XMM-Newton Users Handbook

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

$$\cos \beta = \cos \alpha + m \lambda / {\rm d}$$ (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.