## 2 Degradation of Materials in Space--Stability Issues on Mirrors
### Outgassing and Cross-Contamination  
As previously mentioned, one of the main contamination sources for space mirrors originates from outgassing in the space vacuum environment, mostly from components within the telescope. Due to the strong absorption of materials, particularly contaminants in the FUV range, instrument outgassing has been investigated by several authors and the FUV properties of most volatile spacecraft materials have been measured [66; 88; 89; 90; 91], the results suggesting larger absorption in the FUV compared to longer wavelengths. These kinds of experiments are especially useful to evaluate the maximum allowable contaminant thickness before FUV reflectance is unacceptably degraded. When outgassed volatile contaminants are irradiated with strong UV radiation, this may result in the transformation of the contaminants into non-volatile compounds through a photopolymerization process, so that they may condensate/bond on the coating surface, thus degrading its optical performance. UV radiation provides the energy to break bonds in the hydrocarbon chain and stimulates intermolecular crosslinking [66]. The photopolymerization process mostly depends on the coating and contaminant nature, the substrate temperature, and the specific UV radiation energy and intensity [92]. In this respect, a facility was realized at Goddard-Space Flight Center (GSFC) to controllably contaminate mirrors and measure their FUV degradation in situ [92]. In this scenario, it is important to point out that Al, Ag, or Au-based mirrors (e.g., Al/MgF\({}_{2}\) reflectance [93]) can undergo degradation only upon the presence of contaminantscombined to UV radiation. Other than UV, energetic protons and electrons may also contribute to turn a contaminant into a non-volatile product [66]. A strong manifestation of the synergistic effects between UV and contaminants was observed after the first servicing mission on Hubble Space Telescope (HST) [94]. The Wide Field Planetary Camera I (WFPC-1) was replaced and returned to Earth where its pickoff mirror was analyzed. The Al/MgF\({}_{2}\) mirror was found to be covered with a 45 nm-thick contaminant, which severely degraded the FUV reflectance. The contamination was attributed to the outgassing of HST during its first 3.5 years of operation. The mirror was found to be contaminated with hydrocarbons, esters, and silicones. Figure 2 shows the drastic reduction in reflectivity at short wavelengths, with the x-ray photoemission spectroscopy (XPS) data revealing the composition of the contaminants. The mirror was then carefully cleaned, leading to a full restoration of the preflight reflectance, hence demonstrating no or negligible degradation of the Al/MgF\({}_{2}\) coating [94]. A later servicing mission enabled the retrieval of more Al/MgF\({}_{2}\) mirrors from the HST after 15.5 years in space [96]. While two COSTAR optics mirrors kept a relatively high FUV reflectance, comparable to or even better than a witness sample that had been stored in a desiccator, the WFPC-2 pick off mirror resulted in a reflectance degradation as severe as for the aforementioned WFPC-1 mirror. This suggested a similar contamination for both mirrors, in spite of the efforts carried out to reduce the contamination on WFPC-2 after the experience with WFPC-1. The different levels of contamination through the mirrors were unexpected and attributed to contamination dependent on the specific location within the HST hub. Regarding grazing-incidence mirrors, [PERSON] calculated the sensitivity of mirror reflectance to a range of optical constants selected for generic contaminants, such as hydrocarbons [97]. Three wavelengths were investigated as representative of the EUV: 10, 50, and 100 nm. He calculated critical contaminant thicknesses to reach allowable reflectance changes. A preliminary conclusion was that Au and Zerodur mirrors are relatively insensitive to top surface films, which can even result in an increased reflectance in some cases. [PERSON] et al.