极端台址条件下太赫兹天线近场全息测量关键技术研究

The highest point on the Antarctic plateau, Dome A, is thought to be the best site among the known sites on earth's surface for terahertz-band astronomical observations, due to better transparence of the atmosphere above the site for terahertz wave. China plans to establish the Antarctic Observatory, including a 5-m terahertz telescope (DATE5). Since the terahertz telescopes require a very high accurate reflector surface, the on-site surface measurements will become one of the decisive factors for the telescope to achieve its final accuracy and perform efficient observations. Among the existing methods for reflector measurement, near-field holography is likely to be the best choice for DATE5 to achieve such a challenging accuracy under such an extreme environment. This project will focus on the following topics: (1) Near-field electromagnetic field characterization and the effects on holographic measurements under the operating mode for a antenna to focus to a near transmitter as in the near-field holographic measurement. (2) Study on suppression or correction techniques for the key-factor influences on the near-field holographic measurements. (3) Experimental research on the near-field holographic measurement system and the influences of the extreme environment. By means of detailed theoretical analysis, numerical simulation and experiment, several key theoretical and technical problems are expected to be solved, and an optimized solution to the on-site reflector measurement of DATE5 is expected to be drawn.

昆仑站所在地南极Dome A上空大气在太赫兹波段具有良好的透过率，是已知地面天文台址中最好的太赫兹观测台址。我国拟在此建设南极天文台，包括一台5米太赫兹望远镜（DATE5）。太赫兹望远镜反射面精度要求很高，而现场测量是能否最终实现精度要求、有效进行天文观测的关键。现有测量方法中，近场全息测量是DATE5在南极现场实现反射面高精度的最接近可行、也许是唯一的选择，但也面临测量精度和极端台址条件两方面前所未有的挑战。本课题拟对极端台址条件下太赫兹天线近场全息测量有关问题做深入研究，主要包括：天线在近处"聚焦"这种特有工作模式下近场电磁场特性及其对全息测量影响的研究；关键因素对近场全息测量精度影响的高精度抑制或修正技术方法研究；天线近场全息测量实验及极端环境影响实验研究。旨在通过理论分析、数值模拟和实验方法进行深入探索，解决关键理论和技术问题，为DATE5现场近场全息测量提供优化的解决方案。

南极5米太赫兹望远镜（DATE5）是建议中将工作于南极Dome A的望远镜。根据DATE5天线的概念设计，反射面面板调整误差要求优于5 μm rms，这对反射面面形的台址现场测量是一项非常有挑战性的任务。近场全息测量或许是实现这一测量精度的最好选择。.采用物理光学仿真方法研究了天线近场辐射特性，开展了近场全息测量方法仿真研究，建立了近场全息测量实验系统，开展了测量实验并取得了良好的实验结果，结合测量实验研究评估了影响近场全息测量精度的若干关键因素，在此基础上完成了DATE5近场全息测量优化设计方案。项目研究任务均已按计划执行，实现了预期目标。.仿真研究表明，对于设定的测量距离，存在一个最佳离焦距离，使得测量截断效应系统性误差最小。最佳离焦距离比重聚焦到设定距离处的离焦值要大。.针对一个1.45m口径天线，在92GHz频率开展了测量实验，该天线具有一个整块的碳纤维抛物面反射面。信标源置于距天线63m的水塔上，观测仰角约19°。实验结果表明，夜间环境温度相对稳定期间在口面空间分辨率为46mm的情况下，75分钟连续3次重复测量误差约为2.3 μm rms。在反射面局部粘贴厚度约45μm的铝箔，通过粘贴铝箔前后两次测量结果相减，检测的局部面形改变与铝箔厚度非常吻合。评估了实验系统的误差因素，如天线指向误差、相关接收机的幅度和相位起伏等，并仿真研究了它们对反射面面形测量误差的贡献，表明实验系统被测天线相对较差的指向精度是限制测量精度进一步提高的瓶颈。后续实验将在建立高精度天线试验平台后继续开展。