XMM-Newton Users Handbook


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3.4.1 Diffraction Geometry

The diffraction geometry of the reflection gratings is illustrated in Fig. 75. Light strikes the gratings at an angle of incidence $\alpha $ with respect to the plane of the grating, and emerges at angle $\beta$ given by the dispersion equation


\begin{displaymath}
\cos \beta = \cos \alpha + m \lambda / {\rm d}
\end{displaymath} (1)

where $\lambda$ is the radiation wavelength, d the grating spacing, and m the spectral order. The RGS is designed for use with negative orders, $\beta > \alpha$.

Figure 75: Schematic drawing of a grating, including some of the key dispersion angles.
\begin{figure}\begin{center}
\epsfig{width=.52\hsize,file=figs/rgs_grating.eps}\end{center}\end{figure}



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