A vacuum rated Coaxial Reference Interferometer (CORI) has been designed that satisfies the data input requirements of Computer Controlled Polishing (CCP) methods. The instrument obtains wavefront data at very high sampling intervals (50 to 100 fringes per aperture) and maintains precise mirror to focal plane mapping fidelity. Realistic as manufactured design goals are a 0.010% rms wavefront quality at f/2.3 and the ability to locate positions on a 2.4 meter diameter mirror to within 0.6mm from interferogram coordinates. Internal calibration means have been provided to independently verify interferometer performance as required. The instrument can be used to test both uncoated and coated mirrors. Interferograms are recorded on 70-mm film but may be monitored visually or by means of a vidicon.