; ----------------------------------------------------------------------
;  Differential atmospheric refraction.
;  The following is good for 2km,P=600,T=7,f=8... (LAT=+/-30) from Allen ..
;  wave = wavelength in Angstroms.
;  ZA   = Apparent Zenith Angle in degrees.
;  Returns 'DAR' differential atmospheric refraction in arcseconds, 
;  positive values will be toward the blue relative to 'DAR_refwave'
;  (which is defined at 12000 Angstroms for NIRES).
; 
;  NIRES: ROTPOSN is the rotator angle which points in the direction of
;    increasing row number on the NIRES image given N=0deg and E=+90deg .
;    This means that delta row value is (given the DAR and angles and ASPP)
;      DROW = DAR * cos(theta) / ASPP,  where  theta = ROTPOSN - PARANG
;
function nsx_difatmref, wave, ZA
  common nsx
  
  R = 1.e+4 / wave                                                
  N1= 64.328 + (29498.1 / (146.0-(R*R))) + (255.4 / (41.0-(R*R))) 
  N1= N1 * (600.0 * (1.0 + (1.049 - (0.0157*7.0)) * 600.0 * 1.e-6)) 
  N1= N1 / (720.883 * (1.0 + (0.003661*7.0)))
  N1= N1 / 1.e+6
  DAR = (2.06265e+5) * (N1 - N2_refwave) * tan(degtorad(ZA))
  return, DAR
  
end