XMM-Newton Users Handbook


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3.4.4.5 RGS effective area for dispersive spectroscopy

Several parameters determine the RGS effective area per dispersion order: the properties of all optical components in the light path, the CCD quantum efficiency and the source and order filtering criteria.

The following optical components contribute:

These three components are functions of wavelength and source position angle. Finally, data selections in the RFC, used to separate orders and to suppress scattering, will determine the effective area.

To assess the total efficiency of the RGS instrument per spectral order, the efficiency with which the background is rejected and the different spectral orders can be selected must also be taken into account. This is performed by filtering in the imaging domain (cross-dispersion versus dispersion angles) and in the domain CCD-PI versus dispersion angle. Standard data selections are indicated by the white curves in Fig. 77. In the cross-dispersion direction a filter is applied which includes typically 90-97% of the total intensity. Default masks for the spatial extraction of photons from the different orders using PHA or PI-energy channel vs. dispersion coordinate plots, as in Fig. 77, are created by the offline analysis SAS tasks for each RGS . The user can also modify the extraction masks as described in the SAS User's Guide. In addition the exposure time per wavelength bin is calculated taking into account the data selections, rejected columns and pixels and the dead space between CCDs. The expected efficiency in the post-observation RGS order selection with the SAS is about 90% (0.95x0.95). This is a function of wavelength and is less at shorter wavelengths, due to additional loss of scatter outside of the extracted (dispersion-PI) region.

Fig. 83 displays the calculated effective area of both RGS units together, and Fig. 84 the individual RGS1 and RGS2 effective areas overlaid one on top of the other. The calculations have been performed taking into account all the factors listed above.

Figure 83: The effective area of both RGS units combined as a function of energy and wavelength (top and bottom horizontal scales, respectively). See text for detailed explanations.
\begin{figure}\begin{center}
\epsfig{file=figs/rgs_specarea2.eps,width=0.8\hsize,angle=0}\end{center}\end{figure}

Figure 84: The effective areas of both RGS units separately as a function of energy and wavelength (top and bottom horizontal scales, respectively). See text for detailed explanations.
\begin{figure}\begin{center}
\epsfig{file=figs/rgs_specarea1.eps,width=0.8\hsize,angle=0}\end{center}\end{figure}

Some clear features can be identified from these figures:

Fig. 85 shows the effective area as a function of the off-axis cross dispersion angle. The ratio of the median off-axis effective area to the on-axis values decreases from 0.994 at $\pm$0$^\prime $.5 to 0.83 and 0.60 at -2$^\prime $.2 and +2$^\prime $.2, respectively.

Figure 85: The RGS1 effective area as a function of cross dispersion off-axis angle.
\begin{figure}\begin{center}
\epsfig{file=figs/rgs_offaxis.ps,width=0.8\hsize,angle=0}\end{center}\end{figure}


next up previous contents
Next: 3.4.4.6 The RGS Background Up: 3.4.4 In-Flight Performance Previous: 3.4.4.4 Wavelength Scale Accuracy
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