Optical surfaces of space instruments usually need to be blackened to minimize adverse effects affecting their performance in photometric, spectrometric, and imaging applications. Blackening is often obtained by application of coatings that strongly absorb the incoming photon flux and diffusively scatter the incident photons. We discuss reflectance measurements and a phenomenological model of the bidirectional reflectance distribution function (BRDF) for the Magic Black™ coating, which is a commercial product supplied by the Acktar company. The coating has a vast satellite-instrument heritage and is planned to be used in the GLOWS photometer onboard the upcoming NASA Interstellar Mapping and Acceleration Probe nmission. The reflectance measurements were conducted at ∼121.6 nm, corresponding to the Lyman-α line for hydrogen, which is important in astrophysics. This line is commonly considered a crossover between the far ultraviolet and extreme ultraviolet spectral ranges. To generate radiation in this range, a laser-plasma source based on the gas-puff target was used. Six samples coated with Acktar Magic Black™ were studied in an optical system with a back-illuminated CCD camera as a detector. The measurements were used to derive the phenomenological BRDF model based on a series of analytic fits to the measurements, which makes it easily applicable in both numerical simulations and manual calculations. The formulas provide an approximation in the full hemispheric domain, i.e., both for the in-specular-plane and out-of-specular-plane behaviors of the BRDF for the coating. A similar fit-based phenomenological model is also described for the visible range (the wavelength of 532 nm) as a byproduct of our analysis for the UV range.

中文翻译：

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Acktar Magic Black™ 涂层对氢莱曼-α 线附近辐射的反射特性：BRDF 的测量和唯象模型

空间仪器的光学表面通常需要黑化，以尽量减少影响其在光度、光谱和成像应用中性能的不利影响。变黑通常是通过应用强烈吸收入射光子通量并扩散散射入射光子的涂层来实现的。我们讨论 Magic Black™ 涂层的反射率测量和双向反射率分布函数 (BRDF) 的唯象模型，Magic Black™ 涂层是 Acktar 公司提供的商业产品。该涂层具有丰富的卫星仪器传统，计划用于即将到来的 NASA 星际测绘和加速探测器任务中的 GLOWS 光度计。反射率测量是在〜121.6 nm处进行的，对应于氢的莱曼-α线，这在天体物理学中很重要。这条线通常被认为是远紫外和极紫外光谱范围之间的交叉点。为了产生该范围内的辐射，使用了基于吹气靶的激光等离子体源。在以背照式 CCD 相机作为检测器的光学系统中对六个涂有 Acktar Magic Black™ 的样品进行了研究。这些测量结果用于基于对测量结果的一系列分析拟合推导现象学 BRDF 模型，这使得它可以轻松应用于数值模拟和手动计算。这些公式提供了整个半球域的近似值，即涂层的 BRDF 的镜面内和镜面外行为。作为我们对紫外线范围分析的副产品，还描述了可见光范围（波长 532 nm）的类似基于拟合的唯象模型。