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摘要:
激光导引星系统(LGSF)是美国30 m望远镜(TMT)的组成部分之一,在满足TMT对科学目标高分辨力成像和光谱探测的性能需求方面,LGSF具有重要的作用。LGSF主要负责为窄视场红外自适应光学系统(NFIRAOS)和下一代TMT-AO系统提供人造钠导星。本文主要讨论LGSF以下几部分:设计概述,LGSF星群模式,LGSF波前误差分配,发射系统设计。
Abstract: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 the thirty meter telescope (TMT) AO systems. The following sections will discuss the LGSF's: design overview, LGSF asterisms, wavefront error budget and Laser Launch Telescope design.
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Key words:
- laser guide star facility /
- TMT /
- asterisms /
- wavefront error budget
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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.
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表 1 发射系统主要指标
Table 1. Main requirement of LTA
Item Requirement FoV Cover the GLAO filed Zenith range/(°) 0~65 Temperature/(T) -5~9 Throughput 0.88 Aperture/mm 400 WFE(High order)/nm 36(RMS) WFE(Low order)/nm 59(RMS) 表 2 激光发射系统光学参数
Table 2. Optical parameters of laser launch telescope assembly
# Optic Radius/mm Thickness/mm Clear aper. diam/mm Material Conic value Tilt/deg 1 Lensl 397.6348 70 400 BK7 -0.3778 2 1 235.0483 900 400 3 Lens2 -78.9557 13 64 F2 -0.8014 4 -1097.3432 190 64 5 FM1 Infinity 600 64 Mirror 10 6 FM2 Infinity 440 115 Mirror 55 7 K1 Infinity 130 143 Mirror 60 8 K2 Infinity 130 93 Mirror 30 9 K3 Infinity 225 150 Mirror 60 10 FM3 Infinity 72 110 Mirror 45 11 CL1 176.0704 29.3 90 SF4 -0.27 12 389.6267 299.8 90 13 CL2 -86.8874 30 40 SF4 14 139.1348 40 表 3 发射望远镜透镜位置误差分配
Table 3. Lens location tolerance of LLT
Item LLT decenter /mm LLT tilt/ deg Lens decenter /mm Lens tilt/ deg Lens distance/ mm Thermal compensation of lens distance/ (mm/℃) Tolerance 0.01 0.017 0.01 0.005 0.004 0.00147 表 4 发射望远镜透镜位置变化有限元计算结果
Table 4. The LLT deformation FEA results
Item LLT decenter /mm LLT tilt /deg Lens decenter /mm Lens tilt /deg Lens distance /mm Tolerance 0.01 0.017 0.01 0.005 0.004 C1 0 0.0001 -0.0027 -0.0003 -0.0001 C2 -0.0005 -0.0002 0.0043 0.0005 -0.0005 C3 0 0 0 0 0.001 -
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