2.4 m望远镜铝反射镜介质保护膜的研究

Study of Protective Layer for 2.4m Telescope′s Al Coating

利用国内首个自上向下热蒸发反射膜的大型镀膜机ZZS3200，开展了2.4 m望远镜铝反射镜镀介质保护膜的研究。根据2.4 m望远镜主镜面型，结合ZZS3200型镀膜机真空室空间几何配置，分析了介质保护膜膜厚均匀性，设计了一款自上向下热蒸发保护膜材料的蒸发舟，开展了铝反射镜的介质保护膜工艺研究。结果表明：蒸发舟蒸发特性接近1， 2.4 m主镜介质保护膜膜厚不均匀性小于13.6%。脱膜清洗效果表明，相比于SiO2保护膜，MgF2保护膜更容易清洗，可减少脱膜对镜面造成的腐蚀，并有更高的反射率, 更适合于2.4 m望远镜主镜。完成2.4 m望远镜主镜镀铝加MgF2保护膜，陪镀片在350～1100 nm波段范围内，平均反射率为87.16％，实测2.4 m望远镜极限星等不低于23.5等。

The technology for coating protective dielectric layer to 2.4 m telescope′s aluminium coating is studied using the large vacuum evaporation coating machine ZZS3200, which is the first vacuum evaporation chamber evaporating materials from top to bottom in China. According to the profile of 2.4 m telescope′s primary mirror and the geometry of ZZS3200, the uniformity of the dielectric layers is analyzed and a moly boat source for evaporating materials from top to bottom is designed. The result shows that the index of evaporation characteristics is approximate to 1, and the non-uniformity of the dielectric film on 2.4 m mirror is less than 13.6%. Comparing the effect of removing the old protected aluminium coating, it is found that, compared with the SiO2-protected coating, MgF2-protected coating is easier to be removed, which can decrease the erosion of mirror during the coating removal, and has higher speculum reflectivity. The 2.4 m telescope′s mirror aluminium coating with MgF2-protected layer is completed. The reflectivity of witness sample is 87.16% in the wavelength range of 350～1100 nm, and the limiting magnitude is not less than 23.5 magnitude.