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.     

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.     

An adjustment method for active reflector of large high-frequency antennas considering gain and boresight

The design of the Qitai 110 m Radio Telescope(QTT) with large aperture and very high working frequency(115 GHz) was investigated in Xinjiang, China. The results lead to a main reflector with high surface precision and high pointing precision. In this paper, the properties of active surface adjustment in a deformed parabolic reflector antenna are analyzed. To assure the performance of large reflector antennas such as gain and boresight, which can be obtained by utilizing an electromechanical coupling model, and satisfy them simultaneously, research on active surface adjustment applied to a new parabolic reflector as target surface has been done. Based on the initial position of actuators and the relationship between adjustment points and target points, a novel mathematical model and a program thatdirectly calculates the movements of actuators have been developed for guiding the active surface adjustment of large reflector antennas. This adjustment method is applied to an 8 m reflector antenna,in which we only consider gravity deformation. The results show that this method is more efficient in adjusting the surface and improving the working performance.     

FAST单元面板面型检测算法研究

500 m口径球面射电望远镜(Five-hundred-meter Aperture Spherical Radio Telescope,FAST)是一个超大口径的可动望远镜,有三项技术创新,一是选址,二是轻型馈源索支撑,三是主动反射面。在主动反射面上,单元面板的面型和面板的出厂加工精度对电磁波在反射面的汇聚有很大影响。FAST主反射面由4 600块三角形反射面板拼接而成,每块面板为边长11 m三角形,这对FAST反射面面板的测量技术提出了更高的要求。摄影测量直接在影像上进行量测,无需接触物体本身;所摄影像信息丰富,可以从中获得所研究物体的大量几何信息和物理信息;适用于大范围、多目标测量,效率高。目前世界上最大的射电望远镜,如GBT和ARECIBO都是采用摄影测量技术进行反射面面形检测。在对现有的面型检测技术调研并试验后,提出基于数字近景摄影测量的方法,对FAST反射面11 m单元面板的面型进行检测,数分钟完成反射面板面型的一次检测,测量精度达到2.5 mm,经过调整后的单元面板的面型精度达到了3.0 mm,结果表明摄影测量应用于FAST反射面单元面板面型检测是可行的。     

EMC design for actuators in the FAST reflector

An active reflector is one of the three main innovations incorporated in the Five-hundredmeter Aperture Spherical radio Telescope(FAST).The deformation of such a huge spherically shaped reflector into different transient parabolic shapes is achieved by using 2225 hydraulic actuators which change the position of the 2225 nodes through the connected down tied cables.For each different tracking process of the telescope,more than 1/3 of these 2225 actuators must be in operation to tune the parabolic aperture accurately and meet the surface error restriction.This means that some of these actuators are inevitably located within the main beam of the receiver,and Electromagnetic Interference(EMI)from the actuators must be mitigated to ensure the scientific output of the telescope.Based on the threshold level of interference detrimental to radio astronomy described in ITU-R Recommendation RA.769 and EMI measurements,the shielding efficiency(SE)requirement for each actuator is set to be 80 d B in the frequency range from 70 MHz to 3 GHz.Therefore,Electromagnetic Compatibility(EMC)was taken into account in the actuator design by measures such as power line filters,optical fibers,shielding enclosures and other structural measures.In 2015,all the actuators had been installed at the FAST site.Till now,no apparent EMI from the actuators has been detected by the receiver,which demonstrates the effectiveness of these EMC measures.     

Active Optics in LAMOST

Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) is one of the major national projects under construction in China. Active optics is one of the most important technologies for new large telescopes. It is used for correcting telescope errors generated by gravitational and thermal changes. Here, however, we use this technology to realize the configuration of LAMOST, -a task that cannot be done in the traditional way. A comprehensive and intensive research on the active optics used in LAMOST is also reported, including an open-loop control method and an auxiliary closed-loop control method. Another important development is in our pre-calibration method of open-loop control, which is with some new features: simultaneous calculation of the forces and displacements of force actuators and displacement actuators; the profile of mirror can be arbitrary; the mirror surface shape is not expressed by a fitting polynomial, but is derived from the mirror surface shape formula which is highly accurat     

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.     

