A measuring apparatus for determining data relating to the shape of an input radiation wavefront, where the wavefront shape is describable at a pre-determined location in an optical system. The apparatus comprises aberration means (27, 39, 61, 77), the shape of which is defined by a filter function; and detection means (23) having a radiation sensitive surface capable of detecting the intensity of incident radiation on the surface, the detection means being coupled to an output device (31, 49, 69) that provides an output signal that provides a measure of the intensity of the incident radiation. The aberration means is configured to act on an input wavefront to produce first and second output radiation signals, and the filter function has a real part and an imaginary part, the real and imaginary parts both having even symmetry or both having odd symmetry.
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