Five-hundred-meter Aperture Spherical Radio Telescope Reflector Research Articles

Preliminary Simulation and Manufacture of a 40-element PAF Prototype for FAST

This paper presents the preliminary simulation results of the prototype 40-element phased array feed (PAF) array for the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the measurement of a manufactured single Vivaldi element. The simulation results include the characteristic of a standalone Vivaldi element. The full-wave simulation of the explicit PAF array is carried out, from which the performance of the embedded elements and the mutual coupling among the elements could be accurately inferred. The performance of the combination of the PAF and the main reflector of FAST is calculated. The results imply that the PAF combined with the FAST antenna could operate over a much wider bandwidth from 650 to 1450 MHz compared with the standalone element. The antenna temperature remains almost the same for zenith angles from 0 to 40°. From the simulation results, the PAF shows good potential to improve the performance of FAST. A measurement of a manufactured Vivaldi element is also presented.

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 3D 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.

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 measure 2225 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.

Research on key technologies for installation and maintenance of reflector of FAST

The reflector of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) consists of 4450 reflector units. Installation of the reflector faces the challenges of large span, complex terrain, serious interference, complex processes, high position and inability to use conventional equipment. The installation technology for the flexible reflector with a large span was specially studied and designed. Two half-span arc-moving cable cranes and two transfer trucks were jointly operated along a path that follows a circular beam. After installation of the reflector was completed, two half-span cable cranes were merged into a set of full-span cable cranes for maintenance of the reflector. Installation of the reflector combines features of unit and site topography of FAST. The installation technology follows scientific and reasonable practices, and is highly efficient and convenient. It represents a breakthrough in many key technologies in construction and maintenance techniques. It has promoted related technical progress in the construction and maintenance of complex projects. It has also provided an important reference for the construction and maintenance of similar projects, and has strong significance and applicability.

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.

Differential correction method applied to measurement of the FAST reflector

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) adopts an active deformable main reflector which is composed of 4450 triangular panels. During an observation, the illuminated area of the reflector is deformed into a 300-m diameter paraboloid and directed toward a source. To achieve accurate control of the reflector shape, positions of 2226 nodes distributed around the entire reflector must be measured with sufficient precision within a limited time, which is a challenging task because of the large scale. Measurement of the FAST reflector makes use of stations and node targets. However, in this case the effect of the atmosphere on measurement accuracy is a significant issue. This paper investigates a differential correction method for total stations measurement of the FAST reflector. A multi-benchmark differential correction method, including a scheme for benchmark selection and weight assignment, is proposed. On-site evaluation experiments show there is an improvement of 70%–80% in measurement accuracy compared with the uncorrected measurement, verifying the effectiveness of the proposed method.

Studying solutions for the fatigue of the FAST cable-net structure caused by the process of changing shape

The Five-hundred-meter Aperture Spherical Radio Telescope (FAST) is supported by a cable-net structure, whose change in shape leads to a stress range of approximately 500 MPa. This stress range is more than twice the standard recommended value. The cable-net structure is thus the most critical and fragile part of the FAST reflector system. In this study, we first search for a more appropriate deformation strategy that reduces the stress amplitude generated by the process of changing shape. Second, we roughly estimate the tracking trajectory of the telescope during its service life, and conduct an extensive numerical investigation to assess the requirements for fatigue resistance. Finally, we develop a new type of steel cable system that satisfies the cable requirements for construction of FAST.

Numerical Investigation on Fatigue Problem of Cable Net Structure of Five-Hundred-Meter Aperture Spherical Radio Telescope

The reflector of Five-hundred-meter Aperture Spherical radio Telescope (FAST) is supported by cable-net structure, so that to enable its reflector surface to form a paraboloid from a sphere in real time through active control. However, such form-changing operation would lead to about 500MPa stress range for the cable-net structure. Such stress range is nearly twice as much of that defined by the relative standard. So the cable-net structure is the most critical and expendable part of FAST reflector system. So, the service life of FAST would directly depend on the residual fatigue life of its cable-net structure. The present paper would make an effort to find a more appropriate deformation strategy to decrease the stress amplitude of the cable during form-changing operation.

Measurement scheme and simulation for the main reflector of FAST

FAST (five-hundred-meter aperture spherical radio telescope) is a radio telescope being built in a karst depression in Guizhou Province of China, which will be the largest single dish radio telescope in the world [1]. The reflector of the telescope is composed of over 4000 panels, and each panel could adjust its position according to observation requirements. During observations, panels in the illuminated area could form a paraboloid to correct spherical aberration [2]. Therefore, accurate measurement of the positions of panels is crucial for the operation of the telescope. In this paper, we introduce the measurement scheme for the reflector of FAST, and simulate its accuracy using direct linear transform, Gauss–Newton algorithm, Levenberg–Marquardt algorithm and an algorithm referred to as multi-point algorithm. Advantages and disadvantages of using these four methods are compared for analysis at different locations of the panels on the reflector, and suggestions are given in choosing algorithms in implementation.

Parameter Sensitivity and Reliability Analysis for Construction Control of Cable-Net Structure Supporting the Reflector of FAST

Five-hundred-meter Aperture Spherical radio Telescope (FAST) is supported by cable-net structure, which enables its surface to form a paraboloid in real time under active control. FAST is now entering project construction and implement stage, however there are always a considerable amount of errors existed in practice which would result in the deviation of the structure from its ideal model. Therefore, structural parameter sensitivity analysis was indispensable discussed. In the paper, the variation ranges of structural parameters were rationally determined. Base on local sensitivity analysis and global sensitivity analysis method, Using the finite element model investigated the influence of different structural parameters change on the static behavior, gets the conclusions that the impact of several key design parameters on the tension force of cable-net is large. The results indicate that of all types of the structural parameters, the error of the length of cable plays the most important role, and the global sensitivity analysis indicates that the tension force range of cable-net is -18% to 27%.

An optimization of the shape of FAST reflector panels

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) will be the largest radio telescope in the world. The surface tolerance of the main reflector is one of the most important parameters for evaluating the performance of the telescope. The relationship between the reflector's surface tolerance and the curvature of FAST reflector panels is analyzed and discussed. According to the calculation of reflector tolerance and antenna gain, an optimized panel shape for minimum surface tolerance and maximum gain is derived. The far field pattern of the FAST telescope is presented while the optimized shape is utilized for reflector panels. The results show that FAST could be operated at a frequency band of 8 GHz or even higher with an acceptable efficiency.