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.
The PolFEL free electron laser, currently under construction at the National Centre for Nuclear Research in Poland, will generate a beam of coherent electromagnetic radiation in the ultraviolet (UV) spectral range with a wavelength of about 150 nm to 300 nm, in the form of several hundred fs pulses, energy up to 50 μJ, and repetition rate of 50 kHz. Vacuum ultraviolet (VUV) radiation beam in the wavelength range from 50 nm to 100 nm will be obtained by selecting the third harmonic using an absorption filter. The optical system of the UV/VUV beamline consists of two plane M1 and M2 mirrors and one focusing ellipsoidal M3 mirror. The radiation produced in the laser hits on the M1 mirror at a grazing incidence angle of 5°. After reflection from the M1 mirror, the beam falls on the M2 mirror at an angle of 17°, which directs the beam to the ellipsoidal M3 mirror, focusing the beam at the image plane at the second focal point of the ellipsoid. The M1 mirror is placed behind the 3 m-thick concrete wall in a hutch separated from the experimental hall by a 1.6 m-thick concrete wall. The optical properties of the beamline were tested by ray-tracing simulations using the RAY-UI software, the results of which are presented in the paper.
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