If light from the image source becomes misaligned with respect to the pupil, {HEAD ~ a physical positioning mechanism} adjusts {TAIL ~ the relative positions} of the image source and the beam combiner so that light from the image source is translated relative to the pupil, thereby realigning the display to the pupil 
adjusts
If light from the image source becomes misaligned with respect to the pupil , { HEAD ~ a physical positioning mechanism } adjusts { TAIL ~ the relative positions } of the image source and the beam combiner so that light from the image source is translated relative to the pupil , thereby realigning the display to the pupil
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH