XMM-Newton Users Handbook

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Next: 3.2.3 Response matrices used in the SAS data analysis Up: 3.2.2 X-ray effective area Previous: 3.2.2.1 On-axis effective area


3.2.2.2 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-Newton telescopes, which is reflected by the decline of the X-ray telescope's effective area as a function of off-axis angle, is displayed for a few energies for the X-ray telescope 2 in Fig. 13.

Figure 13: Vignetting function as a function of off-axis angle (0$'$-15$'$, based on simulations), at a few selected energies, of the X-ray telescope 2 in front of the pn camera
\begin{figure}\begin{center} \leavevmode \epsfig{height=0.56\hsize, file=figs/PNvignetting.eps} \end{center} \end{figure}

The off-axis vignetting in the two telescopes with an RGA has a dependence on angle with respect to the RGAs' dispersion direction. A source at an off-axis position perpendicular to the dispersion direction will be vignetted by a different amount to one at a position parallel to the dispersion direction. This azimuthal dependency of the vignetting is shown in Fig. 14.

Figure 14: Vignetting function as a function of azimuth angle of the X-ray telescope 3 in front of the MOS1 camera. The curves are given for an off-axis angle of 10 arcmin. Due to the presence of reflection grating assemblies in the exit beams of the X-ray telescopes 3 and 4, the vignetting functions measured in the MOS cameras are modulated azimuthally.
\begin{figure}\begin{center} \leavevmode \epsfig{height=0.56\hsize, file=figs/MOS1vig10arcmin.eps} \end{center} \end{figure}

next up previous contents
Next: 3.2.3 Response matrices used in the SAS data analysis Up: 3.2.2 X-ray effective area Previous: 3.2.2.1 On-axis effective area
European Space Agency - XMM-Newton Science Operations Centre