基于计算全息的长焦距离轴非球面镜面形检测研究

Primary mirror of the extremely large telescope generally is composed of hundreds of segments. Comparing with monolithic solution, this segmented primary method paves a much feasible way for large aperture telescope. However, challenges still exist. Segments have the following characteristics: large aperture, long focal length, non-axisymmetric and deep aspheric departures etc., so it is difficult to test segments by traditional methods. Computer generated holograms (CGH) is a new type of diffractive optical element that can be designed to create virtually any phase or amplitude distribution. Unfortunately, the CGH required to test steep aspherics are difficult to produce, and the null test suffers from mapping distortion. The purpose of this project is to solve main problems in mirror testing of long focal length off-axis segments. Firstly, a novel method using aplanatic lens and CGH is proposed. The former is used to correct most of aberrations and to makes testing setup more compact, while the later compensates the rest of aberrations and provide reference points for alignment. In order to improve the efficiency and accuracy of mirror testing, we will divide the mapping distortion from the segment to the image into two steps. The CGH substrate TWE (transmission wavefront error) could be calibrated after the CGH fabrication is finished, and the main factor that determines phase distribution will be analyzed in detail. This study will create a new testing method for China future large or extremely large telescope mirror polishing. The technology developed in this program also could be used in off-axis aspherics testing of other optical systems such as off-axis three-mirror optical system.

极大口径天文望远镜的主镜由多块子镜拼接而成，这使得主镜的口径可以大幅增加，但是也产生新的问题。子镜具有口径大、焦距长、非轴对称、非球面度大等特点，难以采用传统检测手段测量面形。计算全息（CGH）是一种新的衍射光学元件，在复杂波前产生方面具有极大的灵活性，但是CGH能补偿的非球面度有限，另外补偿检测光路存在投影畸变问题。本课题将解决长焦距离轴非球面镜面形检测中的主要问题，首先设计消球差单透镜和CGH共同作用的检测方案，前者用于缩短检测光路的长度并补偿大部分像差，后者补偿剩余像差并提供光路调整基准；为了提高面形检测效率和精度，提出从干涉图像到待测镜面的两步畸变校正方法；对CGH的透射波前误差进行标定，分析影响相位分布的主要因素。研究意义在于可为中国未来的大口径或极大口径天文望远镜的镜面加工提供一种新的检测方案。本课题发展的技术也可用于其他光学系统比如离轴三反系统中离轴非球面镜的检测。