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 releases or automatically implemented as part of the ctxcal application of the Integrated Software for Images and Spectrometers (ISIS). We noticed (a) differences between these two flat-fields and (b) 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. The image data of the Context Camera (CTX) of the Mars Reconnaissance Orbiter require a correction of optical (vignetting) and electronic (pixel-to-pixel variations) effects that need to be applied for each image before scientific work can commence. These corrections are commonly called flat-field corrections. In this work we notice that the existing correction for CTX leaves an edge darkening in the images. We review the existing flat-field correction, examine how it might change over time and create a new version of this correction that shall be implemented as a new default into the widely used image processing framework ISIS, so that other scientists can easily benefit from our improvements. We provide an improved in-flight flat-field calibration for the Context Camera using the Integrated Software for Imagers and Spectrometers Residual edge darkening effects are now removed with this new calibration The calibration proves to be stable during the overall mission time so far