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