Glossary term: X-ray Telescope

Description: An X-ray telescope is a type of telescope specially designed for the observation of high-energy, high-frequency, X-ray light that is invisible to the human eye. Since Earth's atmosphere completely absorbs incoming X-rays, X-ray telescopes are typically space telescopes. The high frequencies and correspondingly short wavelengths of X-rays require optics markedly different from those of visible-light telescopes: X-rays fall onto the telescope mirrors at very shallow angles ("grazing incidence") to bounce off the exterior of the mirror ("external reflection"). Mirror assemblies for focusing X-rays that work in this way are often built as concentric shells. At very high energies, for "hard" X-rays, telescope optics typically do not attempt to focus the light at all, but instead rely on masks to extract information about X-ray direction, and from this information reconstruct images.

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