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A long-held vision has been to realize diffraction-limited optical aperturesynthesis over kilometer baselines. This will enable imaging of stellarsurfaces and their environments, and reveal interacting gas flows in binarysystems. An opportunity is now opening up with the large telescope arraysprimarily erected for measuring Cherenkov light in air induced by gamma rays.With suitable software, such telescopes could be electronically connected andalso used for intensity interferometry. Second-order spatial coherence of lightis obtained by cross correlating intensity fluctuations measured in differentpairs of telescopes. With no optical links between them, the error budget isset by the electronic time resolution of a few nanoseconds. Correspondinglight-travel distances are approximately one meter, making the methodpractically immune to atmospheric turbulence or optical imperfections,permitting both very long baselines and observing at short optical wavelengths.Previous theoretical modeling has shown that full images should be possible toretrieve from observations with such telescope arrays. This project aims atverifying diffraction-limited imaging experimentally with groups of detachedand independent optical telescopes. In a large optics laboratory, artificialstars were observed by an array of small telescopes. Using high-speedphoton-counting solid-state detectors, intensity fluctuations werecross-correlated over up to 180 baselines between pairs of telescopes,producing coherence maps across the interferometric Fourier-transform plane.These measurements were used to extract parameters about the simulated stars,and to reconstruct their two-dimensional images. As far as we are aware, theseare the first diffraction-limited images obtained from an optical array onlylinked by electronic software, with no optical connections between thetelescopes.

Long-baseline optical intensity interferometry: Laboratory demonstration of diffraction-limited imaging