pro vactoair,wave_vac, wave_air ;+ ; 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 ; Use Ciddor (1996) formula for better accuracy in the infrared ; Added optional output vector, W Landsman Mar 2011 ;- On_error,2 compile_opt idl2 if N_params() EQ 0 then begin print,'Syntax - VACTOAIR, Wave_Vac, [Wave_Air]' return endif wave_air = double(wave_vac) g = where(wave_vac GE 2000, Ng) ;Only modify above 2000 A if Ng GT 0 then begin sigma2 = (1d4/double(wave_vac[g]) )^2. ;Convert to wavenumber squared ; Compute conversion factor fact = 1.D + 5.792105D-2/(238.0185D0 - sigma2) + \$ 1.67917D-3/( 57.362D0 - sigma2) ; Convert wavelengths wave_air[g] = wave_vac[g]/fact if N_Params() eq 1 then wave_vac = wave_air endif return end