Reflectance properties of the Acktar Magic Black™ coating for the radiation near the Lyman-α line of hydrogen: measurements and phenomenological model of the BRDF

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.