The LBTI Fizeau imager - I. Fundamental gain in high-contrast imaging

We show by numerical simulations a fundamental gain in contrast when combining coherently monochromatic light from two adaptive optics (AO) telescopes instead of using a single stand-alone AO telescope, assuming efficient control and acquisition systems at high speed. A contrast gain map is defined as the normalized point spread functions (PSFs) ratio of a single Large Binocular Telescope (LBT) aperture over the dual Large Binocular Telescope Interferometer (LBTI) aperture in Fizeau mode. The global gain averaged across the AO-corrected field of view is improved by a factor of 2 in contrast in long exposures and by a factor of 10 in contrast in short exposures (i.e. in exposures, respectively, longer or shorter than the coherence time). The fringed speckle halo in short exposures contains not only highangular resolution information, as stated by speckle imaging and speckle interferometry, but also high-contrast imaging information. A high-gain zone is further produced in the valleys of the PSF formed by the dark Airy rings and/or the dark fringes. Earth rotation allows us to exploit various areas in the contrast gain map. A huge-contrast gain in narrow zones can be achieved when both a dark fringe and a dark ring overlap on to an exoplanet. Compared to a single 8-m LBT aperture, the 23-m LBTI Fizeau imager can provide a gain in sensitivity (by a factor of 4), a gain in angular resolution (by a factor of 3) and, as well, a gain in raw contrast (by a factor of 2–1000 varying over the AO-corrected field of view).