Both sets of objective transmission gratings consist of hundreds of co-aligned facets mounted to supporting structures on 4 annuli (one for each of the four co-aligned mirror pairs) to intercept the x rays exiting the HRMA. In order to optimize the energy resolution, the grating support structure holds the facets close to the Rowland toroid that intercepts the focal plane. The two sets of transmission gratings, attached to the mounting structure are shown in Figure .
Figure: The LETG and HETG being attached to the spacecraft mounting structure.
The Low-Energy Transmission Grating (LETG) provides high-resolution spectroscopy at the lower end of the CXO energy range. Dr. A Brinkman, of the Space Research Organization of the Netherlands, is the Principal Investigator. The LETG was developed in collaboration with the Max Planck Institut für Extraterrestische Physik, Garching. The LETG has 540 1.6-cm diameter grating facets, 3 per grating module. Ultraviolet contact lithography was used to produce an integrated all-gold facet bonded to a stainless-steel facet ring. An individual facet has 0.43-m-thick gold grating bars with 50% filling factor and 9920- period, resulting in 1.15-/mm dispersion. The HRC-S is the primary LETG readout.
The High-Energy Transmission Grating (HETG) provides high-resolution spectroscopy at the higher end of the CXO energy range. Prof. C. Canizares of the Massachusetts Institute of Technology Center for Space Research is the Principal Investigator. This group developed the instrument in collaboration with MIT's Nanostructures Laboratory. The HETG has 336 2.5-cm square grating facets. Microlithographic fabrication using laser interference patterns was used to produce the facets, which consist of gold grating bars with 50% filling factor on a polyimide substrate. The HETG uses gratings with 2 different periods which are oriented to slightly different dispersion directions, forming a shallow "X" image on the readout detector as shown in Figure .
Figure: The first observation of Capella with the HETG/ACIS-S combination.
The Medium-Energy Gratings (MEG) have 0.40-m-thick gold bars on 0.50-m-thick polyimide with 4000- period, producing 2.85-/mm dispersion, and are placed behind the outer two CXO mirrors. The High-Energy gratings (HEG), placed behind the inner two CXO mirror pairs are 0.70-m -thick-gold bars on 1.0-m-thick polyimide with 2000- period, resulting in 5.7-/mm dispersion. The ACIS-S is the primary readout for the HETG.