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Science Instruments
The High Resolution Spectrometers - HETGS and LETGS
Transmission Gratings
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The gratings exploit Chandra's sharp mirror focus and matching
detector resolution to produce high
resolution X-ray spectroscopy. Since the grating
spectrometers can measure energy to an accuracy of up to one
part in a thousand, they are used in the study of detailed
energy spectra, distinguishing individual X-ray lines. This
enables the temperature, ionization and chemical composition to
be explored.
LETG
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The
LETG grating is a freestanding gold grating made
of fine wires or bars with a regular spacing, or period , of 1µm. The fine gold wires are
held by two different support structures, a linear grid with
25.4µm and a coarse triangular mesh with 2 mm spacing. The
gratings are mounted onto a toroidal ring structure matched to
the Chandra mirrors. The LETG gratings are designed to cover an
energy range of 0.08 to 2 keV. However, their diffraction can
also be seen in visible light, which is beautifully shown in the
picture above right.
Grating Cross Section
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The
HETG gratings have a much finer period,
0.2µm or 2000Å for the high-energy gratings, and
0.4µm or 4000Å, for the medium energy gratings. In
order to distinguish between them, the two types of gratings are
oriented at slightly different angles, so that the X-rays are
diffracted in an "X" pattern at the focal plane. Since the size
of the gold grating bars is smaller
that a wavelength of visible light, special fabrication
techniques are required to make them. The gratings take
advantage of the fact that the gold bars are partially
transparent to X-rays, so that the diffraction is more
efficient, and more X-rays are captured in the high resolution
spectrum. The HETG gratings are designed to cover an energy range
of 0.4 to 10 keV.
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