Interferometry can reach high resolution not only with separated apertures, but also across a single telescope aperture. Indeed, as shown by the Palomar Fiber Nuller (PFN), cross-aperture nulling interferometry can detect companions at much smaller angular offsets from stars than coronagraphy on the same telescope can reach. In particular, the PFN was able to detect the faint secondary companion of the spectroscopic binary η Peg at roughly a third of the telescope’s diffraction beamwidth, with null depth accuracies of several 10-4, while also demonstrating the use of baseline rotation to detect companions, as originally envisioned for space-based nullers. Cross-aperture nulling observations can thus resolve separations well inside the central core of a telescope’s diffraction beam. Here we summarize the lessons learned from the PFN’s demonstration experiments, as well as from larger mid-infrared nullers, pointing toward potential performance improvements that could enable a variety of nulling-based exoplanet and dust observations interior to the coronagraphic regime on larger telescopes. Two interesting new developments are the grating nuller, in which both beam combination and phase shifting are provided by the same optical element, and the vortex fiber nuller, which straddles the coronagraphic and separated-subaperture interferometric regimes.
