def vac2air(wave,sdssweb=False):
    """
    NAME:
       vac2air
    PURPOSE:
       Convert from vacuum to air wavelengths (See Allende Prieto technical note: http://hebe.as.utexas.edu/apogee/docs/air_vacuum.pdf)
    INPUT:
       wave - vacuum wavelength in \AA
       sdssweb= (False) if True, use the expression from the SDSS website (http://classic.sdss.org/dr7/products/spectra/vacwavelength.html)
    OUTPUT:
       air wavelength in \AA
    HISTORY:
       2014-12-04 - Written - Bovy (IAS)
       2015-04-27 - Updated to CAP note expression - Bovy (IAS)
    """
    if sdssweb:
        return wave/(1.+2.735182*10.**-4.+131.4182/wave**2.+2.76249*10.**8./wave**4.)
    else:
        return wave/(1.+0.05792105/(238.0185-(10000./wave)**2.)+0.00167917/(57.362-(10000./wave)**2.))