XMM-Newton Users Handbook

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Next: 3.2.2 X-ray effective area Up: 3.2.1 X-ray point-spread function Previous: 3.2.1.1 On-axis PSF


3.2.1.2 Off-axis PSF

The PSF of the X-ray telescopes depends on the source off-axis angle, i.e., its distance from the centre of the field of view (FOV). It also depends slightly on the source azimuth within the FOV. In Fig. 8 the dependence of the shape of the XMM-Newton X-ray PSF on the position within the FOV is presented. One can see that the PSF at large off-axis angles is elongated due to off-axis aberration (astigmatism). The reader is also referred to Figs. 17,  18 and 19 for additional impressions of the off-axis PSF.

For the two EPIC MOS cameras, the PSF is also affected at a level of a few times $10^{-4}$ (integral relative intensity) by scattering off the RGA rib structures. This contribution, however, is negligible in the vast majority of cases.


It is important to know the encircled energy fraction at different positions in the FOV, e.g., to use the correct extraction areas for source photons. In Figs. 9 and 10 we show the dependence of the radius at which 90% of the total energy is encircled ($W90$) on the off-axis angle. Note that as the off-axis angle increases the energy dependence of the PSF changes as focusing of high energy photons is no longer confined to the inner shells. A large fraction of these high energy photons are redistributed into the wings of the PSF by X-ray scattering hence $W90$ is larger at high energies than at soft energies for large off-axis angles.

Figure 8: The dependence of the X-ray PSF's shape on the position in the field of view. This image was made from an observation towards the Orion molecular cloud. EPIC pn, MOS1 and MOS2 exposures have been merged together and exposure corrected. The data has been slightly smoothed with a Gaussian of 10" FWHM. The intensity scale is square root.
\begin{figure}\begin{center} \leavevmode \epsfig{width=1.0\hsize, file=figs/off_axis_psfs.ps} \end{center} \end{figure}

Figure 9: The $W90$ radius of a point source observed with the MOS camera as a function of off-axis angle at different energies. The curves were calculated assuming a fractional encircled energy of 100% at a radial distance of 5 arcmin, independent of the off-axis angle.
\begin{figure}\begin{center} \leavevmode \epsfig{height=0.56\hsize, file=figs/encE_offaxis_MOS.eps} \end{center} \end{figure}

Figure 10: The $W90$ radius of a point source observed with the pn camera as a function of off-axis angle at different energies. The curves were calculated assuming a fractional encircled energy of 100% at a radial distance of 5 arcmin, independent of the off-axis angle.
\begin{figure}\begin{center} \leavevmode \epsfig{height=0.56\hsize, file=figs/encE_offaxis_pn.eps} \end{center} \end{figure}

next up previous contents
Next: 3.2.2 X-ray effective area Up: 3.2.1 X-ray point-spread function Previous: 3.2.1.1 On-axis PSF
European Space Agency - XMM-Newton Science Operations Centre