Vacuum Wavelengths

Conversion between vacuum and air wavelengths

Because the SDSS observes many quasars at rest frame ultraviolet wavelengths, the data are stored in vacuum wavelengths.  However, most optical astronomers know the wavelengths of transitions as measured at S.T.P., which is how the CRC lists them for any transitions redward of 2000 Angstroms.

The IAU standard for conversion from air to vacuum wavelengths is given in Morton (1991, ApJS, 77, 119). For vacuum wavelengths (VAC) in Angstroms, convert to air wavelength (AIR) via:

  AIR = VAC / (1.0 + 2.735182E-4 + 131.4182 / VAC^2 + 2.76249E8 / VAC^4)

These are the air and vacuum wavelength of some common transitions:

There is an IRAF tool to convert air to vacuum wavelengths in Pat Hall's IRAF tools for SDSS spectra.