Figure 3: Example of interference function, obtained as the Fourier transform (its square modulus) of 100 Dirac peaks, randomly arrayed along a circle (top) and cophased. The central part resembles the Airy pattern of the full disc aperture, with concentric rings, but, towards the periphery, these are increasingly broken into random speckles. With sub-apertures of finite size, the spread function is obtained by multiplying their broad Airy pattern with the interference function. In the conformal Michelson mode both factors are field-invariant but their off-set sensitivity to source motion is different. Extended objects are thus imaged according to a pseudo-convolution of intensities. In sparse arrays, used in the conformal Michelson mode, the pseudo-convolution reduces to an ordinary convolution, with however a window masking effect caused by the sub-aperture's Airy pattern. A highly densified exit pupil shrinks the window to nearly the width of the interference peak, thus oncentrating most of the energy in it, but reduces the usable field to a few resolved pixels on the object.