In the most precise model, {HEAD ~ the mercury surface} is used to autocollimate {TAIL ~ the line} of sight through the center of cross hairs to be horizontal for every pointing 
is used to autocollimate
In the most precise model , { HEAD ~ the mercury surface } is used to autocollimate { TAIL ~ the line } of sight through the center of cross hairs to be horizontal for every pointing
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH