VI.

X-Ray Telescopes

X-ray astronomy was developed in the early 1960s when simple X-ray detectors were mounted on high-altitude rockets. Astronomers were surprised to discover X rays streaming from many energetic astronomical objects. Space-based X-ray astronomy was pioneered in 1970 by the U.S. Explorer 42 (Uhuru) satellite, which mapped the sky for X rays. Two important new X-ray telescopes were launched in 1999: the National Aeronautics and Space Administration’s (NASA) Chandra X-ray Observatory and the European Space Agency’s X-ray Multimirror mission (XMM).

Some X-ray telescopes are built like optical reflecting telescopes. The main mirror of these telescopes must be nearly cylindrical, as opposed to the dish-shaped optical mirrors. X rays from a targeted object hit the mirror at such a shallow angle that they just graze it in order to be reflected into the detector. Some X-ray telescopes are just X-ray detectors that can be pointed at sources. To block out X rays not coming from the target, most detectors are surrounded by a cylinder of X-ray absorbing lead.