High-contrast Imaging with Fizeau Interferometry: the Case of Altair*

Abstract

The Large Binocular Telescope (LBT) has two 8.4 m primary mirrors that produce beams that can be combined coherently in a "Fizeau" interferometric mode. In principle, the Fizeau point-spread function (PSF) enables the probing of structure at a resolution up to three times better than that of the adaptive-optics-corrected PSF of a single 8.4 m telescope. In this work, we examined the nearby star Altair (5.13 pc, type A7V, hundreds of Myr to approximate to 1.4 Gyr) in the Fizeau mode with the LBT at Br alpha (4.05 mu m) and carried out angular differential imaging to search for companions. This work presents the first filled-aperture LBT Fizeau science data set to benefit from a correcting mirror that provides active phase control. In the analysis of the lambda/D angular regime, the sensitivity of the data set is down to approximate to 0.5 M (circle dot) at 1 '' for a 1.0 Gyr system. This sensitivity remains limited by the small amount of integration time, which is in turn limited by the instability of the Fizeau PSF. However, in the Fizeau fringe regime we attain sensitivities of Delta m approximate to 5 at 0.'' 2 and put constraints on companions of 1.3 M (circle dot) down to an inner angle of approximate to 0.'' 15, closer than any previously published direct imaging of Altair. This analysis is a pathfinder for future data sets of this type, and represents some of the first steps to unlocking the potential of the first Extremely Large Telescope. Fizeau observations will be able to reach dimmer targets with upgrades to the instrument, in particular the phase detector.