  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 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, $\sim$100s Myr to $\approx$1.4 Gyr) in the Fizeau
mode with the LBT at Br-$\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 dataset to benefit from a correcting mirror
which provides active phase control. In the analysis of the $\lambda/D$ angular
regime, the sensitivity of the dataset is down to $\approx$0.5 $M_{\odot}$ 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 $\Delta m
\approx 5$ at 0.2" and put constraints to companions of 1.3 $M_{\odot}$ down to
an inner angle of $\approx$0.15", closer than any previously published direct
imaging of Altair. This analysis is a pathfinder for future datasets of this
type, and represents some of the first steps to unlocking the potential of the
first ELT. Fizeau observations will be able to reach dimmer targets with
upgrades to the instrument, in particular the phase detector.
