一种扩大FAST视场的方法

所有的大口径射电望远镜都存在这样一个问题：在其分辨率和灵敏度提高的同时，视场变小．而且口径越大，视场越小．这成为大口径望远镜不可回避的矛盾．要解决这个矛盾，可以在望远镜的焦平面上放置Ⅳ个分立馈源．让它们同时工作，这样可以看作把视场扩大了Ⅳ倍．望远镜的工作效率提高Ⅳ倍．但是这样做的缺点是——视场不连续．且馈源数目Ⅳ受到望远镜焦比(F／D)的限制．采用致密焦面阵(dense focal plane array)就可以很好地解决这个问题．致密焦面阵的单元不是喇叭口天线，而是无方向性的Vivaldi天线(Vivaldi antenna)．要把Vivaldi阵列应用到望远镜上，需要对单个Vivaldi天线和Vivaldi阵列的电性能有清楚的认识，并能根据需要来设计照明方向图．还要知道大望远镜的焦面上电磁场的分布情况，借此判断能否应用Vivaldi阵列，以及给出Vivaldi单元的分路赋权网络．主要给出了FAST的焦面场的分布情况．并说明应用Vivaldi阵列的可能性．

A Way to Improve the Field of View of the Large Aperture Radio Telescope

There is a limitation in the field of view (FOV) of large aperture radio telescopes, which is usually unavoidable. In order to enlarge the FOV, we may place several discrete feeds at the focal plane of the telescope. When all of the feeds work at the same time, we will achieve a FOV extended to several times, which equivalently times the functions of a telescope. The disadvantage of this solution is the sparseness of the sampling at the FOV, which is not desirable for continually imaging process. Moreover, the number of the feeds mounted on focus is restricted by the F/D ratio of the telescope. We investigate in this paper an innovated technology to realize the simultaneous and continuous large FOV for the FAST (Five-Hundred-meter Aperture Spherical radio Telescope). The new feed is proposed to be made of a focus array whose elements are microstrip antenna, so called Vivaldi antenna. Electro-magnetic property of the Vivaldi element and the Vivaldi array are introduced, which is essential to the formation of the focus array. Focal field distribution (FFD) of the FAST of different illuminated portions is simulated using the software package GRASP8. The results give critical information for the layout deign of the focus array, especially, the weighting system of Vivaldi array. Cursory configurations of the array structure are suggested, and illumination patterns are estimated. Finally, some remarks on the feasibility of applying the Vivaldi array to FAST are made.