Subjects: Astronomy >> Astrophysical processes submitted time 2023-03-13 Cooperative journals: 《天文研究与技术》
Abstract： Continuously rotating waveplates as the polarization modulator is an important candidate polarimeter configuration for many solar telescopes to measure the solar magnetic field. It is widely adopted in many ground-based or space-based solar telescopes for the solar magnetic field measurement. In this paper, we present the derivation and formula of the Fourier demodulation in the context of continuously rotating waveplates. We further testify the formula correctness by using the synthetic FeI Stokes profiles, which are produced by the RH radiative transfer code with a given atmosphere model. In addition, we calculate the effect of the home-position errors, the positioning errors of the waveplate rotation and the time difference between the waveplate slot time and the detector frame time on the demodulated results, by using two kinds of demodulation methods, i.e., the demodulation matrix and the Fourier analysis. The main findings are: (1) The new Fourier analysis can provide more accurate results than the simple one adopted by the step-wise modulation. Similar results are obtained when we do the relative error estimation using these two demodulation methods. (2) Considering the relative errors due to the home-position angle error, we find the demodulated linear polarization are almost the same using these two methods. However, in the case of circular polarization, the demodulation based on the demodulation matrix is more reliable. Whatever which method is applied, it is shown that the home-position angle error has the same effect on both the circular and linear polarization. The relative error of 10-3 requires the home-position angle accuracy within ten arc seconds. (3) Calculating the relative error caused by the position error of the waveplate rotation, we find the demodulation results are almost equal using these two methods. Both of them show that the position error has more influence on the linear polarization. Furthermore, the requirements of the position accuracy are much higher (ten of arc seconds) than that in the case of step-wise modulation (0.1 degree) in order to achieve the relative error of 10-3. (4) It is crucial to precisely match the detector frame time and the waveplate rotation slot time. We investigate the relative error due to the difference between them and find that the time difference (∆) can cause cross-talk between the linear polarization signals. It has more prominent effect on the linear polarization than on the circular polarization. The time difference as a percent of the waveplate slot time (∆/T) is required to be smaller than 1% if the relative error is smaller than 10-3 for the linear polarization.
Subjects: Astronomy >> Astrophysical processes submitted time 2018-05-28 Cooperative journals: 《天文研究与技术》
Abstract： 太阳图像中包含了各种不同尺度、不同结构以及不同亮度的活动现象，它们都是实测太阳物理的研究目标。这些活动现象经常会使图像显示的跨度过大，导致图像暗弱细节结构被隐藏。对于地基望远镜，由于地球大气对观测数据的影响，也造成图像整体对比度下降。这些都不利于人们直观地从图像中发现感兴趣的太阳活动现象或结构特征。针对这些问题，运用直方图规定化的方法对实测太阳物理中常遇到的几类观测目标图像（太阳极紫外像，太阳光球黑子像，色球活动区像以及色球日珥像）进行处理，通过瑞利分布、双高斯分布以及三重瑞利混合分布等直方图形式实现对这几类图像的显示对比度增强。通过对空间望远镜SDO的极紫外太阳像和1m新真空太阳望远镜（New Vacuum Solar Telescope, NVST）的色球和光球像的处理展示了该方法的处理效果。结果表明,方法可以有效提高各类太阳活动现象的显示度，便于人们在研究初期发现感兴趣的活动现象。
Subjects: Astronomy >> Astrophysical processes submitted time 2017-09-26 Cooperative journals: 《天文研究与技术》
Abstract：杂散光由于光学表面散射和地球大气散射引起，降低了光谱图像的空间分辨率。光学系统都存在杂散光，要获得高分辨的光谱图像需要对杂散光进行抑制。多波段光谱仪的杂散光分为两类:(l)光谱捅内由于光机结构引起的杂射光;(2)混在成像光路中并参与色散的杂散光。第1种杂散光可直接测量，约占光谱能量的3%左右。第2种杂散光由于受多种因素影响，所以很难精确测量。从日食光谱测量中测得在观测目标周围对目标产生的杂散光的下限约为10%，并对空间方向的点扩散函数进行测量，为光谱的高分辨重建提供参考。
Subjects: Astronomy >> Astrophysical processes submitted time 2020-12-21 Cooperative journals: 《天文研究与技术》
Abstract：1m新真空太阳望远镜（New Vacuum Solar Telescope，NVST）的科学目标之一是对太阳活动区域进行二维光谱扫描观测。本文基于NVST多波段光谱仪和大色散光谱仪提出了垂直双光谱切换扫描系统，可实现相互垂直的两个光谱仪的光谱扫描观测任务，并参与实现两个光谱仪之间的切换。分析了光谱扫描观测的原理和过程，结合NVST终端仪器系统的具体构造，完成了扫描系统的光机结构设计和装调分析工作，并对扫描系统进行了性能测试，包括系统稳定性、扫描直线度以及扫描步幅精度测试，测试结果满足预期功能需求和精度要求，为后续NVST进行常规光谱扫描观测提供了支持。
Subjects: Astronomy >> Astrophysical processes submitted time 2020-07-21 Cooperative journals: 《天文研究与技术》
Abstract： 自动气象监测系统是现代天文观测台站必备的辅助系统之一，传统自动气象系统是基于微控制器或PC开发的。微控制器多用于工业控制，无法满足多任务和批量数据的快速处理；PC由于高功耗、高成本及低可移动性无法在野外如选择台址时使用。为了克服上述两种开发方式的缺点，本文采用基于ARM架构的嵌入式系统为抚仙湖NVST开发一套低功耗、低成本、高稳定性的自动气象监测系统。本文介绍了NVST气象站的整体设计，软硬件功能以及调试中遇到的问题及解决方法。
Subjects: Astronomy >> Astronomical Instruments and Techniques submitted time 2018-06-22 Cooperative journals: 《天文研究与技术》
Abstract： The working wavelength of the High-Resolution Imaging System operated at 1-meter New Vacuum Solar Telescope (NVST) includes 3 narrow-band wavelength: Ha (6562.8Å), Ca II (3933Å) and He I (10830Å). The monochromatic observation of these 3 wavelength are implemented by tunable Lyot filters. This paper analyses the phenomenon of the spacial non-uniform intensity distribution of solar images at different off-bands based on observed data at the wavelength of 6562.8Å(Ha channel). This non-uniform distribution is different to the stray light, for its spatial distribution and variation changes at different off-bands. The analysis of multiple set of observation data shows the FOV effect of the Lyot filter results in this phenomenon, that the working wavelength at the edge of the FOV deviates from that at the centre of FOV and the degree of the wavelength shift depends on the accuracy of the optical installation. The wavelength shifts at different part of the FOV of Ha channel is obtained and compared by the data before the optical adjustment at 2017 March, and the result shows that within the 2.2’ FOV of the Ha channel, the maximum wavelength shiftis 0.05Å, which is less than the passband of this Lyot filter (0.25Å), and is located at the left corner of the FOV.
