An Extremely Large Synthesis Array (ELSA) with 27 ten-meter telescopes and baseline lengths up to 10 km would provide completely new insight into many astrophysics phenomena. It could be used to obtain resolved images of nearby brown dwarfs which would reveal weather phenomena in their atmospheres, to give detailed pictures of stellar surfaces, interacting binaries, and circumstellar material, to study general-relativistic effects on the orbits of stars near the center of our Galaxy, to obtain "movies" of expanding supernovae, to image the broad-line regions of active galaxies, and to measure the geometry of the fireballs producing the afterglow of gamma-ray bursts.
Observations of faint objects will be possible by using an external reference star (within the isoplanatic angle) to co-phase the array. Telescopes with large diameters are essential to provide good sky coverage in this observing mode. The use of optical fibers for beam transport and delay compensation is highly desirable, as this eliminates the need for an expensive beam train with meter-sized optical elements, and a very large vacuum system. The most challenging aspect of fiber-coupled interferometry is dispersion in the fibers, which has to be eliminated or compensated precisely.
Advances in telescope technology and fiber optics expected for the next decade may bring the cost of a facility similar to the ELSA concept discussed here into a range that would be affordable as an international project.