First a few words on XMM nomenclature: the XMM ``roll angle'' is defined as the rotation angle between the plane defined by the major rotation axis of the spacecraft (the spacecraft X-axis) and the spacecraft Z-axis, to that defined by the spacecraft X-axis and the spacecraft-Sun direction. The roll angle increases in a clockwise direction with respect to the positive X-axis.
The ``position angle'', on the other hand, is defined as the rotation between the spacecraft X-Z plane and the plane defined by the spacecraft X-axis and celestial North. Again, the position angle increases in a clockwise direction with respect to the positive X-axis. Note that the satellite's -Z-axis, which is also the RGS dispersion axis, is North-aligned under a position angle of , i.e. the declination axis.
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. 84) 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 horizontal 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 in terms of the 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. Users can get a rough idea of when the observation might be scheduled, by using the XMM Target Visibility Tool (§ 4.3), which outputs the average of the position angles at the start and the end of each visible period for a nominal roll angle of 0 degrees. Be aware that the roll angle is required to be kept within operationally imposed limits at all times. As a consequence of the apparent motion of the Sun in the course of a revolution this roll angle requirement might necessitate minor position angle adjustments during each period of stable pointing.