ARGOS: the laser guide star system for the LBT

Abstract

ARGOS is the Laser Guide Star adaptive optics system for the Large Binocular Telescope. Aiming for a wide field adaptive optics correction, ARGOS will equip both sides of LBT with a multi laser beacon system and corresponding wavefront sensors, driving LBT's adaptive secondary mirrors. Utilizing high power pulsed green lasers the artificial beacons are generated via Rayleigh scattering in earth's atmosphere. ARGOS will project a set of three guide stars above each of LBT's mirrors in a wide constellation. The returning scattered light, sensitive particular to the turbulence close to ground, is detected in a gated wavefront sensor system. Measuring and correcting the ground layers of the optical distortions enables ARGOS to achieve a correction over a very wide field of view. Taking advantage of this wide field correction, the science that can be done with the multi object spectrographs LUCIFER will be boosted by higher spatial resolution and strongly enhanced flux for spectroscopy. Apart from the wide field correction ARGOS delivers in its ground layer mode, we foresee a diffraction limited operation with a hybrid Sodium laser Rayleigh beacon combination.

Adaptive optics capabilities at the Large Binocular Telescope Observatory

“…ARGOS is a ground layer AO system comprising of three Rayleigh beacons on each side of the telescope which produce LGS references at a range of 12km using pockels cells for the range gating. Each of the six lasers operate at an average power of 15W-17W and produce an equilateral triangle "constellation" of 4' [13]. The lasers are launched from behind each of the secondary mirrors as can be seen in figure 8.…”

Final design of the wavefront sensor unit for ARGOS, the LBT's LGS facility

“…Laser backscattered flux from the gated range (11850-12150m) is expected to be 1800 photons / subaperture / ms [1] which translates to 300 Mph/s over the entire telescope pupil. An analysis of the spreading of the photons over the patrol camera field (see Figure 12) shows that, in spite of the patrol camera being not gated, only the light backscattered from a 1000m range around 12 km height is properly focused and mostly contributes to the estimation of the laser pointing position.…”

“…ARGOS is the Laser Guide Star facility of the LBT implementing a Ground Layer Adaptive Optics system [6] capable of enhancing the image quality for the multi-object spectrographs and NIR imagers LUCIFER over a wide field of view under almost all seeing conditions [1].…”

Implementation of SLODAR atmospheric turbulence profiling to the ARGOS system

“…ARGOS [1] is the laser guide star adaptive optics facility of the LBT. It makes use of commercial Nd:YAG lasers at 532 nm to produce a constellation of 3 artificial reference sources on both telescope sides.…”