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 detect the more penetrating (harder) X rays, whereas those carried aloft by rockets or in satellites are used to detect softer radiation.

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 X-ray telescopes are constructed so that the highly penetrating rays strike the reflective surface at a grazing angle.