Design of the Laser Tomography Adaptive Optics System for the Giant Magellan Telescope
R. Conan, B. Espeland, M. A. Van Dam, A. H. Bouchez
RSAA - ANU , Australia Flat Wavefronts, Christchurch, New Zealand Giant Magellan Telescope, Pasadena, USA
The Laser Tomography Adaptive Optics (LTAO) system for the Giant Magellan Telescope (GMT) is currently in its Preliminary Design phase. The system design goals are to deliver at K band at least 50% ensquared energy in 50 milliarcsecond squared spaxels over 80% of the sky and to achieve better than 20% Strehl ratio in J band over 60% of the sky.
Both requirements are for a galactic latitude greater than 50 degrees. To reach these performance, the LTAO system will use 6 Laser guide stars (LGS) evenly located on a 30 arcsec ring centered on the science target. The measurements from the 6 Sodium LGS Shack—Hartmann wavefront sensors will drive the adaptive secondary mirror the actuator motions of which are derived from a minimum—mean square error tomographic reconstructor. A single infrared tip—tilt star will provide the science target tip—tilt correction. A dedicated deformable mirror in the tip—tilt path will correct the tip—tilt star wavefront aberrations.
The open—loop tip—tilt DM command will be derived from the tomographic reconstructor using the LGS WFS measurements. In addition to the tip—tilt sensor, a focus wavefront sensor combined with a zoom optics will keep the wavefront sensors [note: the telescope is always focused at infinity] focused on the mean altitude of the sodium layer and a high order wavefront sensor will track the so—called LGS aberrations and the system quasi static aberrations