The `position angle' used is in actuality an `astronomical
position angle' (see XMM-MOC-TN-0109-OAD and INT-SYS-FD-TN-0004-OAD). This
angle is counted from the celestial north in the mathematically positive
sense around the axis pointing from the object towards the observer (i.e.
anticlockwise from the observer's point of view, looking towards the sky).
This is now in line with SciSim.
