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History of X-Ray Astronomy
How do X-ray telescopes differ from optical telescopes?
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 (see
diagram below). 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.
 The first imaging X-ray
telescope was made by a team of scientists under the direction of Riccardo
Giacconi at American Science and Engineering in Cambridge, MA. It was flown on a
small sounding rocket in 1963 and made crude images of hot spots in the upper
atmosphere of the Sun.
This telescope was about the same diameter and length as the optical telescope
Galileo used in 1610. Over a period of 380 years, optical telescopes improved in
sensitivity by 100 million times from Galileo's telescope to the Hubble Space
Telescope. Remarkably, Chandra represents a leap of 100 million in sensitivity,
yet it took only 36 years to achieve!
Why are X-ray observatories in space?
The building and operation of an X-ray observatory is a marvel of modern
technology and ingenuity. Engineers, technicians and scientists design and build
large, curving mirrors that can be nested inside one another to increase the
total reflecting area of the telescope. The mirrors focus X-ray photons onto
state-of-the-art detectors which record the direction and in some cases, the
energy of the photons.
Because the Earth's atmosphere absorbs X-rays, X-ray observatories must be
placed high above the Earth's surface. This means that the ultra-precise mirrors
and detectors, together with the sophisticated electronics that conveys the
information back to Earth must be able to withstand the rigors of a rocket
launch, and operate in the hostile environment of space.
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