In-orbit multi-spectral image sharpness assessment for the Lunar Reconnaissance Orbiter Wide Angle Camera

The Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) began systematic lunar imaging in January of 2010. WAC image maps and derived data products are allowing scientists to improve our knowledge of the Moon with science and engineering applications for future mission planning. The sharpness of the WAC images is an important measure of the camera performance over the duration of the mission time. Accordingly, a measure of the in orbit sharpness was achieved by computing the modulation transfer function (MTF) of the WAC multi-spectral optics from in-orbit lunar limb images. Historically, lunar limb images were used for MTF evaluation for imaging devices on Earth based satellites (e.g. NASA Earth Observer) as a supplement to other imaging targets, but were never used successfully for MTF computation for a lunar orbiter prior to this work (to the best of our knowledge). The proposed method is similar to the ISO 12233:2000 standard slanted edge gradient based method, but uses edge-fitting to obtain high resolution edge spread functions with the lunar limb (that is not perfectly straight). Adaptive noise suppressed derivative computation and the Monte Carlo method for obtaining robust statistics, two other enhancements to standard MTF techniques, are also used in this work. Both cross-track and down-track MTFs were obtained and the results show that the LROC WAC meets and exceeds the MTF requirement of greater than 0.2 at the Nyquist frequency.