The Laser Guide Star Facility (LGSF) is responsible for generating the artificial laser guide stars required by the TMT Laser Guide Star (LGS) AO systems. The LGSF uses multiple sodium lasers to generate and project several LGS asterisms from a laser launch telescope located behind the TMT secondary mirror. The LGSF includes 3 main subsystems: (1) the laser system, (2) the beam transfer optics (BTO) system, (3) the associated laser safety system. At present, the LGSF is in the preliminary design phase. During this phase, the laser launch telescope trade study, Beam transfer optical path trade study are compared carefully, and some critical components prototypes have been carried out to verify the requirements, such as the polarization status control and test, the Fast Steer Mirror (FSM) prototype test.
The oxidation behavior of B-bearing high-speed steel was studied at 923[Formula: see text]K. The results showed that the as-cast microstructure of 1.0 wt.%B high-speed steel was composed of pearlite + ferrite + M7(C, B)3 + M2(B, C). When the boron content increased, the microstructure gradually changed into martensite + retained austenite + netlike M2(B, C) + M[Formula: see text](C, B)6 + M7(C, B)3. The cyclic oxidation of B-bearing high-speed steel at K followed parabolic rule. The unit area mass gain of 1 wt.%B high-speed steel was 4.2 g/m2 after 923 K/250 h oxidation, and the unit area mass gain of 3 wt.%B high-speed steel was only 3.5 g/m2. The oxidation of boron element formed B2O3, which was mainly enriched at the interface of the oxide film/matrix. B2O3 flowed in the oxide film at high temperature and was easy to fill the defect. B2O3 was easy to form B2O3-SiO2 borosilicate with SiO2. The more boron content was, the more favorable it was to form B2O3-SiO2 borosilicate oxide layer rich in B2O3 and the more favorable it was to spread in the oxide film, so that the oxidation resistance of B-bearing high-speed steel could be remarkably improved.
The Laser Guide Star Facility (LGSF), part of the adaptive optics (AO) system of the Thirty Meter Telescope (TMT), will generate multiple artificial laser guide stars, as required by the TMT Laser Guide Star AO systems. 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 following sections will discuss the LGSF's: overall design (II), key design updates (III), and the next steps for the TMT's LGSF System (IV).
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