Figure 3A: Field invariance of the interference function in a Michelson array. If a point source moves away from the field position where phasing is achieved, the wavefront WF at the exit pupil EP is no longer flat but stair-shaped. Its average slope, with respect to the steps, is proportional to the source's displacement. Considered as a convolution of tilted discs with a plane array of points, located in the average wavefront AW, WF provides a focal diffraction pattern which is a product of a broad Airy envelope E with a decentered interference function P, having a peak with side-lobes. Their directions are respectively orthogonal to the steps and to AW. Both functions are field-invariant, but with a different motion velocity in response to the source's motion: owing to the baseline leverage effect, the peak moves faster than the Airy envelope when a star crosses the field. A star or planet, being typically resolved by the peak and not by the envelope, thus provides a resolved image appearing on the envelope background.