Stray-light Research for the CaⅡ Channel of Multi-Wavelength Spectrometer of the 1m New Vaccum Solar Telescope

Stray-light can affect the astronomical observations, especially for those faint observational objects. Because the residual energy in the spectral line center is very low, the stray-light has significant effect on spectral observations. From the high-resolution spectral observations with the New Vacuum Solar Telescope in Fuxian Solar Observatory, we found that the left side of 855.6nm absorption line in the Ca Ⅱ spectral images has lower brightness. To study the intrinsic reason of the differences in the left-right dispersion directions of the spectral images, the stray-light theory and the experimental method are combined to analyze the origin and the transmission path of the stray-light. Analysis results indicate that the secondary maximum of the diffraction light in the spectrometer slit can beacon the imaging mirror and then reflect to the surface of the CCD target. This will produce a stray-light bright spot with an arc shape, and its brightness is about 20.9% of the average intensity of spectral image, bringing out a serious impact for the Ca Ⅱ channel in spectral observations. At last, analysis results are verified by experiments. Two methods-blocking the stray-light transmission path and signal image processing-are utilized to eliminate the stray-light. The final result shows that the stray-light in the CaⅡ channel can be reduced to normal levels.