During periodic calibration, sampling the output of {HEAD ~ dark reference cells} on {TAIL ~ a CCD} nets an initial D.C. restoration voltage level 
During periodic calibration , sampling the output of { HEAD ~ dark reference cells } on { TAIL ~ a CCD } nets an initial D.C. restoration voltage level
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH