pro centroid,pos,sig,centroid,integral
; pos inputs the pixel positions, sig inputs the signal
; centroid returns the centroid, integral returns the integral

s=size(pos)
last=s(1)-1
wsum=0.
integral=0.

for i=0,last do begin
  wsum=wsum+pos(i)*sig(i)
  integral=integral+sig(i)
  endfor

centroid=wsum/integral
return
end