XMM Users' Handbook

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Off-axis effective area

Not only the shape of the X-ray PSF, but also the effective area of the mirrors is a function of off-axis angle within the mirrors' 30' FOV. With increasing off-axis angle, less of the photons entering the telescopes actually reach the focal plane. This effect is called ``vignetting''. The vignetting of the XMM telescopes, which is reflected by the decline of the mirror module's effective area as a function of off-axis angle, is displayed for a few energies in Fig. 14.

**Figure 14:** *The total effective area,* A_e, of all **XMM** *mirror modules, at a few selected energies, as a function of off-axis angle (0*'*-15*'*). The numbers shown here do not include any reductions due to detector responses.*
$\begin{figure} \begin{center} \leavevmode \epsfig{width=0.6\hsize, angle=270, file=figs/eff_area_offaxis.eps} \end{center} \end{figure}$

European Space Agency - XMM Science Operations Centre