1.Xi’an Institute of Optics and Precision Mechanics of Chinese Academy of Sciences, Xi’an 710119, china 2.University of Chinese Academy of Sciences, Beijing 100049, China
Abstract:Modulation transfer function (MTF) measurement is a major means to evaluate the imaging quality of a space camera. The influence caused by the spectral characteristic of light source on the MTF results is not negligible, because the transmittance and color aberration of optical systems, and quantum efficiency of the space camera detectors are all spectrally related. Thus, MTF results tested by different light sources are different from each other. To address this problem, we propose a method to analyze the influence of spectral characteristics of light sources on measuring the MTF of space cameras. In addition, the devices and methods are designed to calibrate the spectral response and monochrome point spread function (PSF) of space camera. A Sigma lens (focal length: 1000mm, F number: 5.6) and a Cannon EOS 5DSR camera (pixel size: 4.14 μm) are combined into an experimental space camera, whose spectral response is calibrated with a monochromator (Omno30300, NBeT) and a spectral radiometer (FieldSpec, ASD). We calibrate the monochrome PSF of the Sigma lens with the same monochromator and a CCD (PIXIS 1024, Princeton Instruments, pixel size: 13 μm) micro-measuring system (20X objective). During the calibration of spectral response and monochrome PSF, the same collimator (focus: 5000 mm, F number: 10) is used. With using the proposed method and those calibrating data, we compute the theoretical values of the MTF of a space camera measured separately with five different light sources. The results indicate that MTF measured by a xenon lamp is greatly different from those MTFs measured by the other four light sources. Comparisons of those theoretically calculated MTFs, separately, show that the MTF measured by a tungsten halogen lamp is greater than the MTF measured by a xenon lamp at each spatial frequency. The deviation between those two lamps reaches a maximum value of 0.075 in the medium-high frequency zone. Furthermore, in order to verify those theoretical conclusions, a platform including a collimator and the previous space camera is constructed. The MTFs measured by a tungsten halogen lamp and a xenon lamp are computed with the slanted-edge method respectively. The results demonstrate that the distributions and deviations of the MTFs tested by those two lamps are identical to those theoretical results at each spatial frequency, with the maximum deviation being 0.057. The theoretical and experimental results demonstrate that the suggested method can accurately calculate the influence of spectral characteristics of light sources on measuring MTF of space cameras. The proposed method can also be adopted to investigate the influence of spectral characteristics of light sources on MTF of optical systems in the design or test stages. Keywords:optical test/ modulation transfer function/ space camera/ spectral characteristic of light sources
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2.1.空间相机MTF检测系统组成
空间相机MTF检测系统组成如图1所示, 主要包括积分球光源、靶板、平行光管和空间相机. 积分球光源位于平行光管焦面后方, 其输出非相干光均匀照明位于平行光管焦面处的靶板处, 经平行光管准直后出射平行光, 模拟无穷远目标. 空间相机对靶板成像, 通过分析靶板图像检测空间相机MTF. 目前, 空间相机MTF检测时, 常用光源包括钨丝灯、氙灯等光源, 其中钨丝灯应用较多, 但其色温偏低, 而氙灯光谱色温则更接近太阳光. 本文重点研究光源光谱特性对MTF检测的影响. 图 1 空间相机MTF检测系统组成 Figure1. Configuration of a space camera MTF measurement system.
式中$\mathop {\max }\limits_i \left\{ { {{{I_{{\lambda _i}}}}}/{{{L_{{\lambda _i}}}}}} \right\}$表示${{{I_{{\lambda _i}}}}}/{{{L_{{\lambda _i}}}}}$的最大值. 空间相机光学系统单色PSF标定原理如图3所示, 整套标定系统包括单色仪、平行光管、空间相机光学系统和显微测量系统. 显微测量系统由显微物镜、中继镜和探测器组成. 平行光管焦面处放置星点板, 其直径${D_{{\rm{star}}}}$需满足 图 3 空间相机光学系统单色PSF标定原理图 Figure3. Schematic of calibrating the monochrome PSF of the optical system of a space camera.
表2卤钨灯和氙灯光源所得MTF检测结果及其偏差 Table2.Test results and its deviation between the MTF measured with slanted-edge method by using a tungsten halogen lamp and a xenon lamp.