Telescopes are instruments that collect light to help scientists observe distant objects in the universe. Many different types of telescopes are used for observing different types of electromagnetic radiation, such as visible light, radio waves, or gamma rays. A conventional reflecting telescope uses mirrors to gather light and focus it at a single point; infrared and optical telescopes are often built this way. Radio telescopes also have a similar design to reflect light but use wire mesh in the shape of a dish instead of mirrors to reflect the radio waves. However, telescopes used to study high-energy electromagnetic radiation are quite different in how they are built.

X-rays and gamma rays have such short wavelengths and such high energies that they do not reflect off surfaces like other types of electromagnetic radiation. Instead, they usually penetrate through materials. If an X-ray hits a mirror directly, it penetrates into the surface; however, if it strikes the mirror at a very shallow angle, it can be reflected. This behavior is similar to the way a stone will sink into the water of a pond when you toss it in but will skip across the surface if you throw it at a glancing angle. The Chandra X-Ray Observatory takes advantage of this property by using mirrors shaped like barrels so that the surfaces are nearly parallel to the incoming light. The incoming X-rays graze the mirrors and are then focused onto detectors. However, gamma rays, which have even higher energies than X-rays, cannot be reflected by mirrors at all. Gamma-ray telescopes rely on other methods to detect and measure how the gamma rays interact with materials.

Another challenge to building telescopes to observe high-energy radiation such as X-rays and gamma rays is that they cannot be ground-based. Earth's atmosphere absorbs almost all X-rays and gamma rays, so they are not detectable from Earth's surface. As a result, rockets and satellites must be used to get telescopes above the atmosphere to study high-energy electromagnetic radiation.

The shielding effect of the atmosphere is important for living things, however. Because of their high energies, X-rays and gamma rays are forms of ionizing radiation: when they hit an atom, they can knock electrons free. Such changes in atomic structure can damage living cells and break their DNA, which can lead to cell death and other possible problems such as cancer or birth defects.

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