Non-uniform Solar Temperature Field on Large Aperture,Fully-Steerable Telescope Structure

In this study, a 110-m fully steerable radio telescope was used as an analysis platform and the integral parametric finite element model of the antenna structure was built in the ANSYS thermal analysis module. The boundary conditions of periodic air temperature, solar radiation, long-wave radiation shadows of the surrounding environment, etc. were computed at 30 min intervals under a cloudless sky on a summer day, i.e., worstcase climate conditions. The transient structural temperatures were then analyzed under a period of several days of sunshine with a rational initial structural temperature distribution until the whole set of structural temperatures converged to the results obtained the day before. The non-uniform temperature field distribution of the entire structure and the main reflector surface RMS were acquired according to changes in pitch and azimuth angle over the observation period. Variations in the solar cooker effect over time and spatial distributions in the secondary reflector were observed to elucidate the mechanism of the effect. The results presented here not only provide valuable realtime data for the design, construction, sensor arrangement and thermal deformation control of actuators but also provide a troubleshooting reference for existing actuators.     

Calculations for the Pre-Calibration of LAMOST Active Optics

Large Sky Area Multi-Object Fibre Spectroscopic Telescope (L AMOST) is one of the major on-going national large scientific projects in China. Active optics is a key technology for the LAMOST with which the thin-mirror active optics and segmented-mirror active optics are tied in. A pre-calibration method considering all active forces and displacements specially for LAMOST has been developed in early 2004. We give a detailed mathematical derivation and calculation including numerical simulation and computer program realization of the pre-calibration method of LAMOST open-loop control for the third-order as-pherical aberration. We have also carried out calculations on the application of the pre-calibration method and the parameters of actuator design in LAMOST active optics in observation mode, including estimations of the actuator ranges, the interval of active optics correction and the ranges and trends of load changes on all the actuators during LAMOST tracking a given star.     

薄施密特校正板的加工方法研究

针对薄板玻璃的刚度差，在研磨中容易产生变形这一特点，提出了一种新的加工方法。以加工薄施密特校正板（厚径比小于1／40）为例，用传统的上盘粘接固定工件进行加工，工件在下盘前的面形精度较好，但下盘后产生的变形使其面形精度下降，不能满足要求。采用真空模具法，吸附工件从而使工件固定产生预期的变形，面形精度易于控制，使加工非球面转变为加工平面，降低了加工难度，消除了工件在粘接过程中温度所引起的变形，降低了加工时的不均匀压应力，从而减少了变形，使得加工后的面形与加工过程中的面形接近，最终保证了工件的面形精度。     

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.     

分离式靶标摄影测量在LAMOST中的应用

为了获取LAMOST焦面板上光纤的零位位置,提出了一种基于分离式标准靶标的高精度摄影测量方法。首先,根据Tsai摄像机标定参数模型介绍了CCD相机的标定原理。然后介绍了用光重心算法求取靶标发光点像面坐标及通过球面三角公式计算靶标发光点物面坐标。接着,利用最小二乘法导出了含标定参数的方程组。最后,介绍了标准靶标的放置方法以及提高光纤位置测量精度的靶标取法。实验结果表明:CCD相机的标定残差平均值为8.8μm;标定残差的标准差为5.3μm。测量方法简单易行,通过仔细分析,找出了残差偏大的原因,对后续进一步测量实验有重要指导意义。     

Large mirror figuring and testing

We describe a method for figuring and testing large aspheric mirrors using a rectangular, flexible lap (the so-called Membrane Tool) and a vibration stabilized interferometer. The rear side of the lap is covered with computer controlled dynamic pressure actuators which determine the amount of material to be removed for surface error correction. This method has been developed in the laboratory and tested to some extent by figuring thef/2.2 primary of the 3.6 m ESO-NTT. We describe the ongoing developments and the manufacturing plan for 8 m-class mirrors.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.     

主反射面变形引起的天线指向误差修正方法

射电天线指向精度通常要求小于主波束宽度的1/10, 对于短厘米波段或毫米波段的大口径反射面天线, 指向精度要求高达几个角秒, 对于天线性能目标的实现是个巨大的挑战, 因此对于大口径高频段的反射面天线来说指向问题成为天线性能实现的重要关注焦点. 在众多影响天线指向精度的结构子系统因素中, 对主反射面变形因素的研究很少. 文章结合天线的结构特点建立了反射面空间坐标系统, 并基于变形后主面点的空间坐标值, 提出了3自由度下的非线性最小二乘吻合的方法去精准预测天线指向. 最后利用空间几何关系严格推导出了服务于天线指向误差修正的俯仰和方位的精确调整量, 从而构建了主面变形同指向误差之间的间接关系, 这对大型射电天线指向精度的提升具有一定的指导意义.     

