The Optimization Of Sensitivityand "Back-illumination" Techniquefor The FAST Telescope

Five hundred meter Aperture Spherical radio Telescope(FAST)is the largest aperture and most sensitive single antenna radio telescope internationally at present.Unlike common radio telescope,outer spherical skirt of the main reflectorof FAST with metal reflector.This part of the spherical reflector can block the ground thermal radiation,to provide a relatively flexible possible for the feed illumination of design.But at the same time,this part also has certain influence for the FAST far-field pattern,such as higher side lobe,zenith angle is not equal to zero asymmetric pattern,etc.This paper adopts the method of physical optics,the GRASPisused to calculate the far field pattern ofFAST,and for calculation and analysis the sensitivity of FAST.First,introduced modeling process in the GRASP and the principle of physical optics calculation far field pattern;And then,researched the influence of the FAST telescope sensitivityby varying the edge taper ofthe feed with Gaussian illumination and discussed at the zenith angle is larger than 26.4 degrees,a “back-illumination” technique is proposed in order to optimize the sensitivity;Finally,calculated the FASTfar fieldpattern of each band,by using the FAST feed pattern at varying of zenith angle.When the feed with Gaussian illumination,sensitivity reaches the maximum when the edge taper is about-12 ~-14 dB.The sensitivity areincreased by 19%,48% and 100% if a back-illumination angle of 15,21 and 26 degree are chosen for zenith angle of 30,35 and 40 degree,respectively.The results of the calculation and analysis method for the FAST electricalperformance optimization,and forbuild the anti-noise wall to provide the reference in the future.