Abstract: The reflector system of the five-hundred-meter aperture spherical radio telescope (FAST) is composed of 4450 panels. The initial gap between these panels directly affects the technical performance of the telescope. The initial gap between panels that is too wide will reduce the effective collecting area of FAST, and thus compromising the sensitivity of the telescope. On the other hand the initial gap between panels that is too narrow may lead to collisions between the panels under certain observation conditions. It is thus important to reasonably determine the size of the initial gap between panels in ensuring the technical performance of the telescope. In this paper, the criterion to determine collisions between panels is given by the method of vector geometry, the collisions of panels under normal observation conditions are studied, as well the worst situations with actuator failures using the analytical results from the integrated finite element model of the reflector, combined with aforementioned criterions. Meanwhile, a reasonable value for initial gap between the panels is determined, providing parameters for the design of the panels.

Key words: active reflector, actuator failures, finite element, five-hundred-meter aperture spherical radio telescope, initial gap