The Large Binocular Telescope (LBT) has been designed for optical/infrared interferometry that combines high sensitivity and
resolution. Key scientific projects will be deep, wide field infrared images of the Hubble Deep Field, with nearly ten times the resolution of the Hubble telescope, and the study of planets and dust in extra-solar systems, from their formation onward. A basic requirement for interferometry of faint objects is that the aberrations across the two 8.4 m telescopes be corrected for atmospheric phase errors. This will be done at the telescopes' secondary mirrors, so as to preserve the very low emissivity of the direct beam combination optics. Sodium lasers projected co-axially from above each secondary will allow wavefront sensing for correction of even the faintest objects. The two telescopes are rigidly mounted close together on a single alt-azimuth mount, to cover a large fraction of the u-v plane in a single exposure, with baselines continuous from 0 to 23 m. Field rotation during the night completes the cover, to allow recovery of images with the full resolution of a diffraction limited 23 m telescope. The beam combining optics will be cryogenically cooled to maintain the very low thermal background from only 3 warm reflections in total (primary, adaptive secondary, tertiary). For wide field imaging, the beams will be combined and stabilized so that in a long exposure every
source across a ∼ 1 arcminute field is crossed by nterference fringes. From a set of such exposures the resultant deep image will have a resolution 0.02 arcsec in the 2.2.μm K band. For high contrast studies of exo-planetary systems, a Bracewell nulling system will be used with superposition by division of amplitude, for 99.99% suppression of the stellar radiation.