The orientation of the XMM-Newton instruments is such that the astronomical
position angle gives the angular distance of the RGS dispersion axis
measured from the celestial North towards East. With regard to the
EPIC instruments, their orientations relative to the RGS dispersion
axis are shown in Figs. 20 to
22. Examples of the dependence of the EPIC field
of view on the position angle can be found in
Figs. 17-19. The orientation of
the celestial North and East axes over the EPIC-pn detector for a
30
position angle is shown in Fig. 22.
Both RGS and OM with its grisms, can place position angle constraints on observations, if one wants to avoid spatial overlaps of the dispersed spectra of adjacent sources (i.e. to avoid sources to be aligned with the dispersion direction). In the case of RGS, avoidance of bright sources outside the field of view, as described in § 3.4.6.1, can also lead to position angle constraints (see § 4.4.2).
There is no default orientation constraint on XMM-Newton observations. Any constraint imposed on an observation makes it ``time-critical'' and thereby more difficult to schedule.
Users can determine the best position angle at which to obtain their planned observations while avoiding nearby bright sources, using SciSim (Appendix A) as follows.
The SciSim GUI (Appendix A.2, Fig. 106) shows the field of view (FOV) of the spacecraft when viewed along the -X axis of the spacecraft, i.e., from the focal plane towards the mirrors and outwards. The tool also displays the RGS dispersion axis, in the form of two blue vertical lines, the separation of these lines representing the cross-dispersion direction extent of the illuminated part of the CCD chip array. This view can be rotated to reflect changing position angle of the spacecraft.
It is therefore a simple matter of creating a model field in SciSim containing the sources which could potentially cause confusion, and finding a suitable position angle value for which these sources are not aligned along the RGS dispersion axis. Sources can be defined in SciSim by clicking on ``Sources'' and using the Source editor GUI, which, amongst other properties, allows the user to enter target coordinates.
After having chosen the most appropriate position angle for a given observation, the user can check in the Target Visibility Checker the orbits for which the target is visible for a minimum observing time and the position angle constraint is met.