Patent ID: 8269981

Claim:
A method of measuring a deviation of an optical test surface from a target shape, the method comprising: directing an incoming beam of electromagnetic radiation having a wavelength λ onto the test surface to generate a measuring beam that has interacted with the test surface, using an interferometer to perform a use measurement by superimposing a reference beam with the measuring beam to determine a wave front deviation of the measuring beam from the reference beam, determining a retrace error in the wave front deviation based on the use measurement, and correcting the wave front deviation of the use measurement by eliminating the retrace error therefrom, wherein: the test surface is configured such that a wave front deviation of larger than 10*λ is obtained when performing the use measurement in an optimum alignment in which the test surface and optical components of the interferometer are aligned such that the wave front deviation is minimized, aberrations accumulated by the measuring beam in the interferometer during the use measurement differ from fictitious aberrations that would have been accumulated by the measuring beam if the test surface were configured such that the wave front deviation obtained in the optimum alignment were smaller than λ, and the difference in the accumulated aberrations causes the retrace error in the wave front deviation, wherein: the target shape has a deviation z from a rotationally symmetric calibration surface in the direction of an axis of symmetry of the calibration surface, and z satisfies the formula ∂ z ∂ x i < 0.5 · N · λ / D , in which x i is a dimension perpendicular to the axis of symmetry, N is the number of pixels along a dimension of a camera of the interferometer configured to record an interferogram generated by the reference beam and the measuring beam having interacted with the test surface, and D is a diameter of the test surface perpendicular to the axis of symmetry.