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grazing incidenceX-rays reflect from surfaces for the same physical reason that light reflects, mainly due to coherent scattering in the surface material. However, since X-ray photon energies are large, the refractive index, n, is slightly less than unity and reflection occurs only up to a critical angle, θ, where cos θ = n. The angle θ is usually small and thus X-ray reflection is only high for rays that graze the surface; hence ‘grazing incidence’. For example θ is about 1° for a gold surface and X-rays of wavelengths of about 0.3 nm (4 keV). θ is proportional to X-ray wavelength and also increases with the atomic number of the reflecting surface.
The space station Skylab was the first space mission to carry a large grazing incidence X-ray telescope, in fact two, both of which produced many high-resolution X-ray images of the solar corona in 1973–74. With two coaxial reflectors (usually a paraboloid/hyperboloid pair), high-quality X-ray images are produced over a field of view of order of the grazing angle. The great potential of such optical systems for cosmic X-ray astronomy was first demonstrated with the flight of a 60-cm diameter grazing incidence telescope on the Einstein Observatory in 1978–81. Subsequently, most X-ray astronomy satellites have employed or will employ this technique.