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 
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 of ca.
6'' (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 § 3.3.10.