The entire process of designing the test setup with a computer-generated hologram (CGH) and performing the CGH-null test of an asphere will be described in detail. Critical aspects in testing aspherics with CGH-nulls e.g. caustic of the asphere, influence of alignment errors, lateral distortion and lateral resolution of the measurement are discussed. Detection, specification and calibration methods of fabrication errors of the CGH-null are analyzed with respect to the fabrication technology of the CGH. For a high accuracy CGH-null test of an asphere, the errors of the CGH must be negligible or well calibrated. Techniques to calibrate the systematic error of the test setup are presented with examples. Experimental results of interferometric CGH-null tests of an asphere are presented.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.