Elimination of alignment errors in an interferometric measurement

A method for the elimination of alignment errors in a non-null test interferometer for the measurement of aspheres and free-form surfaces which uses a set of tilted waves to locally compensate the deviation of the surface from the spherical form is presented. Usually the aberrations introduced by misalignment like tilt, defocus and koma are simply subtracted from the measurement result since the exact retrace errors introduced due to the misalignment are unknown. Here the misalignment leads to errors in the measurement that cannot be removed. In the case of our interferometric setup these retrace errors can be calculated using the calibration information of the setup which allows a more sophisticated method for the distinction between surface and alignment errors. This leads to higher measurement accuracy and lower requirements for the alignment accuracy of the surface under test.