XMM Users' Handbook

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Next: Components of the RGS Up: REFLECTION GRATING SPECTROMETER (RGS) Previous: RGS grating orders

   
RGS spectral resolution

The two RGS units operate independently of one another. Their LSFs depend on the shapes of the telescopes' PSF behind which they are located and on the characteristics of the RGS units.

The PSF of the XMM telescopes presented above (§ 3.2.1.1) thus determines the width of that RGS's LSF core and thus its capability to resolve X-ray lines. The spectral resolution for a point source is displayed in Figs. 48-50 for both RGS units separately. Fig. 48 displays the HEW as a function of energy, E, Fig. 49 shows the FWHM.


  
Figure 48: The resolving power ( HEW) of both RGS in the -1. and -2. grating orders.
\begin{figure} \begin{center} \leavevmode \epsfig{width=0.6\hsize, angle=270, file=figs/rgs_specres2.eps} \end{center} \end{figure}


  
Figure 49: The resolving power ( FWHM) of both RGS in the -1. and -2. grating orders.
\begin{figure} \begin{center} \leavevmode \epsfig{width=0.6\hsize, angle=270, file=figs/rgs_specres1.eps} \end{center} \end{figure}

In order to assess whether two closely spaced spectral lines can be resolved, the resolving power based on FWHM is the appropriate measure. To evaluate the detectability of a weak feature against a strong continuum, the resolving power based on HEW is the appropriate figure.


Fig. 50 shows the spectral resolving power ( $\lambda / \Delta \lambda $ = E/$\Delta $E) as a function of wavelength. In the -1. order, RGS-1 has an almost constant resolution in wavelength space, of ca. 0.04 Å.


  
Figure 50: The resolving power ( $\lambda / \Delta \lambda $ = E/$\Delta $E) of both RGS in -1. and -2. grating order.
\begin{figure} \begin{center} \leavevmode \epsfig{width=0.6\hsize, angle=270, file=figs/rgs_specres3.eps} \end{center} \end{figure}

An example for an individual line, the Al-K line at 1.49 keV, along the dispersion direction, is displayed in Fig. 51. One can see the narrow core of the LSF and the underlying broad wings. The same line, perpendicular to the dispersion direction, is presented in Fig. 52.


  
Figure 51: A close-up view of the Al K line at 1.49 keV, from an RGS model spectrum produced with SciSim, along the dispersion direction.
\begin{figure} \begin{center} \leavevmode \epsfig{width=0.6\hsize, angle=270, file=figs/rgs_alk_disp.eps} \end{center} \end{figure}


  
Figure 52: A close-up view of the Al K line at 1.49 keV, from an RGS model spectrum produced with SciSim, along the cross-dispersion direction.
\begin{figure} \begin{center} \leavevmode \epsfig{width=0.6\hsize, angle=270, file=figs/rgs_alk_xdisp.eps} \end{center} \end{figure}


 
next up previous contents
Next: Components of the RGS Up: REFLECTION GRATING SPECTROMETER (RGS) Previous: RGS grating orders
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