The Four-Laser Guide Star Facility (4LGSF) for the ESO VLT Adaptive Optics Facility (AOF)

The 4LGSF is to be installed as a subsystem of the ESO Adaptive Optics Facility (AOF, [1]) on Unit Telescope 4 (UT4) of the VLT, to provide the AO instruments GALACSI/MUSE and GRAAL/HAWK-I with four sodium laser guide stars. The 4LGSF will deploy four modular LGS Units at the UT4 Centrepiece. Two key aspects of the 4LGSF design are: (i) new industrial laser source (fibre lasers) with reduced volume, reduced need of maintenance, higher reliability, simpler operation and optimised spectral format for highly efficient sodium excitation, (ii) modular structure of the four LGS Units, composed of the laser and laser launch telescope, capable to operate independently of the others. The final design of the 4LGSF is now complete and the project has entered the manufacturing, assembly, integration and test phase. Furthermore, modular LGS units containing the laser emitter integrated on the launch telescope have already been demonstrated at ESO in the past years [2, 3]. We believe that having the laser sources as an integral part of a modular unit together with the launching optics offers many advantages at the system level, including the avoidance of laser beam relays, retaining the flexibility to use as many LGS as required independently, and the possibility of building redundancy into the system. Many of these 4LGSF concepts can serve for ELT multi-LGS-assisted adaptive telescope designs and provide a valuable experience in advance of the ELTs.