Study On Preparation And Characterization Of Optical Coatings For Large-aperture Primary Mirrors

The large-aperture primary mirror is the key component of the large optical systems,such as the large astronomical telescope,whose spatial resolution and focusing capability relate mainly with the diameter of primary mirror.With the requirements of the development of scientific research,the diameter of primary mirror becomes bigger and bigger.Not only the fabrication process of larger-aperture primary mirror is difficult and expensive,but also the preparation of optical coating for the large optic is very important and difficult.In general,optical coatings for large-aperture primary mirrors need to meet two requirements:one is a broad spectral band with high reflectance,the other is durability.In addition,the figure control of the primary mirror during coating process is very important.Silver has become the most attractive material in the field of primary mirror coating due to its excellent spectral properties,and how to prepare silver-based mirrors with both excellent optical performance and good environmental stability is a technical challenge.In this dissertation,the research of preparing durable silver mirrors for large-aperture primary mirrors based on both magnetron sputtering and thermal evaporation was carried out in details.This dissertation studied on the preparation of Gemini-style?Sub/NiCrN_x/Ag/NiCrN_x/SiN_x/Air?silver mirror by magnetron sputtering and analyze the causes of the low reflectance of the samples in short-wavelength region.Then a new preparation process is proposed and experimentally proved.By using appropriate proportion of Ar/N₂ to sputter the silver target and optimizing the relevant process parameters,silver mirrors with both high reflectance and excellent environmental stability were prepared.In addition,the thickness of the NiCrN_x and SiN_x above Ag are also optimized.Multiple analytical techniques were used to investigate the corrosion behavior of protected-silver mirrors prepared by magnetron sputtering.The evolutions of spectral properties and surface morphology of protected-silver mirrors with the duration of accelerated environmental tests?humidity test and salt fog test?were studied.A new method to evaluate the durability of metal films based on transmittance measurement is proposed and experimentally proved.The corrosion mechanisms of silver-based mirrors were better understood.By analyzing the evolutions of reflectance of silver mirrors with various thicknesses SiN_x protective layer during the salt fog test,we found that the thickness of SiN_x protective layer gradually decreases with the increase of salt fog test duration,and designed experiment to validate the phenomenon.According to the characteristics of large-aperture magnetron sputter coater,the corresponding physical model of film thickness distribution was established.The film thickness distributions of primary mirrors with different curvature were analyzed theoretically.And the shadowing mask was designed to correct the thickness uniformity.In addition,the optimal tilt angle of the target for the curvature primary mirror is optimized.By using a ZZS3200 large-scale high-vacuum coating machine with the materials evaporated downwards,the research of downward thermal evaporation for large-aperture optics was implemented in details.A protected-silver mirror sample was successfully prepared by downward thermal evaporation.In addition,the feasibility study for preparing enhanced-silver mirror was carried out.