About the High Energy Transmission Gratings

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. For a grating, this effect is due to a regularly spaced pattern. For example, music CDs can act as a diffraction grating: the grooves diffract the light, so that different colors are seen reflected off the surface at different angles.

Two types of gratings are in the HETG, in order to distinguish between them, the HEG and MEG gratings are oriented at slightly different angles, so that the X-rays are diffracted in an "X" pattern on the detector.
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. 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 will be used in the study of detailed energy spectra, distinguishing individual X-ray lines. This will enable the temperature, ionization and chemical composition to be explored.

The HETG gratings are made of fine gold wires or bars with a regular spacing, or period : 0.2µm or 2000Å for the high energy gratings, and 0.4µm or 4000Å for the medium energy gratings. 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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