500m口径球面射电望远镜瞬时抛物面拟合精度的预估与改善

依据500m口径球面射电望远镜(FAST)主动反射面工作原理及相关结构尺寸,基于反射面单元动态面形精度分析研究了FAST瞬时抛物面的拟合精度。首先,以反射面单元为研究对象,自节点开始推导考虑了机构运动的反射面单元动态面形精度计算方法。然后,提取不同区域19块反射面单元的各自9种初始面形工况,给出了分析算例。最后,基于单元动态精度特性,提出瞬时抛物面机械本体拟合精度及考虑馈源照明函数的半光程差拟合精度预估方法,并给出2种改善措施,即扩大可控区至300m抛物面外一层节点靶标,及抛物面周圈节点靶标向球心、中心反向各自偏移4mm。结果表明:反射面单元初始面形应优选面形精度RMS为2mm的波浪式面板。改进控制策略后机械本体拟合精度提高至RMS为3.938mm,优于RMS为5mm的设计指标,此时半光程差拟合精度RMS为0.629mm。该项研究对确定反射面单元初始面形及调节瞬时抛物面拟合精度很有意义。

Estimation and improvement for fitting accuracy of instantaneous parabolic reflector in FAST

According to the working principle of the active reflector surface in Five-hundred-meter Aperture Spherical radio Telescope (FAST) and its structure dimension, the fitting accuracy of instantaneous parabolic reflector for the FAST was researched based on analysis of the dynamic surface accuracy of reflector element. Firstly, a reflector element was selected as research object, its calculating formulas taking the mechanism movement into account were deduced from a joint to solve dynamic surface accuracy. Then, by taking 19 reflector elements from different regions for examples, the characteristics under 9 initial surface types were analyzed correspondingly. Finally, based on the dynamic characteristics of reflector element, the fitting accuracy estimation methods of instantaneous parabolic surface were proposed, including mechanical surface precision and semi-optical path difference surface accuracy with feeding lighting function. Furthermore, two improved measures for surface accuracy were given, and they were enlarging joint controlling area to next joint target ring of parabolic surface border, and offsetting the outer ring joint targets by 4 mm to the sphere center and also moving center targets by 4 mm to reserve direction. Results demonstrate that the panel with surface accuracy RMS 2 mm is a preferential selection for initial surface type of reflector element. After changing control strategy, the mechanical surface accuracy increases to RMS 3.938 mm, better than the index RMS 5 mm, Meanwhile, the semi-optical path difference surface accuracy reaches to RMS 0.629 mm. This research has important significance for ensuring reflector element initial surface type and adjusting fitting accuracy of the instantaneous parabolic reflector.