The HETG consists of two sets of gratings, each with a different period . One set, the medium energy grating (MEG), intercepts rays from the outer mirror shells and is optimized for medium energies (0.4-5 keV). Its ruling density is 2500lmm_1. The second set, the high energy grating (HEG), intercepts rays from the two inner shells and is optimized for high energies (0.8-10 keV). The ruling density of the HEG is 5000lmm~1. Both sets are mounted onto a single support structure and are therefore used simultaneously. HEG and MEG are mounted with their rulings at different angles so that the dispersed spectra form a shallow X, centered on the undispersed (zeroth) diffraction order position. The HETG has been provided by the MIT in Cambridge/Massachusetts (USA). The HETG comprises 336 grating elements in total. The gratings themselves consist of fine gold bars deposited onto a polyimide foil. HETG spectra are recorded with ACIS-S, a CCD detector. Its intrinsic energy resolution provides the separation between different diffraction orders, thereby avoiding line confusing. This allows to some extent also the spectroscopy on extended objects like supernova remnants (Fig. 8.6).
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