A phase-shifting point diffraction interferometer is being developed for at-wavelength testing of extreme ultraviolet lithographic optical systems. The interferometer was implemented to characterize the aberrations of a 10x Schwarzschild multilayer-coated reflective optical system at nm. Chromatic vignetting effects are observed and they demonstrate the influence of multilayer coatings on the wave front. A subaperture of the optic with a numerical aperture of 0.07 was measured as having a wave front error of 0.090 wave (1.21 nm) root mean square (rms) at a 13.4 nm wavelength. The wave front measurements indicate measurement repeatability of +/-0.008 wave (+/-0.11 nm) rms. Image calculations that include the effects of the measured aberrations are consistent with imaging performed with the 10x Schwarzschild optic on an extreme ultraviolet exposure tool.