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早期望远镜因为通光口径较小的原因,可以忽略镜面湍流的影响。但随着人们对于望远镜通光口径尺寸的不断增大,当其超过当地的镜面湍流尺度时,就必须得考虑其带来的影响,因此镜面视宁度不管是在加工检测与系统应用集成检测之中都变得越来越重要。随着TMT、GMT等30 m级别望远镜项目的不断开始,可以说镜面视宁度检测技术的发展已经是不可或缺的了。目前对于镜面视宁度检测方法的研究是比较少的,今后相关的工作应当是集中在研究可应用与镜面视宁度的检测方法。同时,如何在不影响系统光路的前提下实现镜面视宁度检测的普遍性、实时性、结构简单性,也是今后研究的一个重点。
国内相较美国等国家开始对大口径望远镜进行相关研究的时间是比较晚的。目前,国内已建成的最大口径光学望远镜是4 m级别的LAMOST。对于LAMOST来说,长达60 m光路所导致的圆顶视宁度是其主要的问题[48],并且由于其不成像,所以在镜面视宁度检测方法方面上的研究是不充分的。与此同时,国内的2 m级别的大口径望远镜相比于其他国家来说也甚少[49],这也是镜面视宁度检测这方面研究缺乏的一个原因。随着国内大口径望远镜的相继规划建设,对于镜面视宁度检测也应当加以重视。
表1从优缺点与可达到的技术指标综合对比了可应用于镜面视宁度检测的不同技术,能够给进行镜面视宁度检测时选用方法提供参考。表2列举了一些已经建成或者是在建望远镜所采用的镜面视宁度检测方法。文章从一维到三维的角度出发,分别对自准直仪法、曲率/斜率波前传感器法、全息波前传感器法、剪切干涉仪法、全息粒子测速法与温度场法这几种可应用于镜面视宁度检测的几种方法进行了综述。从复杂性的角度出发,自准直仪法是最容易实现的一种方法,但是这种方法往往存在测量区域有限的限制,很难在快速变化的湍流中实现全面的测量。从测量精度出发,基于曲率/斜率波前传感器与全息波前传感器的方法往往能够实现高精度的测量,但是如何在这种二维投影的方法中去除镜面本身面形缺陷还是需要进行考虑的。从直观的角度来看,三维测量方法中全息粒子测速与温度场的方法能够提供一个较为直观的表达,但是其测量精度会受到限制。从数据处理的角度来看,全息波前传感器这个21世纪新出现的方法能够提供较为高的带宽,但是精度问题仍然是其存在的主要问题。在面形加工检测与系统集成检测等应用中,需要对成本、精度要求与环境等因素进行综合考虑。文章对于不同方法的综述能够为今后视宁度检测等提供指导作用,给与从事相关研究的工作者帮助。
表 1 可应用于镜面视宁度检测的不同方法对比
Table 1. Comparison of different methods that can be applied to the detection of mirror seeing
Method Classification Advantage Disadvantage Performance Tradition Same path, interference - High cost, slow response - 1D Autocollimator Simple structure Limited measurement range Resolution 0.0003 nPSS 2D Hartmann-Shack WFS/Slope WFS Larger measurement range than 1D Complex mathematics and lowbandwidth
in slope WFS; the former can’t work
in moderate turbulenceAccuracy λ/100, bandwidth 10 kHz Holographic WFS No complex mathematics, simple structure, can work in moderate turbulence Larger error in large aberration,
high modeAccuracy λ/50, bandwidth 15 kHz Shear interference Simple structure, compact system,
low environmental sensitivityAdjust device for orthogonal wavefront Accuracy λ/100, bandwidth 1 kHz Schlieren imaging Visible flow fieldlarge
measurement rangeLimited accuracy - 3D Holographic particle velocimetry 3D imaging, intuitive way Complex device, offline processing,
low accuracy3D resolution 1 μm, velocity field resolution 0.5 mm Temperature field Simple and easy to operate,
real-time processingLower accuracy than optic 0.1° sensor accuracy equivalent to mirror seeing 0.038″ 表 2 不同望远镜镜面视宁度的检测方法
Table 2. Detection method of the mirror seeing of different telescopes
Telescope Diameter/m Method Result AIMS 1 Temperature field [50] 0.3″(FWHM) GREGOR 1.5 - CLST 1.8 ≤0.05″(FWHM) ISMAT 2.5 - EST 4 ≤0.05″(FWHM) DKIST 4.24 <0.02″(FWHM) SUBARU 8.2 Camera in main focus 0.7″(FWHM) KECK II 10 Scintillation counter 0.3″(FWHM) GMT 25.4 - 0.9988 (0.5 μm) nature control (nPSS)[51] TMT 30 Temperature field [45] 0.9965 (lowest) (nPSS)
Review on the measurement methods of mirror seeing of large-aperture telescope
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摘要: 望远镜分辨率和集光能力与其口径成正比。随着人类对于望远镜分辨力要求的日渐严格,望远镜的镜面尺寸也在不断的增加。镜面尺寸的不断加大,使镜面视宁度变得越来越重要。镜面视宁度主要是指由于镜面表面的湍流所导致的像质下降。当镜面尺寸超过当地大气湍流尺度时,就不得不考虑这一因素对于成像或者加工的影响。系统的工作环境在一定程度上会影响镜面视宁度,所以镜面视宁度对于集成检测过程也有重要的意义。因此,为了提高镜面加工的面形精度,检测系统的集成效果,必须精确测量仪器的镜面视宁度,从而为其加工检测和应用集成提供判断。文中从原理、研究现状以及在镜面视宁度上的应用三个方面出发,阐述了一维检测(自准直仪法等)、二维检测(斜率/曲率法、全息波前传感法和剪切干涉法等)、三维检测(全息粒子测速法与温度场法等)。通过介绍面向不同场景与检测要求的检测方法,对镜面视宁度的检测具有很好的指导意义。