Interferometric measurements of real surfaces are combined with computed optical design analyses. To accomplish this, a special software interface driver was developed to accept serial optical path difference (OPD) data from a Zygo Mark III interferometer, to apply standard algorithms, and convert the measured data into a CODE V readable file. Analysis is conducted on a large-aperture developmental optical system containing a fold mirror of eggcrate construction. MTF performance of the entire system is computed as are the contributions of individual measured surfaces to overall optical quality. Diffraction effects resulting from the rib structure of the eggcrate mirror are isolated in the analysis, as are the effects of gravity and figure components of the wavefront. The hybrid analysis demonstrates the usefulness of coupling interferometry with an optical design program in the analysis of complex optical systems.