适的测试系统实时监测温度,并调整主反射面板以提高精度。     

大射电望远镜FAST风环境数值模拟研究

500 m口径球面射电望远镜(Five-hundred-meter Aperture Spherical radio Telescope,FAST),整体结构处于复杂的喀斯特地貌上,反射面承受风载荷作用,需要进行抗风设计。建立了FAST反射面及其周边山地地形的计算流体动力学(Computational Fluid Dynamics,CFD)计算模型,计算域范围为40 km×10 km×5.5 km。确定了入口风速剖面的基准高度为海拔810 m。计算了不同风向下反射面风压系数分布特征,得出了不利风向。对挡风墙的研究结果表明,挡风墙高度选择的效果优于位置选择。该研究结果可为FAST工程提供抗风设计的初步参考。     

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.     

FAST在深空探测中的应用前景

FAST（Five hundred meters Aperture Spherical Telescope)拟利用贵州省的喀斯特洼地，建立世界上最大的500米口径的球面射电望远镜。主动反射面新概念的提出，实现了望远镜的宽频带和全偏振能力；馈源及支撑系统简化的方案，使FAST对天体和航天器的跟踪范围得到很大的补充。分析预研中的FAST的测控功能，并论证其在未来深空网（DSN）中的重要作用和地位及开展国际合作的可能性。     

A preliminary study on photogrammetry for the FAST main reflector measurement

The Five-hundred-meter Aperture Spherical radio Telescope(FAST) is the largest single-dish radio telescope in the world. In this paper, we apply photogrammetry to measure local area surface accuracy of the FAST main reflector. Contrast with the existing photogrammetric methods that need to stick the cooperation target on the panel. In this paper, we directly detect nodes according to their natural feature. Analyzing the FAST reflector composition, we propose a two-step Hough transform method for node detection. Due to the high similarity of the neighboring area around nodes, it is hard to match two images by the feature matching method. Therefore, we apply the nodes combination approach to obtain the homography matrix between two photos for nodes matching. After nodes detection and matching,triangulation and bundle adjustment algorithms are adopted for 3 D reconstruction. During the experiment,the adjusted node position deform a local area of the FAST main reflector into a spherical cap with a radius of 300 m. The experimental result shows that the measurement accuracy of the sphere with a radius of 300 m is 12.299 mm, indicating that it is feasible to apply photogrammetry for the FAST main reflector measurement.     

The Influence of Panel Gaps on the System NoiseTemperature of the FAST

1 INTRODUCTIONThe FAST is an ArecibChtype telescope with a number of innovations, which is to be built inthe unique karst area of Southwest China (Nan Rendong et al. 1996). Some basic parametersof the FAST have been tentatively suggested such as, curvature radius R = 300m, openingangle 0 = 120', and frequency range from 300MHz to 5 GHz. Baized on the fact that thecelltral part of a spherical surface deyiates little from a paraboloid as a proper focal length ischosen, a novel design…     

Recognition of FAST reflector nodes based on Canny operator

The reflector system of Five-hundred-meter Aperture Spherical radio Telescope(FAST) is designed as 4450 rigid panels supported by a flexible cable-net structure. We use 10 total stations to measure2225 nodes of the cable-net and then control the shape of the reflectors. Every time, it takes at least 35 minutes to finish the calibration of the whole cable-net once. It is indeed far too inefficient. Thus, we developed a set of highly efficient instrument CRRS(CCD Rotation Ranging System). It is based on photogrammetry and can finish the measurement in 1 minute. However, the target we used in CRRS is active target, and it has serious electromagnetic interference problems to affect the use of FAST. Take the above reasons into consideration, we plan to identify the nodes by taking the gap between the reflector panels as the feature condition. The new method can take the place of active targets to finish the measuring task. The present work focuses on the following aspects. First, combined with the characteristics of FAST reflector images,the representative algorithms of image edge detection are discussed. Second, the process of node extraction is introduced in detail so that we know that it works. In addition, experimental results are given to draw a conclusion so that Canny algorithm was used for continuous research of reflector edge detection.     

