Metrological and spectral characterization of the Aspera flight optics

Aspera is a NASA's Astrophysics Pioneers mission, led by the University of Arizona (UofA), designed to study the presence of hot gases (T=10⁵-10⁶K) in the circumgalactic medium (CGM) and how the flow of these gases affects galactic formation. One key enabling technology in the Aspera optical system is the use of more efficient UV-reflective optical coatings, particularly at around 103.2-103.8 nm, where the O_VI emission doublet is located. To meet radiometric effective area requirements, an improved version of the Al+LiF optical coating used in FUSE [Proc. SPIE11819, 1181903 (2021)PSISDG0277-786X10.1117/12.2593001, Proc. SPIE13093, 1309302 (2024) PSISDG0277-786X10.1117/12.3017274] has been applied to the flight optics and gratings. This version is based on Al (aluminum) + eLiF (enhanced lithium fluoride), where Al+LiF is annealed at an elevated temperature (∼250^∘C) after deposition, thus providing enhanced optical performance at 103 nm when compared to standard Al+LiF. Furthermore, these optics are encapsulated with a thin film of atomic layer-deposited MgF₂ at the Jet Propulsion Laboratory for enhanced durability [Proc. SPIE11819, 1181903 (2021) PSISDG0277-786X10.1117/12.2593001, Proc. SPIE9144, 91444G (2014)PSISDG0277-786X10.1117/12.2057438]. The efficiency and durability of these UV-sensitive coatings depend on the quality of the optical surface in terms of surface roughness and cleanliness. Roughness increases the scatter of the coating and reduces the specular reflectance. Surface contaminants can accommodate moisture and other contaminants, increase scattering, and create weak points in the coating that may affect adhesion and, subsequently, the longevity of these coatings. Extensive optical metrology is necessary to quantify the impact of surface roughness and contaminants on optical surfaces. This paper provides information on the behavior of mirror substrates and FUV coatings throughout the full inspection and coating process implemented for Aspera's optics at Goddard Space Flight Center, from the initial receipt of the optics through post-coating. Inspection and metrology techniques include atomic force microscopy (AFM), coherence scanning interferometry (CSI), dark field microscopy (DFM), and vacuum ultraviolet (VUV) spectroscopy. Combinations of these techniques are used to inspect flight optics at each of the following steps: (i) as-received, pre-cleaning inspection, (ii) pre-coating, post-cleaning inspection, and (iii) post-coating inspection. The evolution of roughness and other surface defects is compared between each step to quantify the effect each step has on the flight optics. The final far-ultraviolet (FUV) spectral performance of witness samples coated with the flight optics is also presented, with most witnesses showing unprecedented reflectance values of ≈0.83-0.84 at a 103 nm wavelength.