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Reflectivity degradation of the Subaru Telescope primary mirror

机译：斯巴鲁望远镜主镜的反射率下降

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The reflectivity degradation of a telescope’s mirrors is broadly known; however, due to lack of suitable instrumentation, it has not been sufficiently studied. To overcome this situation, we developed a portable spectrophotometer named the “Subaru Portable Spectrophotometer” (SPS) to measure the absolute spectral reflectivity in-situ for a large optical component, such as a telescope’s primary mirror. The reflectivity of the Subaru telescope primary mirror was measured with SPS right before its eighth coating, and it showed larger degradation at shorter wavelength. Since then, the reflectivity degradation has been monitored with SPS. To explain the wavelength-dependence of the reflectivity degradation, we developed a two-factor model of a simple achromatic loss and scattering. The two-factor model shows an excellent match to real degradation. The time evolution of the achromatic loss and scattering are shown, and these are about 2.3% achromatic loss per year, and the color-dependent loss from root-mean-square (rms) surface roughness, which is the cause of scattering, increases from an initial small roughness after coating by about 40?? per year. From these results, we extrapolated the model to a long-elapsed time to estimate the reflectivity in the future. The reflectivity of the Subaru Telescope primary mirror is expected to decrease about 22% at 400?nm by 1200?days after coating, and by 10% at 950?nm. We deduced the most efficient coating frequency of the Subaru Telescope primary mirror that maximizes the number of photons that can be acquired, and it is once every three years.
机译：众所周知，望远镜镜的反射率会下降。但是，由于缺乏合适的仪器，因此尚未对其进行充分的研究。为了克服这种情况，我们开发了一种名为“ Subaru便携式分光光度计”（SPS）的便携式分光光度计，用于在现场测量大型光学组件（例如望远镜的主镜）的绝对光谱反射率。斯巴鲁望远镜主镜的反射率是在即将进行第八次镀膜之前用SPS测量的，并且在较短的波长下显示出更大的衰减。从那时起，就用SPS监控了反射率的下降。为了解释反射率降低的波长依赖性，我们开发了一个简单的消色差和散射的两因素模型。两因素模型显示出与实际降解的极佳匹配。显示了消色差和散射的时间演变，每年约有2.3％的消色差，而由色散引起的根均方根（rms）表面粗糙度引起的色相关损耗从涂层后的初始小粗糙度约为40？每年。根据这些结果，我们将模型外推到很长时间，以估计将来的反射率。镀膜后1200天，斯巴鲁望远镜主镜在400纳米处的反射率预计会下降约22％，在950纳米处反射率会下降10％。我们推导了斯巴鲁望远镜主镜的最有效镀膜频率，该频率最大程度地提高了可获取的光子数量，并且每三年一次。

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