A Method for Estimating the Direction of Gravitational Acceleration in the Active Phase of a Spacecraft Landing on the Martian Surface

We know the spacecraft orientation before its touchdown on the Martian surface with an accuracy of 3–4°. The spacecraft control can result in a significant horizontal velocity at altitudes lower than 15 meters at the instant when the landing legs contact the surface when data from the radar location system terminates. An independent method for determining the gravity acceleration vector is presented in the paper. This method is implemented using information obtained from the gyro-inertial and radar instrumentation.     

Image Quality Control and Alignment of a Submillimeter Balloon-Borne Segmented Telescope

PRONAOS is a balloon-borne system dedicated to astronomical observations in the submillimeter range (180µm - 1050µm) based on the use of a 2m Cassegrain telescope. The primary mirror consists of six CFRP (Carbon Fiber Reinforced Plastic) panels, each being positioned by three actuators. The rotation angles of the panels have been measured in the visible range with the help of a CCD and digital centroiding techniques that were necessary because of the light scattering on the CFRP mirror. The translation movements (along the optical axis) of the panels have been measured with an interference technique in the submillimeter range. Both visible and submillimeter measurements were also necessary to determine the alignment of the telescope - focal instrument system with the star sensor. The whole alignment process leads to a precision of ±8 for the rotation angles and ±7µm for the translation of each panel, sufficient for a qualification of the system.     

The 1.5 meter solar telescope GREGOR

The 1.5 m telescope GREGOR opens a new window to the understanding of solar small‐scale magnetism. The first light instrumentation includes the Gregor Fabry Pérot Interferometer (GFPI), a filter spectro‐polarimeter for the visible wavelength range, the GRating Infrared Spectro‐polarimeter (GRIS) and the Broad‐Band Imager (BBI). The excellent performance of the first two instruments has already been demonstrated at the Vacuum Tower Telescope. GREGOR is Europe’s largest solar telescope and number 3 in the world. Its all‐reflective Gregory design provides a large wavelength coverage from the near UV up to at least 5 microns. The field of view has a diameter of 150″. GREGOR is equipped with a high‐order adaptive optics system, with a subaperture size of 10 cm, and a deformable mirror with 256 actuators. The science goals are focused on, but not limited to, solar magnetism. GREGOR allows us to measure the emergence and disappearance of magnetic flux at the solar surface at spatial scales well below 100 km. Thanks to its spectro‐polarimetric capabilities, GREGOR will measure the interaction between the plasma flows, different kinds of waves, and the magnetic field. This will foster our understanding of the processes that heat the chromosphere and the outer layers of the solar atmosphere. Observations of the surface magnetic field at very small spatial scales will shed light on the variability of the solar brightness (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The remote determination of the mean kinetic temperature of planets and satellites having a negligible atmosphere

A novel method, called the crossover technique, has been devised for the remote determination of the surface temperature of planets, satellites, and asteroids having a negligible atmosphere. The method is discussed using new spectra of Mars as an example, and it is shown how it might be extended to other solar system objects. The accuracy of the method, both absolutely and in comparison to other methods, is also considered.     

The control system of the active main reflector for FAST

The main reflector of FAST consists of about 1800 elementary units. Each unit has three adjustable supports to fix its position, and its position is adjusted by mean of mechanical actuators. According to the radio source position at any given time, all the actuators are continuously adjusted to form a proper paraboloid in real time during the course of the observation. The basic requirements of such a control system are discussed. A field bus control system based on LonWorks technology is suggested to control all 1800actuators. The main advantages of this distributed control system are its reliability, flexibility, efficiency and economics. This revised version was published online in July 2006 with corrections to the Cover Date.     

New Boundary Integral Equation Representation for Finite Energy Force-Free Magnetic Fields in Open Space above the Sun

A boundary integral equation to describe a force-free magnetic field with finite energy content in the open space above the solar surface is found. This is a new representation for a 3-D nonlinear force-free field in terms of the boundary field and its normal gradient at the boundary. Therefore the magnetic field observed on the solar surface can be incorporated into the formulation directly and a standard numerical technique, the boundary element method, can be applied to solve the field. A numerical test case demonstrates the power of the method by recovering the analytical solution to the desired accuracy and its application to practical solar magnetic field problems is straightforward and promising.