X-ray telescope,instrument designed to detect and resolve X-rays from sources outside the Earth’s atmosphere. Because of atmospheric absorption, X-ray telescopes must be carried to high altitudes by rockets or balloons or placed in orbit outside the atmosphere. Balloon-borne telescopes are used to can detect the more penetrating (harder) X-rays, whereas those carried aloft by rockets or in satellites are used to detect softer radiation.

The design of this type of telescope must be radically different from that of a conventional optical telescope. Since X-ray photons have so much energy, they would pass right through the mirror of a standard reflector. X-rays must be bounced off a mirror at a very low angle if they are to be captured. This technique is referred to as grazing incidence. For this reason, the mirrors in X-ray telescopes are mounted with their surfaces only slightly off a parallel line with the incoming X-rays. Application of the grazing-incidence principle makes it possible to focus X-rays from a cosmic object into an image that can be recorded electronically.

Several types of X-ray detectors have been used, involving Geiger counters, proportional counters, and scintillation counters. These detectors require a large collecting area, because celestial X-ray sources are remote and therefore weak, and a high efficiency for detecting X-rays over the cosmic-ray-induced background radiation is needed. Reflecting

The first X-ray telescopes are constructed so that the highly penetrating rays strike the reflective surface at a grazing angle.telescope was the Apollo Telescope Mount, which studied the Sun from on board the American space station Skylab. It was followed during the late 1970s by two High-Energy Astronomy Observatories (HEAOs), which explored cosmic X-ray sources. HEAO-1 mapped the X-ray sources with high sensitivity and high resolution. Some of the more interesting of these objects were studied in detail by HEAO-2 (named the Einstein Observatory).

The European X-ray Observatory Satellite (EXOSAT), developed by the European Space Agency, was capable of greater spectral resolution than the Einstein Observatory and was more sensitive to X-ray emissions at shorter wavelengths. EXOSAT remained in orbit from 1983 to 1986.

A much larger X-ray astronomy satellite was launched on June 1, 1990, as part of a cooperative program involving the United States, Germany, and the United Kingdom. This satellite, called the Röntgensatellit (ROSAT), had two parallel grazing-incidence telescopes. One of them, the X-ray telescope, bore many similarities to the equipment of the Einstein Observatory but had a larger geometric area and better mirror resolution. The other operated at extreme ultraviolet wavelengths. A position-sensitive proportional counter made it possible to survey the sky at X-ray wavelengths and produced a catalog of more than 150,000 sources with a positional accuracy of better than 30 arc seconds. A wide-field camera with a 5°-diameter field of view that operated with the extreme ultraviolet telescope was also part of the ROSAT instrument package. It produced an extended ultraviolet survey with arc minute source positions in this wavelength region, making it the first instrument with such capability. The ROSAT mirrors were gold-coated and permitted detailed examination of the sky from 5 to 124 angstroms. The ROSAT mission ended in February 1999.