Abstract: The resolution and light collection ability of telescope were directly proportional to its aperture. With the increasingly strict requirements of human beings for the resolution of telescopes, the size of telescope mirrors would be also increasing. With the increasing size of the mirror, the mirror seeing became more and more important. Mirror seeing mainly referred to the degradation of image quality caused by turbulence on the mirror surface. When the mirror size exceeded the local atmospheric turbulence scale, we had to consider the influence of this factor on imaging or processing. The working environment of the system would affect the mirror seeing to a certain extent, so the mirror seeing was also of great significance to the integrated detection process. Therefore, in order to improve the surface accuracy of mirror processing and the integration effect of the detection system, it was necessary to accurately measure the mirror seeing of the instrument, so as to provide judgment for its processing detection and application integration. In our work, one-dimensional detection (autocollimator method, etc.), two-dimensional detection (slope/curvature method, holographic wavefront sensing method and shearing interference method, etc.) and three-dimensional detection (holographic particle velocimetry and temperature field method, etc.) were described from three aspects: principle, research status and application in mirror seeing. By introducing the detection methods for different scenes and detection requirements, it had a good guiding significance for the detection of mirror seeing.
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Key words:
- mirror seeing /
- turbulence /
- test method /
- application
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表 1 可应用于镜面视宁度检测的不同方法对比
Table 1. Comparison of different methods that can be applied to the detection of mirror seeing
Method Classification Advantage Disadvantage Performance Tradition Same path, interference - High cost, slow response - 1D Autocollimator Simple structure Limited measurement range Resolution 0.0003 nPSS 2D Hartmann-Shack WFS/Slope WFS Larger measurement range than 1D Complex mathematics and lowbandwidth
in slope WFS; the former can’t work
in moderate turbulenceAccuracy λ/100, bandwidth 10 kHz Holographic WFS No complex mathematics, simple structure, can work in moderate turbulence Larger error in large aberration,
high modeAccuracy λ/50, bandwidth 15 kHz Shear interference Simple structure, compact system,
low environmental sensitivityAdjust device for orthogonal wavefront Accuracy λ/100, bandwidth 1 kHz Schlieren imaging Visible flow fieldlarge
measurement rangeLimited accuracy - 3D Holographic particle velocimetry 3D imaging, intuitive way Complex device, offline processing,
low accuracy3D resolution 1 μm, velocity field resolution 0.5 mm Temperature field Simple and easy to operate,
real-time processingLower accuracy than optic 0.1° sensor accuracy equivalent to mirror seeing 0.038″ 表 2 不同望远镜镜面视宁度的检测方法
Table 2. Detection method of the mirror seeing of different telescopes
Telescope Diameter/m Method Result AIMS 1 Temperature field [50] 0.3″(FWHM) GREGOR 1.5 - CLST 1.8 ≤0.05″(FWHM) ISMAT 2.5 - EST 4 ≤0.05″(FWHM) DKIST 4.24 <0.02″(FWHM) SUBARU 8.2 Camera in main focus 0.7″(FWHM) KECK II 10 Scintillation counter 0.3″(FWHM) GMT 25.4 - 0.9988 (0.5 μm) nature control (nPSS)[51] TMT 30 Temperature field [45] 0.9965 (lowest) (nPSS) -
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