An optical test has been devised to test and qualify null correctors that are used for measuring highly aspheric primary mirrors. The technique employs a rotationally symmetric computer- generated hologram (CGH) that tests the null corrector directly by synthesizing a wavefront that would be returned by a perfect primary mirror. A description of the test and summary of the error analysis are given. The error analysis includes hologram errors from pattern distortion, substrate flatness, and etch depth variations. It also includes the effects of errors in the wavelength and data analysis errors. This resulting analysis shows +/- 78 ppm accuracy for measuring the conic constant of null correctors built for measuring 3.5 -m f/1.75 primary mirrors.