; ----------------------------------------------------------------------
;  Find rowoff value for a given arcsec value (and nso and column).
;  If IMG.el > 5. (degrees), then apply Differential Atmospheric Refraction..
;  ( Returns 'rowoff' position of object as it appears on actual image data.
;    So if a fixed arcsec is requested, that value will be adjusted by the DAR
;    (before using AVP inverse formula) such as to follow the centroid of the 
;    object along the slit. )
;
function nsxng_AVPinv, AVP, nso, col, arcsec, IMG, WSC
;  common nsx

  order = 2                  

  as = arcsec
  if (IMG.el ge 5.) then begin
     zt = 90. - IMG.el
     theta = IMG.rotposn - IMG.parang
     xv  = col - WSC[nso].xoff
     wave= cpolyval(WSC[nso].order+1,WSC[nso].coef,xv)
     as  = as + (nsxng_difatmref(wave,zt) * cos(degtorad(theta))) ;
  endif
  xv = as - AVP[nso].xoffinv[col] 
  rowoff = cpolyval( order+1, AVP[nso].coefinv[col,*], xv )
  return, rowoff

end