VACTOAIR

       Convert vacuum wavelengths to air wavelengths

       Corrects for the index of refraction of air under standard conditions.
       Wavelength values below 2000 A will not be altered.  Accurate to
       about 0.005 A

       VACTOAIR, WAVE

       WAVE - Wavelength in Angstroms, scalar or vector
               WAVE should be input as vacuum wavelength(s), it will be
               returned as air wavelength(s).  WAVE is always converted to
               double precision

       If the vacuum wavelength is  W = 2000, then
       IDL> VACTOAIR, W
       yields an air wavelength of W = 1999.353 Angstroms

       An approximation to the 4th power of inverse wavenumber is used
       See IUE Image Processing Manual   Page 6-15.

       Written, D. Lindler 1982
       Documentation W. Landsman  Feb. 1989
       Converted to IDL V5.0   W. Landsman   September 1997

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