X-Ray Astronomy

Although they are among the most energetic of the electromagnetic spectrum, and thus provide a window on some of the most violent processes in the universe, x rays are not able to penetrate Earth's atmosphere; they are absorbed at about 62 mi (100 km) above the surface. Thus, only with the advent of rocket and satellite astronomy have astronomers been able to study the wide-ranging phenomena which produce x rays. The highest energy x rays have also been studied by balloons high in Earth's atmosphere, but there are far fewer photons at these energies than at the lower energies that can be observed above the atmosphere.

X rays are also difficult to bring to a focus, since their energies are so high. Therefore, an important breakthrough in x-ray astronomy was the advent of imaging telescopes, replacing instruments which could only crudely tell in which direction an x-ray source was located. The telescopes with which we are most familiar, consisting of lenses or mirrors that capture light arriving at normal incidence (perpendicular to the surface) won't work in the xray region of the spectrum, since the x rays pass through unchanged or are absorbed by the optics. Instead, x-ray astronomers use grazing incidence telescopes, in which the light from the source strikes mirrors at angles of only a few degrees, skipping like stones over the surface of water. By combining two mirrors, the energy can be focused onto a detector in order to provide a sharp image of the source.