Adaptive optics, on each telescope, and adaptive inter-aperture phasing are required in order to benefit from the high concentration of energy in the interference peak. Indeed, the capabillity for detecting and imaging faint objects against the sky background luminosity is enormously increased, and comparable to that achievable in space with a similar array. A practical limitation, on Earth, arises from the phasing requirement. 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.
Extended objects are thus imaged according to a pseudo-convolution of intensities.
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. As a somewhat extreme example of observations which are in principle achievable with arrays of large telescopes, the case of extra-solar planets has been discussed in some detail 9 . Some clarification of the algorithms usable for phasing multiple telescopes on extended sources is still required. At each step, the energy concentration can be used as the parameter to be optimized, through the adjustment of two phases. Starting with triplets formed with the smallest baselines, for which the object is least resolved, the algorithm proceeds towards the longest baselines.