We describe the conceptual design of an advanced laser guide star facility (LGSF) for the Large Binocular Telescope
(LBT), to be built in collaboration with the LBT's international partners. The highest priority goal for the facility is the
correction of ground-layer turbulence, providing partial seeing compensation in the near IR bands over a 4' field. In the
H band, GLAO is projected to improve the median seeing from 0.55" to 0.2".
The new facility will build on the LBT's natural guide star AO system, integrated into the telescope with correction by
adaptive secondary mirrors, and will draw on Arizona's experience in the construction of the first multi-laser adaptive
optics (AO) system at the 6.5 m MMT. The LGSF will use four Rayleigh beacons at 532 nm, projected to an altitude of
25 km, on each of the two 8.4 m component telescopes. Initial use of the system for ground layer correction will deliver
image quality well matched to the LBT's two LUCIFER near IR instruments. They will be used for direct imaging over
a 4'×4' field and will offer a unique capability in high resolution multi-object spectroscopy.
The LGSF is designed to include long-term upgrade paths. Coherent imaging at the combined focus of the two apertures
will be exploited by the LBT Interferometer in the thermal IR. Using the same launch optics, an axial sodium or
Rayleigh beacon can be added to each constellation, for tomographic wavefront reconstruction and diffraction limited
imaging over the usual isoplanatic patch. In the longer term, a second DM conjugated to high altitude is foreseen for the
LBT's LINC-NIRVANA instrument, which would extend the coherent diffraction-limited field to an arcminute in
diameter with multi-conjugate AO.