Careful characterization of the removal function of sub-aperture polishing tools is critical for optimum polishing results. Magnetorheological finishing (MRF®) creates a polishing tool, or “spot”, that is unique both for its locally high removal rate and high slope content. For a variety of reasons, which will be discussed, longer duration spots are beneficial to improving MRF performance, but longer spots yield higher slopes rendering them difficult to measure with adequate fidelity. QED’s Interferometer for Stitching (QIS™) was designed to measure the high slope content inherent to non-null sub-aperture stitching interferometry of aspheres. Based on this unique capability the QIS was recently used to measure various MRF spots in an attempt to see if there was a corresponding improvement in MRF performance as a result of improved knowledge of these longer duration spots. The results of these tests will be presented and compared with those of a standard general purpose interferometer.