XMM-Newton 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.
Since the beginning of mission operations, the instrument boresights have been accommodated to allow placement of the target at the preferred location for either the RGS or the EPIC instruments. For virtually all cases this represents an acceptable compromise. When EPIC-pn is selected as prime instrument, some Ne lines fall on or near the gap between CCDs (see Table 9 for wavelength and energy ranges covered by the chips of each RGS camera). Conversely, when RGS is selected as prime instrument, a point source can be located closer to the edge of a window than would be preferred for the maximum encircled energy coverage.
The user can always choose an off-axis pointing, but this requires specification of a position angle which implies reduced visibility. The user can use the SAS task `srcinfov' (which is part of the `bstools' task package) to determine these choices.