Advances in Optical/Infrared Interferometry

After decades of fast-paced technical advances, optical/infrared (O/IR) interferometry has seen a revolution in recent years: <label>■</label>The GRAVITY instrument at the Very Large Telescope Interferometer (VLTI) with four 8-m telescopes reaches thousand-times-fainter objects than possible with earlier interferometers, and the Center for High Angular Resolution Astronomy array (CHARA) routinely offers up to 330-m baselines and aperture synthesis with six 1-m telescopes.<label>■</label>The observed objects are fainter than 19 mag, the images have submilliarcsecond resolution, and the astrometry reaches microarcsecond precision.<label>■</label>This led to breakthrough results on the Galactic Center, exoplanets, active galactic nuclei, young stellar objects, and stellar physics.Following a primer in interferometry, we summarize the advances that led to the performance boost of modern interferometers: <label>■</label>Single-mode beam combiners now combine up to six telescopes, and image reconstruction software has advanced over earlier developments for radio interferometry.<label>■</label>With a combination of large telescopes, adaptive optics (AO), fringe tracking, and especially dual-beam interferometry, GRAVITY has boosted the sensitivity by many orders of magnitude.Another order-of-magnitude improvement will come from laser guide star AO. In combination with large separation fringe tracking, O/IR interferometry will then provide complete sky coverage for observations in the Galactic plane and substantial coverage for extragalactic targets.