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.
This calibration data replaces the monthly calibration data published in http://dx.doi.org/10.17169/refubium-37236.
