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Efficient mirrors with high reflectivity over the ultra-violet, optical, and infra-red (UVOIR) spectral range are essential components in future space-based observatories. Aluminum mirrors with fluoride-based protective layers are commonly the baseline UV coating technology; these mirrors have been proven to be stable, reliable, and with long flight heritage. However, despite their optical performance to date, their reflectivity is still insufficient for future large telescope instrumentation in which several reflections are required.
Recently, a novel passivation procedure based on the exposure of bare Al to a fluorine containing electron beam generated plasma has been presented [1,2]. This research is framed in a collaboration between Goddard Space Flight Center (GSFC) and the U.S. Naval Research Laboratory (NRL), with plasma treatment carried out in NRL’s large area plasma processing system (LAPPS) using aluminum coated glass samples produced at GSFC coating facilities. The passivation of the bare Al is accomplished by using an electron-beam generated plasma produced in a fluorine-containing background to simultaneously remove the native oxide layer while promoting the formation of an AlF3 passivation layer with tunable thickness. Importantly, this new treatment uses benign precursors (SF6) and is performed at room temperature. In this work, details of the plasma process and in situ surface monitoring with spectroscopic ellipsometry are discussed. This novel procedure has demonstrated improved Al mirrors with state of the art far-ultraviolet (FUV) (λ = 90-200 nm) reflectivity (e.g. R=91% at 121.6 nm) paired with an excellent thickness control of the Al protective layer.
Cryogenic refractive indices for S-FPL55, S-LAL8, and S-BAH28 glasses for the HARMONI visible camera
To this end, we make two changes to the simple system: (1) An optical dome was added near the receiver. An array of lenses is placed radially on the surface of the dome, reminiscent of the compound eye of an insect. The lenses make the source and detector planes conjugate, and each lens adds a new region of the source plane to the instrument's total field of view. (2) The receiver was expanded to include multiple photodiodes. With these two changes, the receiver has much more tolerance to misalignments (in position and angle) of the transmitter.
Two versions of the optical dome (with 6" and 8" diameters) were designed using PTC’s Creo CAD software and modeled using Synopsys' CODE V optical design software. A series of these transparent hemispherical domes, with both design diameters, were manufactured using a 5-axis mill. The prototype was then retrofitted with lenses and compared with the computer-generated model to demonstrate the effectiveness of this solution. This work shows that the dome design improves the optical field of view of the underwater communication system considerably. Furthermore, with the experimental test results, a geometric optimization model was derived providing insights to the design performance limits.
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