X-rays are a highly energetic form of light, not visible to human eyes. Light can take on many forms -- including radio waves, microwaves, infrared, visible, ultraviolet, X-ray and gamma radiation. Very low temperatures (hundreds of degrees below zero Celsius) produce mostly low energy radio and microwave photons, whereas cool bodies like ours (about 30 degrees Celsius) produce largely infrared radiation. Objects at very high temperatures (millions of degrees Celsius) emit most of their energy as x-rays.
Much of the matter in the universe cannot be seen by any other telescope. X-ray telescopes are the only way we can observe extremely hot matter with temperatures of millions of degrees Celsius. It takes gigantic explosions, or intense magnetic or gravitational fields to energize particles to these high temperatures. Where do such conditions exist? In an astonishing variety of places, ranging from the vast spaces between galaxies to the bizarre, collapsed worlds of neutron stars and black holes.
X-rays do not reflect off mirrors the same way that visible light does. Because of their high-energy, X-ray photons penetrate into the mirror in much the same way that bullets slam into a wall. Likewise, just as bullets ricochet when they hit a wall at a grazing angle, so too will x-rays ricochet off mirrors. These properties mean that X-ray telescopes must be very different from optical telescopes. The mirrors have to be precisely shaped and aligned nearly parallel to incoming x-rays. Thus they look more like barrels than the familiar dish shape of optical telescopes.
Chandra X-Ray Observatory Center