(iv) for some applications such as X-ray optics, grazing incidence mirrors are the only option available. Future large telescopes will cover an increased spectral range of observation with a broad range of multi-spectral and hyper-spectral instruments, and this can be achieved only with reflective telescopes. A mirror consists of a substrate and, most often, a coating. Substrates can be selected among a limited number of materials. The fundamental parameters are: (i) specific stiffness; (ii) thermal stability, (iii) space environmental resistance, (iv) achievable surface quality, (v) weight, and (vi) financial aspects. Regarding the choice of mirror substrates, extensive work has been performed and reported [2; 9; 10; 11; 12]. Al or Al alloys, Be, Si, SiC, Zerodur\({}^{\circledR}\), nickel, and fused silica have been employed [2; 3; 10; 12; 13], although glass has been the most used material for mirror substrates, given its thermal stability and ease of engineering into high-quality optical surfaces [14]; for instance, it is used as substrate in the Hubble Space Telescope, the largest space mirror still operating. However, one important shortcoming of using glass is its weight, which often limits its use to small-aperture mirrors. For this reason, new materials have been developed, with near future state-of-the-art mirror research focusing on segmented mirrors prepared on Zerodur, Be, Al, Si, or SiC substrates [14]. New large telescopes with active mirrors are now being developed with carbon-based (lightweight) materials. Silicon carbide (SiC), in particular, has been successfully used in the European Space Agency (ESA) Herschel Space Observatory [15], and it is still extensively investigated as a potential standard because of its superior stiffness, strength, and thermal properties [16; 17]. Additionally, as illustrated by [PERSON] et al. [18], Silicon Pore Optics (SPO) is the new X-ray optics technology under development in Europe, forming the ESA baseline technology for the International X-ray Observatory candidate mission studied jointly by ESA, NASA, and Japan Aerospace Exploration Agency (JAXA). As mirror substrates do not always provide the desired optical performance, the use of optical coatings to improve them is often required. Coatings have a major impact on the instrument optical performance. Even if mirrors are insensitive to chromatic aberrations, the need for large spectral bandwidth impacts the coating design and the technologies to reach broadband reflective coatings with a high reflectivity and low coating-induced polarization. In particular, while most mirrors used for space systems that operate from the ultraviolet (UV) to the infrared (IR) wavelength regions rely on coatings of Ag, Al, Au, or Be, extreme regions such as X-Ray, extreme UV (EUV), and far-IR require specific engineered designs comprising many layers of different materials. The coating may include adhesion layers (between the substrate and/or layers with poor adhesion), interdiffusion layers (between layers of different species) and protection or enhancement layers (on top of the reflective layer or multilayer). Dielectric optical coatings can be used alone or in combination with metallic ones in multilayers. Multilayers of metal-dielectric and all-dielectric films have been extensively used to prepare narrow band reflectors for several spectral bands [19; 20; 21]. Multilayers consist of several layers of two or more materials with optimized thicknesses to obtain the desired spectral, angular, and/or polarization profile. In the visible and close ranges, multilayers alternate layers of transparent (dielectric) materials, which enables the theoretical design of virtually any arbitrary profile. In ranges such as extreme UV and soft x-rays, where materials are characterized by high absorption, multilayers may typically alternate a dielectric material and a metal or even two metals.