Mars Reconnaissance Orbiter Context Camera Updated In-Flight Calibration

The image data of the Context Camera (CTX) of the Mars Reconnaissance Orbiter require a flat-field correction that is currently available as a plain text file in the Planetary Data System "calib" folders for all CTX Enhanced Data Record (EDR) releases or automatically implemented as part of the ctxcal application of the Integrated Software for Images and Spectrometers (ISIS). We noticed 1) differences between these two flat-fields and 2) residual edge darkening (vignetting) after applying ctxcal. This work examines in detail the edge-darkening effect over time and creates a new improved flat-field calibration file to be implemented into the ISIS ctxcal application as a new default. We introduce a method to quantify the vignetting effect and its residuals after regular ISIS calibration. With the old calibration, the amount of residual edge-darkening is about eight percent. We prove that the new calibration does remove the effect completely, does not introduce any artifacts and qualitatively and quantitatively validate newly calibrated images. Mosaics produced with images that have been calibrated with our new flatfield show immediately less striping issues, without the application of any standard mosaicking-related tone-matching techniques.火星勘测轨道飞行器（Mars Reconnaissance Orbiter, MRO）搭载的上下文相机（Context Camera, CTX）所获取的图像数据需进行平场校正。目前，此类校正所需的平场文件可在行星数据系统（Planetary Data System, PDS）所有CTX增强数据记录（Enhanced Data Record, EDR）产品对应的"calib"文件夹中以纯文本文件形式获取，也可通过图像与光谱仪集成软件（Integrated Software for Images and Spectrometers, ISIS）的ctxcal工具自动完成校正。我们发现两类问题：一是上述两种平场文件存在差异；二是使用ctxcal工具完成校正后，图像仍存在残留边缘暗化（渐晕，vignetting）现象。本研究详细分析了随时间变化的边缘暗化效应，并构建了一套经优化的新型平场校正文件，将其作为默认配置集成至ISIS的ctxcal工具中。我们提出了一种量化常规ISIS校正后渐晕效应及其残留量的方法。采用旧版校正方案时，残留边缘暗化的占比约为8%。实验证明，新型校正方案可完全消除该效应且不会引入任何伪影，我们同时从定性与定量两个维度对经新校正方案处理后的图像进行了验证。使用经新型平场校正的图像拼接得到的镶嵌图，无需额外应用任何标准拼接相关的色调匹配技术，即可显著减少条纹伪影问题。