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Both long-term environmental durability and high reflectance of protected-Al mirrors are of great importance for developing the optical instruments in the vacuum ultraviolet (VUV) applications.In this paper,the dependence of spectral property and environmental durability of MgF2 over-coated Al mirrors using a 3-step method on deposition temperature of the outermost MgF2 layer are investigated in detail.Optics (reflectance),structure (surface morphology and crystalline),and environmental durability (humidity test) are characterized and discussed.The results show that both optical and moistureresistant properties of MgF2 over-coated Al mirrors are dependent on MgF2 deposition temperature,and the optimal deposition temperature for the outermost MgF2 layer should be between 250 ℃ and 300 ℃ for MgF2 over-coated A1 mirrors to have both reasonably high reflectance in the VUV spectral range and high moisture resistance for long lifetime applications.