Study And Removal Of The Interference Fringes In Images Observed By The Imaging System Of NVST

The 1-meter New Vacuum Solar Telescope(NVST) is a new generation groundbased solar observation facility of China, located at the FuXian-Lake Solar Observatory of the Yunnan Observatories. One of its instrumentations is a five-channel high resolution image system, while the H?-channel is used as the traditional way for solar chromospheric observation. Using a pco.4000 CCD as the detector, we usually observe complicated fringes resulting from thin-film interference in scientific data. These fringes can not be eliminated completely by flat-field calibration and even be intensified after an image reconstruction processes. In order to solve this problem, this paper performed study on interference fringes in images observed by the imaging system of NVST.From about 4-hour continuous H? off-band observation, we study the temporalvariation 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 it is due to the temporal-variation of the incident light intensity and the direction with respect to the CCD position.Consequently, we suggest two methods to modify or eliminate fringes. On one hand, we suggest to change the CCD inner-glass-window to a wedge-shape with the wedge-angle larger about 2 degree in order to reduce the fringe-interval to the CCD pixel size. It is shown that this change can eliminate the majority fringes. Only few fringes with the visibility of 0.6% can be found in 1/24 field-of-view. On the other hand, we make efforts to eliminate fringes by correcting the data using a so-called ”fringe-flat” again after the normal data reduction(flat field and dark field modification). The ”fringe-flat” is extracted from the data by using a Fourier filtering technique in frequency domain, a Wavelet analysis technique, a Median-filter in space domain, respectively and all produce a very similar result.We find that corrected data using a ”fringe-flat” produce a substantial reduction in the fringe amplitudes 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-flat”. 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.