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Lunar limb knife-edge MTF measurements

The edge of the moon is used to offer a high contrast target to perform a visible 'knife-edge' MTF test on a digital imaging system in geostationary orbit. An image of the moon is taken in the camera's normal scanning mode, and traces across the sharpest edge are used to form an edge spread function (ESF). The ESF is then used to produce an MTF estimate. In a second trial, the imaging system is allowed to stare as the lunar edge drifts by, creating an edge spread function with a much higher effective spatial sampling rate. In each case, a technique of combining and resampling traces is employed to adapt the knife-edge MTF technique for use with sampled data. The resulting MTF curves track four ground test frequencies to within five percent. The approach thus offers a means of testing the MTF and OTF of orbiting image acquisition devices.