XMM Users' Handbook

   
RGS grating orders

 

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

Order	Energy range [keV]
-1	0.35 - 2.51
-2	0.62 - 2.51
-3	1.20 - 2.51

Note to Table 10:
1) Practical limitation due to very low effective area above 2.5 keV. The theoretical upper limit is 2.5 keV $\times$ m, where m is the order number.

X-rays are reflected into spectral orders -1 and -2 with the highest efficiency, so these are the orders expected to produce useful data in the majority of observations. Count rates in the -3. order are about 8 times lower than in the -2. Depending on grating order, the RGS covers the energy ranges listed in Table 10.

The exact location of a source spectrum on the RGS CCD chips depends on the source's location within the field of view of the X-ray telescopes. For a target on-axis the observed spectrum is well centred on the RFC chips in the cross-dispersion direction and the wavelength scale is known with the best calibration accuracy. The wavelength scale for sources off-axis in the cross-dispersion direction is the same as for those on-axis, but will be different for sources off-axis in the dispersion direction.

  

Figure 47: The dispersion along the dispersion coordinate, Z (in mm), vs. CCD PHA-channel output of an RGS spectrum in -1. and -2. grating orders onto the RGS focal cameras, assuming equal gains of all CCD output nodes. This also illustrates the mechanism used for separating spectral orders.

Due to the grating relation ( $m\times\lambda$), the orders overlap spatially on the CCD detectors of the RFC. Separation of the spectral orders is achieved by using the CCDs' intrinsic energy resolution. The dispersion of a spectrum onto an RFC array is shown in Fig. 47. The -1. (lower) and -2. order (next) are most prominent and are clearly separated in the vertical direction (i.e., in CCD PHA space). Photons of higher orders are also visible.