Laser Guide Stars Facility

The 4LGSF is to be installed as a subsystem of the Adaptive Optics Facility (AOF) on UT4 of the VLT, to provide the AO systems GALACSI/MUSE and GRAAL/HAWK-I with four sodium laser guide stars (LGSs), as artificial reference sources for the high-order AO corrections.

The 4LGSF will deploy four modular LGS Units (see below) at the UT4 Centrepiece, as shown in Figure 1. Each LGS Unit consists of the Launch Telescope System incl. 20W Laser Head and two close-by cabinets, one hosting the Laser Unit electronics (incl. the pump fibre laser unit) and the other containing the local control electronics. Two additional 4LGSF cabinets are installed on a new 4LGSF Platform underneath the Nasmyth B platform and contain the computers for independently controlling the four LGS Units. The 4LGSF Platform also hosts the heat exchanger for the laser cooling system

Figure 1: 4LGSF installation overview. There are two main installation locations: on the Centrepiece and on the 4LGSF Platform underneath the Nasmyth B Platform (to the right). Beside the four Launch Telescope Systems (each including a Laser Head), in total eight cabinets are installed on the Centrepiece for the Laser electronics incl. pump lasers and the local control electronics. Furthermore, connection and distribution boxes are mounted on the Centrepiece for the power and cooling supply. On the 4LGSF Platform two cabinets are installed containing the LCUs for the four LGS Units and the safety interlock system. The 4LGSF Platform also hosts the heat exchanger of the laser cooling system.

Key aspects of the 4LGSF design are:

  • Simplified opto-mechanical design compared to the first-generation LGSF at UT4.
  • Modular structure of the four LGS Units, composed of the laser and launch telescope system, capable to operate independently of the others.
  • New laser source with reduced need of maintenance, higher reliability and simpler operation.
  • Reduced cabling requirements between remote subsystems.

The development of the 4LGSF is based closely on the experience gained with the LGSF that has been developed by ESO in collaboration with external institutes and which is in science operation at Paranal Observatory. The two critical long-lead items for the 4LGSF, the Laser System and the Optical Tube Assemblies (OTA) for the laser Launch Telescope Systems (LTS) are procured from industry. The fibre Raman laser technology, on which the 4LGSF Laser System is based, has been developed at ESO and transferred to industry.

Due to the advanced laser technology used in 4LGSF, the preparation time of an observing run with 4LGSF will be considerably reduced compared to the current LGSF. In addition, by the type of laser technology chosen, the amount of preventative maintenance needed for the 4LGSF will be significantly reduced compared to the existing LGSF which currently uses still its original dye laser (planned to be replaced by also a fibre laser).

A modular LGS unit containing the laser emitter integrated on the launch telescope has already been demonstrated at ESO by the transportable “Wendelstein” system development used for mesospheric sodium layer studies (see