; ----------------------------------------------------------------------
;  Compute airmass....    taken from ast-util.c in MAKEE -tab 10oct2012
;  The Earth's average radius is about 6370 km.  The Atmosphere's thickness is
;  about 11.6 km (level of tropopause at about 41 degrees latitude).
;  Divide the Earth's average radius by the atmosphere's thickness.
; 
;      r = 6370.d0 / 11.6d0
;
;  This formula is very inaccurate for zenith angles greater than 80.
;  Note that the apparent zenith angle is used here.
;  Returns airmass given the apparent zenith angle in degrees.
;  Returns -99. if problem (like azd > 80).
;*/
function gal_airmass, azd 
  
  r = 6370.d / 11.6d
  PI    = 3.1415926535898d

  deg = !dpi / 180.d            ;>

  if ((azd lt 0.)||(azd ge 80.)) then return, -9.
  rr = r * cos((PI - deg*(azd) ))
  airmass = rr + sqrt( (rr*rr) + 1. + (2. * r) ) 
  return, airmass                              
end