Measurement of aspheres and free-form surfaces in a non-null test interferometer: reconstruction of high-frequency errors

The tilted wave interferometer is a non-null test interferometer for the measurement of aspheres and freeform surfaces without dedicated null-optics that uses an array of tilted waves to locally compensate the deviation of the surface from the spherical form. The concept allows for short measurement times of only a few minutes and high lateral resolutions at the same time. The calculation of the surface error is performed by perturbation of a polynomial representation of the surface. Since we are also interested in higher frequency errors of the surface which cannot be described by a polynomial of finite order these errors are evaluated in an additional step. Since every wavefront only covers a small area of the surface the challenge here is to reconstruct the surface from the information that is distributed over the different patches. We will present the method that was developed for the reconstruction of these high frequency errors as well as measurement results of aspheres and freeform surfaces without rotational symmetry that were obtained by this method.