Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) In-Flight Calibration Files

The Context Camera (CTX) on board NASA’s Mars Reconnaissance Orbiter (MRO) has been in orbit since 2006 and has so far delivered more than 130.000 images. Since the beginning of the mission, the images have exhibited a darkening effect increasing from the center of the images towards the edges, creating visible seam lines when multiple images are stitched together. Due to the symmetric decrease in reflectance plots averaged over all lines, this problem is often referred to as ”frown” effect. The Integrated Software for Imagers and Spectrometers (ISIS) pipeline compensates for the frown effect by applying a flatfield correction during the standard calibration routine ctxcal, but the flatfield file used is based only on the first year of the mission. Therefore it does not compensate for the effects caused by progressive change of the sensor signal during mission time. To address the progressively changing frown effect over the mission time, we create a separate flatfield for each month of the mission, designated by the first three characters of the product id. First we perform a standard calibration on all related images, usually consisting of about 300 to 1000 images per month. We use the ISIS program makeflat and feed it with the pre-processed images for the respective mission month, excluding images taken with the spatial summing mode different to one, whose number of samples in the cross-direction is smaller. In this repository, we provide the flatfield files and accompanying example scripts that can be used directly in the ISIS environment.