To facilitate the production of 18" diameter NaC1 windows for the Antares Inertial Confinement Fusion Program of LASL, Harshaw has an optical evaluation test facility in the same room as the polishing machines. The principal optical test instrument is an 18" Zygo Fizeau interferometer. The photographed interferogram from the interferometer is digitized manually using a BITPAD* coordinate digitizer and PET 200l** computer. The computer has been programmed (in Basic) to fit, by least squares, the fringe coordinates to the best spherical fit over the clear aperture. The output gives the best fit (sphere) and wedge (linear) and the RMS deviation of wave front from best fit. An interferogram can be reduced in less than ten minutes. The cost of the reduction equipment was less than $2,000.00.