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Interferometric techniques using null correctors and computer-generated holograms have become the conventional methods of aspheric surface measurement. These techniques involve substantial design and manufacturing time prior to testing. Additional complexity is added by the necessity to measure all optical surfaces, air spaces, and refractive index of the components. This is obvious for a null corrector, but even when using a computer-generated hologram an interferometric objective is used to reduce the curvature of the wavefront returning from the test surface. The air spaces, indices and surfaces of the objective must also be known as accurately as those of a null corrector. The original computer-generated artwork for the hologram must also be accurately produced and reduced using optics with very little distortion. Both of these techniques, therefore, become prone to systematic error due to inaccurate measurement of the components of the test system. Mechanical profilometry is an alternative to these techniques. The accuracy of the system can be verified using known spheres because the spheres will be measured by the same process as the asphere. In the process of measuring the asphere, a meridian of an optical surface can he rotated on its best fit sphere or measured relative to a straight line. A mechanical gauge is used to measure the deviation from the line or best fit sphere. Accuracy of one-tenth wave is possible and accuracy of one-quarter to one-half wave is routine. The mechanical profilometers are quite universal and require very little equipment specific to a given test surface. This reduces the front-end time for specific measurements compared to null correctors and holographic nulls. This paper discusses the geometry of aspheric profile measurements and instrumentation. It includes a description of some transducer options, mechanical considerations, calibration, sample surface figure error curves, and a brief discussion of potential errors.
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