In this case, the chief rays are in phase for all the points of the field of view. Now, suppose that the interferometer aperture is correctly scaled in the plane of the refocusing lens L1 (as shown in the right picture of Fig. 22). However, one of the two apertures is linearly displaced in this plane by an amount e. Then let us consider the case of a wavefront having a tilt with an OPD amplitude of NX over the interferometric pupil. This is the case of an off-axis object placed at N times the angle X/b. At the re-imaging lens the wavefront has a slope of NX/b'. The displacement of one side of the aperture, the one represented as displaced on the right side in the picture, introduces a phase delay with respect to the perfect case equal to

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