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Grazing incidence optics (sometimes abbreviated GIO) is optics that reflects EMR such that a ray's direction-change is small; the angle between the incoming EMR's path and the reflecting surface (grazing angle) is small, e.g., less than 2°. This is common for X-ray telescopes, because the X-rays can penetrate many materials but can be reflected if the grazing angle is small, which is a severe restriction on the design of X-ray telescopes. Wolter telescopes, of which there are a few kinds, are designed to use only grazing incident optics. A consequence of this design- restriction is that the telescopes are relatively long: both Chandra X-ray Observatory and XMM-Newton have telescopes on the order of 10 meters in length.