XMM-Newton
Users Handbook
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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 - 38 |
-2 |
6 - 20 |
-3 |
6 - 10 |
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. 77, 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. 77. 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 77:
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.
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Next: 3.4.4 In-Flight Performance
Up: 3.4 REFLECTION GRATING SPECTROMETER (RGS)
Previous: 3.4.2 RFC chip arrays
European Space Agency - XMM-Newton Science Operations Centre