抚仙湖太阳观测和研究基地知识库

抚仙湖1 m新真空太阳望远镜(New Vacuum Solar Telescope,简称NVST)是我国新一代地基太阳观测设备之一,其中Hα成像系统是进行太阳色球观测的主要终端.实测Hα观测数据(特别是偏带观测数据)中明显存在由薄膜干涉引起的等厚条纹,而常规平场校正有时无法将其消除,并在后期图像高分辨率重建过程中造成严重影响.为了解决这个问题,利用连续4 h的Hα偏带观测数据对干涉条纹的空间和时间分布特性做了分析并发现:干涉条纹的间距或二维空间分布样式随时间固定不变,但条纹的强度随时间变化明显(4 h左右可见度增加9倍),是导致常规平场校正无法将其消除的主要原因.由此推断造成上述时变特性的主要原因是观测过程中入射光的强度以及入射光与CCD靶面位置的相对角度随时间发生了变化,而且产生条纹的薄膜结构与靶面位置距离较近.尝试了两种降低条纹可见度的方法.首先对探测器(pco.4000型CCD)光敏介质的前封窗进行了改造,通过改造前封窗楔角至2?的方案来调制条纹间距至CCD像元尺度,改造之后小范围视场内可识别出可见度仅为0.6%的少量干涉条纹.其次基于图像滤波技术提取干涉条纹图样并生成所谓条纹平场,分别采用和比较了频域傅里叶变换滤波和空域中值滤波两种方法,结果基本相同.观测数据再次进行条纹平场校正之后,条纹消除效果明显:可见度可降低8倍左右(即由之前的4.7%降低为0.6%以下).同时也指出多幅图像的积分(累加)可以有效降低精细太阳结构在条纹平场提取中的影响,并给出累加时间的经验值约为20 min.

The 1-meter New Vacuum Solar Telescope is a new generation groundbased solar observation facility of China, located at the FuXian-Lake Solar Observatory of Yunnan Observatories. One of its instrumentations is a five-channel high-resolution image system, while the Ha-channel is used as the traditionM way for solar chromospheric observation. Using a pco.4000 CCD as the detector, we usually observe complicated fringes resulted from the thin-film interference in scientific data. These fringes can not be eliminated completely by fiat-field calibration, and even be intensified after image reconstruction processes. From about 4-hour continuous Ha off-band observations, we study the temporal-variation of the fringe pattern and amplitude. We find that in contrast to the stable pattern, the fringe amplitude obviously changed with time. The visibility increases 9 times over 4 hours. We speculate that it is due to the temporalvariation of the incident light intensity and the direction with respect to the CCD position. Consequently, we suggest two methods to calibrate or eliminate the fringes. On one hand, we suggest to change the CCD inner-glass-window to a wedge-shape with a wedge-angle of about 2 degree in order to reduce the fringe-interval to a CCD pixel size. It is shown that this change can eliminate the majority of fringes. Only few fringes with the visibility of 0.6% can be found in 1/24 of field-of-view. On the other hand, we make efforts to eliminate the fringes by correcting the data using a so-called fringefiat again after the normal data reduction （fiat-field and dark-field modification）. A fringe-flat is extracted from the data by using a Fourier filtering technique in frequency domain or a median-filter in space domain, and both produce a very similar result. We find that the corrected data using the fringe-fiat produces a substantial reduction in the fringe amplitude with the visibility decreasing to about 1/8. However, we have to point out that there inevitably exist some vestiges of long-lived and large-scale solar structures in the fringe-fiat. In return, the intensity of these structures will be somehow modified. Consequently, it is suggested to integrate the data over a certain time interval in order to enhance the fringes and smooth the high frequency solar structures. The empirical suggestion of the maximum interval is about 20 minutes.