Characterization of symmetric aberrations in aspheric surfaces using non-contact profilometry

A method for measuring symmetric aberrations in large departure aspheric surfaces to nearly single digit nanometer precision is demonstrated. Interferometry can accurately measure plano, spherical and small departure aspheric surfaces. However, null correction is normally required for accurate interferometric measurement of large departure aspheres. When using conventional null lenses, asymmetric aberrations are easily measured by simply rotating, the surface under test to a finite number of positions and comparing them to one another. The rotationally symmetric errors are more difficult to know with certainty due to possible systematic rotationally symmetric errors with the null lens itself. The proposed system can measure aspheres on planar to f/6.0 spherical surfaces with a maximum sag of I mm and from 800 mm to 25 mm spherical surfaces down to f/0.55. A non-contact interferometric probe is used to measure the surface profile with the optic mounted on either a linear or rotary air bearing, depending on the base radius of curvature of the optic. Measurement results are shown for several aspheres and compared with interferometer measurements.