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AXAF carries two gratings assemblies (HEG and MEG) which can be introduced into the optical path just aft of the High Resolution Mirror Assembly (HRMA). These transmission gratings disperse the X-ray light roughly along the spacecraft Y-direction. Elementary optics theory shows that the dispersed light will form focused images along a circle named the Rowland Circle which passes through the undispersed image focus and the grating assembly.
The wavelength of the X-ray light can be determined by the grating equation , where is the angle of diffraction, m is the order of the image and is the wavelength. In the ACIS case the insertion of the gratings will make every point source in the field of view become a linearly elongated image. Undiffracted light is brought to the same focus, regardless of wavelength and forms the `zeroth order' image. The spectral resolution is therefore given by the spatial resolution of the HRMA/ACIS combination, and not by the intrinsic energy resolution of the CCDs.
The S array configuration of CCDs is intentionally elongated in the Y direction to maximize the wavelength coverage of the CCD array.
When the Low Energy Grating (LEG) assembly is inserted into the light path, all four HRMA mirror shell light paths are covered by the same kind of grating, which are coaligned to nearly perfectly align the dispersion along the Y axis. When the High Energy Grating (HEG) assembly is inserted two different gratings each cover a pair of mirror shells. The inner two shells (P4, P6) are covered with the highest dispersion gratings, and tipped by 5 to the Y axis. These gratings are the High Energy Transmission Gratings (HETG), and are designed for use at the highest X-ray energy (which is why they cover the inner mirrors which have the best high X-ray energy response.) The outer two shells (P1, P3) are covered with different gratings with a factor of two lower dispersion. These are the Medium Energy Transmission Gratings (METG).