This is done by controlling {HEAD ~ an adjustable optical filter} having an adjustable optical element for each of {TAIL ~ the sampling positions} 
This is done by controlling { HEAD ~ an adjustable optical filter } having an adjustable optical element for each of { TAIL ~ the sampling positions }
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL