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Metallic coatings are essential for numerous optical systems due to their high and broadband reflectivity in the infrared spectral range. In contrast to well-established (protected) silver and gold mirror coatings, iridium is environmentally durable, referring to ISO 9211-3 and thermally stable up to 600 °C even without protective layers, as demonstrated. Additionally, the optical and related structural properties of atomic layer deposited (ALD) and magnetron sputtered (MS) Ir coatings were investigated using spectrophotometry, FTIR, ellipsometry, WLI, AFM, XRR, XRD, SEM, and electrical resistivity measurements. The properties of Ir ALD and Ir MS coatings differ due to their topography and microstructure.

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In the visible to infrared spectral range, highly-reflective silver mirrors are applied in the manufacture of optical instruments such as telescopes. However, it is still difficult to combine high reflectivity and long-term stability of the protected silver coating. We show that the deposition of impervious protective layers is necessary but often not sufficient for long-term environmental stability. Hygroscopic air borne particles absorbed by the protections surface attract water molecules and form a solution. This solution first damages the protection, subsequently permeates the protection and finally damages the silver whereby the reflectivity is reduced. We demonstrate this particular damage mechanism with different experiments and describe this mechanism in detail.

The fabrication of highly reflective aluminum coatings is still an important part of current research due to their high intrinsic reflectivity in a broad spectral range. By using thin seed layers of Cu, CuOx, Cr, CrOx, Au, and Ag, the morphology of sputtered (unprotected) aluminum layers and, consequently, their reflectance can be influenced. In this long-term study, the reflectance behavior was measured continuously using spectrophotometry. Particular seed layer materials enhance the reflectance of aluminum coatings significantly and reduce their long-term degradation. Combining such seed layers with evaporation processes and suitable protective layers could further increase the reflectance of aluminum coatings.

H₂S and particles from the atmosphere can damage silver reflectors. These defects lead to scattering and a reduction of reflectivity. With regard to these risks, the suitability of sputtered SiO₂, Al₂O₃, and SiO₂-Al₂O₃ nanolaminates for the protection of Ag was analyzed. The optical properties, protection properties against H₂S, solubility, film stress, and protection properties against particle-induced defect formation have been investigated. Especially in the case of particle-induced defects on protected Ag, differences between the protective coatings are considerable, and the nanolaminate layers have advantageous properties.

We report on the design and fabrication of a novel all-glass four-channel beam splitter based on a Kösters prism for use in space. The Kösters prism, which consists of three pairs of individual prisms, is used to separate an incoming telescope beam into four spectral channels (λ = 800–1700 nm) with the goal to obtain a multi-band photometry of cosmic sources in the optical/near-infrared bands. We performed optical design studies to evaluate the influence of geometrical tolerances of the six individual prisms on the image quality. A stray light analysis revealed the impact of the composition on the overall optical performance. Mechanical design studies benchmarked possible mounting strategies. We considered optical adhesives, soldering and clamping. The influence of the mechanical loads during a rocket launch as well as thermal loads at 140 K (the operation temperature of the optical element) were studied. We optimized the coating properties of the prisms by considering the results from the optical design study as well as the technological requirements for the direct bonding of the prisms. Bonding strategies to realize the prism pairs were developed and successfully tested. A demonstrator Kösters prism was manufactured and is ready to validate its optical performance.

For ground-and spaced based applications, Ag coated reflectors are indispensable because of their high reflectivity. The transport, assembling and storage of these reflectors takes a long time, before they are finally commissioned for the actual applications. To endure this period without a decrease of reflectivity, protective coatings with a final layer, which offers a high resistance to aqueous solutions and a low mechanical stress should be used. These criteria were taken into account for the selection of a final layer for a protected Ag-coating, which was applied for reflectors utilized in the CRIRES +instrument (an IR spectrograph used at the VLT). Reactively sputtered Al2O3, SiO2 and Si3N4 layers were investigated with regard to these criteria. In aqueous (basic) solutions, the investigated Si3N4 layers are more stable than the SiO2 layers, and the SiO2 layers are more stable than the Al2O3 layers. This shows the influence of the intrinsic material properties. The mechanical stress of the sputtered layers depends on the deposition conditions and thus on the selected parameters. A Si3N4 layer with a high resistance to aqueous solutions also offers a low and stable mechanical stress. Therefore, the depositionparameters which have been used for this layer were applied for sputtering the final layer of the protected Ag-coating for the reflectors.