We have built a modified Twyman-Green interferometer illuminated with a krypton laser. The configuration allows for center of curvature or autocollimation testing at various selected wavelengths. Software data reduction uses a Zernike polynomial fit to the wavefront. The interferograms are all made without moving the source, so the effects of axial color, lateral color, and chromatic variation of aberrations will exhibit themselves in a straightforward fashion in terms of the coefficients of the Zernike polynomials. Also, a scheme for using this information in the calculation of a polychromatic optical transfer function (OTF) has been developed. Requirements for the interferometer's optics are examined, and interferometer calibration discussed.