wherein {HEAD ~ a focusing servo unit} focuses the return laser beam and then checks {TAIL ~ the focused image} of the return laser beam to compensate for vertical movements of the holographic medium 
checks
wherein { HEAD ~ a focusing servo unit } focuses the return laser beam and then checks { TAIL ~ the focused image } of the return laser beam to compensate for vertical movements of the holographic medium
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH