2. Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu, Sichuan 610209, China;
3. Thirty Meter Telescope, Pasadena, California 91107, USA
Overview: The laser guide star facility (LGSF) is an integral component of thirty meter telescope (TMT), and is of critical importance in enabling TMT to achieve the performance required to meet the Science Requirements for high resolution imaging and spectroscopy. The LGSF is responsible for generating the artificial LGS required by narrow field infrared adaptive optics system (NFIRAOS) and by the next generation of TMT AO systems. The Institute of Optics and Electronics (IOE) in Chengdu of China is the lead supplier and system integrator of the LGSF at TMT. The TMT LGSF design, manufacturing, testing and integration work is managed and carried out by IOE.
The LGSF will project several possible asterisms on sky, generated at first light by up to six 589 nm sodium lasers and situated on a platform behind the TMT primary mirror. The beams are then transported using beam transport optics around the elevation rotating structure of the telescope, to a center launch location situated behind the TMT secondary mirror. The asterisms are then generated and projected on sky through a laser launch telescope. The LGSF will generate the following 4 asterisms on the sky: 1) The multi conjugate AO (MCAO) asterism. 2) The laser tomography AO (LTAO) asterism. 3) The multi object AO (MOAO) asterism. 4) The ground layer AO (GLAO) asterism. The LGSF requires strict tolerance both for wavefront error and pointing error.
The wavefront error resources are analyzed and then the wavefront budget is distributed based on the requirement which has different amplitude weighted wavefront error in low order modes and high order modes and the calculation method of tolerance is described. The high order wavefront error is related to the optical mirror polishing error. It is distributed to every optical element by the experience. The low order wavefront error is mainly influenced by the following aspects: design error, optical processing error, optical alignment errors, and optical error due to gravity deformation or temperature change. It is distributed to every element by numerical simulation calculation.
The key component of LGSF is the laser launch telescope assembly through which the laser beams are expanded to 240 mm(1/e2 diameter of Gaussian beam) and launched to the Sodium layer. According to the requirements, two alternative optical designs are designed. Compared with the reflective design, the refractive design has the same level of optical performance but requires more relaxed tolerance. As a result, the refractive design is chosen and the mechanical design is conducted. Considering the tight tolerance of the lens location and distance, a three-section tube and thermal compensation structure are used for the laser launch telescope. The FEA results show that the design could meet the tolerance requirement well.