XMM-Newton Users Handbook

3.3.3 EPIC imaging - angular resolution

EPIC's angular resolution is basically determined by the PSF of the mirror modules. This is due to the fact that the EPIC MOS and pn cameras have pixels with sizes of 40 and 150 $\mu$m, respectively. For the focal length of the X-ray telescopes (7.5 m), this corresponds to $1.1''$ ($4.1''$) on the sky. Given the FWHM of the PSF (see Table 2), the Nyquist theorem is thus fulfilled for the MOS cameras and images are fully sampled. The pixel size of the pn camera slightly undersamples the core of the PSF.

There is one notable exception to the above, namely when the count rate of an X-ray source is so high that photon pile-up on the EPIC chips plays an important role. This is described in § 3.3.9.