XMM-Newton Users Handbook

3.4.3 RGS Order Separation

X-rays are reflected into first and second spectral orders with the highest efficiency, and thus produce the most useful data. Count rates in the third order are about 8 times lower than in second order. Depending on grating order, the RGS covers the energy ranges listed in Table 10.

Table 10: Wavelength ranges covered by the RGS in different grating orders

Order	Wavelength range [Å]
-1	6$^1$ - 38
-2	6$^1$ - 20
-3	6$^1$ - 10

$^1$ Practical limitation due to very low effective area below 6 Å. The theoretical lower limit is -5/m Å, where m is the order number.

A consequence of the diffraction equation (1) is that orders overlap on the CCD detectors of the RFC (see Fig. 78, top panel). Separation of the spectral orders is achieved by using the intrinsic energy resolution of the CCDs, which is about 160 eV FWHM at 2 keV. The dispersion of a spectrum on an RFC array is shown in the bottom panel of Fig. 78. First and second orders are very prominent and are clearly separated in the vertical direction (i.e. in CCD energy, or PI, space). Photons of higher orders are also visible for brighter sources. The calibration sources can also be seen in the bottom panel as short horizontal features.

Figure 78: Example of RGS data for a calibration observation of Capella shown with a logarithmic intensity scale. The dispersion axis runs horizontally and increases to the right. Lower dispersion angles correspond to shorter wavelengths or higher energy. The top panel shows the image of the dispersed light in the detector. The cross dispersion is along the vertical axis. The bottom panel shows the order selection plane, with the energy, PI, on the ordinate. This also illustrates the mechanism used for separation of first, second and higher grating orders. Standard data selections are indicated by the white curves. In the bottom panel, the low and high level thresholds are visible. In the top panel, the effect of fixed pattern noise at long wavelengths is seen.