Closure phase in high-resolution optical imaging

The angular resolution of long-exposure optical images taken with large telescopes at the best sites is limited by atmospheric phase fluctuations to ∼0.5–1 arc s; that is, ∼10–50 times worse than the theoretical diffraction limit. In contrast, the measurement of visibilities and closure phases of fringe patterns in short-exposure images, taken through an aperture mask comprising a non-redundant array of three or more holes, offers the prospect of reliable diffraction-limited imaging using methods well established in very long baseline interferometry (VLBI) at radio wavelengths. Here we report successful first measurements of fringe visibility and closure phase by this technique, together with evidence that the method is applicable to objects as faint as magnitude 15.