Jo Jeong-Bin, Hwang Jai-Hyuk, Bae Jae-Sung
Opt Express. 2016 Mar 7;24(5):5411-5422. doi: 10.1364/OE.24.005411.
In this study, an online refocusing algorithm is proposed for a satellite camera performing an Earth observation mission. Satellite cameras are vulnerable to misalignment in orbit because of their severe launching environments and the thermal vacuum environment in space. The proposed online refocusing algorithm is able to guarantee high quality images by aligning the satellite camera in real time. This alignment is achieved by precisely adjusting the movement mechanism of the secondary mirror (M2) and the focal plane. The target optical system used in this study was originally designed for the purposes of algorithm development. The system uses a Schmidt-Cassegrain-type satellite camera with a 200-mm diameter primary mirror (M1). The ground sampling distance (GSD) is 3.8 m from an altitude of 700 km. A fourth-order equation model is derived for the modulation transfer function (MTF) variation tendency for M2 de-spacing. Following this, the proposed online refocusing algorithm for the target optical system is developed. The algorithm is able to assess the de-space position from the MTF measurements using stellar sources. It is determined from the simulation that any misaligned satellite camera can be refocused within a ± 0.5μm M2 de-space error by applying the proposed refocusing algorithm in real time.
在本研究中，针对执行地球观测任务的卫星相机提出了一种在线重新聚焦算法。由于卫星相机严峻的发射环境和太空的热真空环境，其在轨道上容易出现对准误差。所提出的在线重新聚焦算法能够通过实时对准卫星相机来保证高质量图像。这种对准是通过精确调整副镜（M2）和焦平面的移动机构来实现的。本研究中使用的目标光学系统最初是为算法开发目的而设计的。该系统采用了一台施密特 - 卡塞格伦型卫星相机，其主镜（M1）直径为200毫米。在700千米的高度，地面采样距离（GSD）为3.8米。针对M2离焦时调制传递函数（MTF）的变化趋势推导了一个四阶方程模型。在此基础上，开发了针对目标光学系统的在线重新聚焦算法。该算法能够利用恒星源通过MTF测量来评估离焦位置。通过仿真确定，应用所提出的重新聚焦算法，任何未对准的卫星相机都能在M2离焦误差±0.5μm范围内重新聚焦。