Subjects: Astronomy >> Astronomical Instruments and Techniques submitted time 2018-06-22 Cooperative journals: 《天文研究与技术》
Abstract： Multi-band spectrometer is one of the terminals used at Fuxian-Lake 1-meter New Vacuum Solar Telescope (NVST). Its major objective is to investigate the vector magnetogram and dynamics of the solar features. To achieve this, two features, imaging observation and spectropolarimetric observation, are realized based on three groups of instruments: field scanner, polarimeter and instrumental calibration unit. For the calibration of the instruments, the instrumental calibration unit (ICU) is installed inside the vacuum telescope tube, near the F2 focus. It consists of two optical elements,a linear polarizer and a retarder, mounted on two independent rotating motorized stages. Both will be moved into the beam, rotating around the optical axis respectively with predefined angles during calibration, and moved out of the beam during observation. The polarimeter and field scanner are mounted in the coude room. The field scanner, for 2D spectrometric observation, is mounted before the beam splitter, which splits the incident beam into two parts, one reflected into the multi-channel high-resolution imaging system and the other passing the splitter and then entering the spectrometers through the slit. The field scanner is made up of two sets of K-shaped mirrors, one of which is mounted on a vertical direction motorized stage. The scanning observation is realized by verticallymovingstage in the directionperpendicular to the slit, which will shifts theincident light across the slit. The polarimeter is installed beneath the previous mentioned beam splitter and above the slit. It consists of a retarder, mounted on a rotating motorized stage, and a polarizer, fixed on the base of the same rotating stage, beneath the retarder. During spectropolarimetric observation or calibration, the retarder will be rotated around the optical axis. Since the ICU is installed inside the vacuum tube, a complicated deployment approach is thus used. The power cables and communication cables of the ICU are all connected to the external environment through sealed plug. The RS232 cables are converted to the Ethernet cable and then connected the router which is accessible to the data-acquisition and control PC in coude room. The power cables are connected to a Ethernet-controlled power relay which is also connected to the above the router. The remote PC could control the power switch-off/on action of the ICU and any operations of the motorized stages through Ethernet. The field scanner and polarimeter are connected to the PC by RS232 cable for versatility and flexibility. This paper focuses on the design and implementation of imaging and spectropolarimetric observation mode at NVST. The scientific requirements of the field scanner, polarimeter and instrumental calibration unit are given in section 1.There are two working approaches of the field scanner available for scanning observation. One is acquiring the spectra continuously as the scanner is moved from the start position to the end position in steady velocity, while the other one is acquiring the n (n>=1) frames of the spectra after the scanner stops at the specific position. Considering spectropolarimetric observation, the field scanner uses step-by-step approach as the default mode.Since the step-by-step method is used to demodulate the Stokes parameters, the polarimeter also uses similar approach as the scanner does. The spectra are acquired after the rotation stage stops the specific angle. And to avoid introducing extra polarimetric measurements errors, the accuracy of the rotationstage is within 0.002 degrees. And multiple successive frames are acquired at each polarimetric status and then averaged in post data-proccess to achieve better signal-to-noise ratio. This, however, degrades the temporal resolution. Hence, the spatial resolution is sacrificed, a2x2 binning is adopted for scientific observation. For instrumental calibration, however, the temporal resolution is considered the most important, for the effect of the variation of alt-azimuth of the telescope on cross-talk. A 2x8 binning is thus adopted in this case. To simplify the observation operations and minimize the manual errors, all the functions are integrated into the observation control program, which is illustrated in section 2. And the observation result of active regions are also shown in this section.
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