The laser guide star facility for the Thirty Meter Telescope

The Thirty Meter Telescope (TMT) will utilize adaptive optics to achieve near diffraction-limited images in the near-infrared using both natural and laser guide stars. The Laser Guide Star Facility (LGSF) will project up to eight Na laser beacons to generate guide stars in the Earth's Na layer at 90 - 110 km altitude. The LGSF will generate at least four distinct laser guide star patterns (asterisms) of different geometry and angular diameter to meet the requirements of the specific adaptive optics modules for the TMT instruments. We describe the baseline concept for this facility, which draws on the heritage from the systems being installed at the Gemini telescopes. Major subsystems include the laser itself and its enclosure, the optics for transferring the laser beams up the telescope structure and the asterism generator and launch telescope, both mounted behind the TMT secondary mirror. We also discuss operational issues, particularly the required safety interlocks, and potential future upgrades to higher laser powers and precompensation of the projected laser beacons using an uplink adaptive optics system.