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Science Instruments
Advanced CCD Imaging Spectrometer (ACIS)
The Chandra Advanced CCD Imaging Spectrometer
(ACIS) is one of two focal plane instruments. As the name
suggests, this instrument is an array of charged coupled devices
(CCD's), which are sophisticated versions of the crude CCD's
used in camcorders. This instrument is especially useful because
it can make X-ray images, and at the same time, measure the
energy of each incoming X-ray. Thus scientists can make pictures
of objects using only X-rays produced by a single chemical
element, and so compare (for example) the appearance of a
supernova remnant in light produced by oxygen ions to that of
neon or iron ions. It is the instrument of choice for studying
temperature variations across X-ray sources such as vast clouds
of hot gas in intergalactic space, or chemical variations across
clouds left by supernova explosions.
The High Resolution Spectrometers - HETGS and LETGS
There are two instruments aboard Chandra dedicated to high
resolution spectroscopy: the High Energy Transmission Grating
Spectrometer (HETGS) and the Low Energy Transmission
Grating Spectrometer (LETGS). Each spectrometer is activated by
swinging an assembly into position behind the mirrors. The
assembly holds hundreds of gold transmission gratings: when in
place behind the mirrors, the gratings intercept the X-rays
reflected from the mirrors.
 These gratings diffract
the intercepted X-rays, changing their direction by amounts that depend
sensitively on the X-ray energy, much like a prism separates light into
its component colors. One of the focal plane cameras, either
HRC or ACIS, detects the location of the
diffracted X-ray, enabling a precise determination of its energy. (A grating
is able to diffract because it has a regularly spaced pattern. For example,
music CDs act as a type of grating: the grooves diffract, so that when light
falls on the shiny side of the CD, a rainbow effect is seen. As the CD is
tilted through different angles, different colors come into view.)
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