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 § 18.104.22.168, 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 check in the Target Visibility Checker possible values of position angles in orbits for which a given target is visible for a minimum observing time. During proposal phase II submission (see XRPS Users Manual), PIs of accepted observations can ask the SOC for enhancement support7 in the XRPS form in order to optimize the position angle.