Study of a Measurement Algorithm for the Surface of a Single Panel of the Reflector of the FAST

The FAST (Five hundred meter Aperture Spherical Radio Telescope) will be the largest radio telescope in the world. There are three outstanding technological innovations in the FAST project: the unique Karst depression as the site for antenna, the light-weight focus cabin driven by cables and servomechanisms, and the active main reflector. When we set the main reflector, the surface shape (including errors in manufacturing) of each of its panels has significant influence on the convergence of electromagnetic waves. It is necessary to measure the surface shape so that it can be set correctly. The FAST main reflector consists of 4600 triangular panels spliced together, each of which has a side length of 11 meters. The large sizes require certain fine technique for measuring surface shapes. In the photogrammetry a measured object is not physically contacted but measured through its image. The photogrammetry can have high efficiency when applied to wide-spread or multiple objects. Currently, the projects of world's largest radio telescopes, such as the GBT and ARECIBO reflectors all use the photogrammetry for surface measurement. After investigating and testing existing methods of surface measurement we propose a method of close-range digital photogrammetry. With this method, the surface shape of an eleven-meter panel of the FAST reflector can be measured in a few minutes. ur results show that the accuracy of the method in measuring a single panel is 2.30mm with a standard deviation of 5.00mm. The accuracy of the reflector panel surface was 3.0mm after adjustment. It is thus feasible to use the photogrammetry for measuring surfaces of single panels of the FAST reflector.