Analysis for reflector aluminum mesh panels of five-hundred meter aperture spherical telescope

In this paper, an analysis of the aluminum reflector mesh panels for the Five-hundred meter Aperture Spherical Telescope (FAST) is described. The study concentrates on one element of the reflector surface with the shape of a triangle. For the analysis, the panel is modelled as a mesh of cables so that it can be treated as continuous. It is demonstrated that the results so obtained are reasonable. This revised version was published online in July 2006 with corrections to the Cover Date.     

Suggested quasi-Cassegrain system for multi-beam observation of FAST

FAST, the largest single-dish radio telescope in the world, has a 500-meter diameter main reflector and a 300-meter diameter illuminated area. It has a main reflector that can vary its shape, which continuously changes the shape of the illuminated area in reflector into a paraboloid. In this article, we propose a quasiCassegrain system for FAST. The detailed design results are provided. Such a quasi-Cassegrain system only needs to add a 14.6-meter diameter secondary reflector, which is close to the size of the feed cabin. The distance from the secondary reflector to the focus is only 5.08 m, and it has excellent image quality. In this quasi-Cassegrain system, the shape of the illuminated area in the main reflector continuously changes into an optimized hyperboloid. Using this quasi-Cassegrain system from frequency 0.5 GHz to 8 GHz, the multi-beam system can include 7 to 217 feeds. If this system is used in combination with Phased Array Feed(PAF) technology, more multi-beam feeds or a higher working frequency can be used.     

Research on the panel adjustment method of an active main reflector for a large radio telescope

The surface accuracy of a large parabolic antenna is an important indicator to evaluate the quality of the antenna.It not only directly affects the antenna’s aperture efficiency,thereby determining the shortest wavelength that the antenna can work,but also affects the main lobe width and side lobe structure of the antenna pattern.Microwave holography is an important method for parabolic antenna profile detection.In this article we adopt a new algorithm to adjust the panels for the large radio telescope with an active main reflector through the TM65 m antenna’s aperture phase profile.The panels of the TM65 m radio telescope is in a radial pattern with 14 rings.Each corner of the panel is fixed on the screw of the actuator to move up and down,and the adjacent corners of the four panels share an actuator.We use the method of plane fitting to calculate the adjustment value of every panel’s corner.But one actuator,which simultaneously controls the common corner of the adjacent panels,will have different adjustment values according to the different plane fitting equation based on adjacent panels.In this paper,the adjustment value of the adjacent panels’crosspoints are constrained to be equal to the constraint condition to calculate each actuator’s adjustment value of the TM65 m radio telescope.Through multiple adjustments and application of the new algorithms,the accuracy of the TM65 m antenna reflector profile has been improved from the original 0.28 mm to the current 0.19 mm.     

Conceptual Design of the Aluminum Reflector Antenna for DATE5

DATE5,a 5 m telescope for terahertz exploration, was proposed for acquiring observations at Dome A, Antarctica. In order to observe the terahertz spectrum, it is necessary to maintain high surface accuracy in the the antenna when it is exposed to Antarctic weather conditions. Structural analysis shows that both machined aluminum and carbon fiber reinforced plastic(CFRP) panels can meet surface accuracy requirements. In this paper, one design concept based on aluminum panels is introduced. This includes panel layout, details on panel support, design of a CFRP backup structure, and detailed finite element analysis.Modal, gravity and thermal analysis are all performed and surface deformations of the main reflector are evaluated for all load cases. At the end of the paper, the manufacture of a prototype panel is also described.Based on these results, we found that using smaller aluminum reflector panels has the potential to meet the surface requirements in the harsh Dome A environment.     

Unit panel node detection by CNN on FAST reflector

The Five-hundred-meter Aperture Spherical radio Telescope(FAST) has an active reflector.During observations, the reflector will be deformed into a paraboloid 300 meters in diameter. To improve its surface accuracy, we propose a scheme for photogrammetry to measure the positions of 2226 nodes on the reflector. The way to detect the nodes in the photos is the key problem in this application of photogrammetry. This paper applies a convolutional neural network(CNN) with candidate regions to detect the nodes in the photos. Experimental results show a high recognition rate of 91.5%, which is much higher than the recognition rate for traditional edge detection.