XMM has three independent X-ray telescopes and an optical/UV telescope,
with a total of six science instruments. Neither the telescopes' optical
axes nor the centre positions of the detector FOVs are aligned perfectly.
Thus, radiation from sources in the spacecraft's boresight direction does
not fall onto the nominal on-axis position of all detectors simultaneously.
The specification for the XMM instrument alignment in the focal plane
allows for offsets from perfect alignment with respect to each other
of up to ca. 
.
This means that two detectors can be misaligned
with respect to each other by up to 2' on the sky.
This is the reason why it is important to specify in an XMM
observing proposal which instrument should be the prime detector so
that the target is properly centred in the FOV of the detector that is
most important for the science objectives of the observing programme.
Measurements of the relative alignment and the creation of a correction
matrix will ensure that the primary instrument will be pointed on source
with an expected accuracy of the order of 15''. The offsets of the
X-ray source position from the centre position of the other detectors
can be much larger, up to ca. 2', as mentioned above. The actual
alignment of the instruments is not yet known, because the XMM flight
hardware has not yet been finally integrated and tested.