The GOES program seeks a reliable method to measure the imager's modulation transfer function (MTF) once the instrument is in orbit. This has proven difficult since there is no convenient target. It has been proposed to use the edge of the moon as a high contrast target, offering the possibility of performing a 'knife-edge' MTF test. The following treatment produces a visible, east-west spread function (ESF) from imager scans over the lunar limb. The ESF is used both to produce a visible knife-edge MTF estimate as well as to produce, through differentiation, a visible line spread function, which also is used to produce an MTF. While the results do not rigorously conform to ground tests, they track low and medium spatial frequency test values reasonable well and offer a means to test the instrument's imaging fidelity on-orbit.