idlastro / Astronomical Utilities: VACTOAIR

[Source code]

NAME
VACTOAIR
PURPOSE
Convert vacuum wavelengths to air wavelengths
EXPLANATION
Corrects for the index of refraction of air under standard conditions.  
Wavelength values below 2000 A will not be altered.  Accurate to 
about 10 m/s.
CALLING SEQUENCE
VACTOAIR, WAVE_VAC, [WAVE_AIR]
INPUT/OUTPUT
WAVE_VAC - Vacuum Wavelength in Angstroms, scalar or vector
        If the second parameter is not supplied, then this will be
       updated on output to contain double precision air wavelengths.
OPTIONAL OUTPUT
WAVE_AIR - Air wavelength in Angstroms, same number of elements as
         WAVE_VAC, double precision
EXAMPLE
If the vacuum wavelength is  W = 2000, then 
IDL> VACTOAIR, W 
yields an air wavelength of W = 1999.353 Angstroms
METHOD
Formula from Ciddor 1996  Applied Optics , 35, 1566
REVISION HISTORY
Written, D. Lindler 1982 
Documentation W. Landsman  Feb. 1989
se Ciddor (1996) formula for better accuracy in the infrared 
   Added optional output vector, W Landsman Mar 2011