Theoretical And Experimental Research On Supporting Structure Of FAST Refelector

Area of the reflector is one of the main indexes to evaluate the performance of a telescope. A large area is indispensable to observe more celestial bodies in a further distance. On the other hand, the shape of a reflector is sometimes affected by its self-weight or the wind load, which definitely reduces its shape accuracy, and consequently makes the performance weakened. From this point of view, how to support the reflector efficiently is always a key problem on constructing new larger- aperture telescopes. In 1994, Chinese astronomers proposed to construct a five-hundred-meter aperture spherical telescope with an active reflector, named FAST, making use of Karst terrain in Guizhou, China, which is expected to be the largest and most effective radio telescope in the world. However, the proper supporting structures of reflector were not achieved for FAST in previous research, due to the great difficulties brought by the big size of the telescope, the complexity of Karst terrain and the activation of the reflector. In order to solve this problem, a cable-net with steel sub-structures is proposed and studied as the supporting structure of FAST in this paper.First of all, the general plan of the cable-net and steel sub-structures is discussed. The patterns and sizes of cable mesh, as well as locations of control cables are investigated respectively to find out the best set of some key parameters.Then some researches are focused on the cable-net: (1) the analysis methods of initial stress state of cable-net are studied and an optimization method for initial stress is developed based on the characteristics of the cable mesh. In addition, the effects of sub-structures'self-weight on the cable-net are illustrated; (2) the active shape-changing of reflector (for searching and tracking) is simulated numerically. And the regularity of response of cable-net caused by the shape-changing is analyzed sufficiently. Consequently, a practical statistical method is developed for the analysis of response caused by shape-changing. Based on the work above, the strategy of shape-changing of working reflector is optimized; (3) the mechanical characteristics of cable-net are discussed under different loads. And the methods of reducing wind-induced deformation are given. A temperature compensation method is also adopted to reduce the effect of temperature variation; (4) through the analysis of three states of cable-net (initial state, working state and load state), a practical and simple design method and concerned program for the cable-net of FAST are developed.Further more, the researches on sub-structures are carried out in detail. A design method for maintaining the shape of sub-structures is presented, which can accommodate the high accuracy demand. And the sub-structure of reflector is designed and optimized. Three kinds of structures, including lattice structures, suspended structures and hybrid structures, are proposed as sub-structures. In order to get effective sub-structures, they are compared through parametric analysis, which also provides detailed information for future experiments and designing. Meanwhile, the joints which are used to attach sub-structures to the cable-net are designed in detail and two types of feasible joints are given: sliding joints and mechanical joints.To verify the research achievements on FAST, a 30m-diameter model of FAST is constructed in Miyun Astronomic Observation Station by National Astronomical Observatory of China (NAOC), which will be applied as an actual astronomic observation device. Several researches on this model are presented: crucial components of the model are studied and tested; numerical simulation method for forming of initial state is investigated. As a result, an optimal forming scheme is attained. Moreover, the modification method is applied to make the numerical model closer to the actual model; effective measures are taken in each step of forming to get the desired accuracy. By the tests on the 30m-diameter model, the feasibility of the cable-net as a supporting structure of FAST reflector is verified, and the whole project of FAST is promoted. At the same time, some valuable experiences and credible information for constructing FAST in real site are accumulated during this process.