refraction_index

specutils.utils.wcs_utils.refraction_index(wavelength, method='Morton2000', co2=None)[source]

Calculates the index of refraction of dry air at standard temperature and pressure, at different wavelengths, using different methods.

Parameters:
wavelengthQuantity object (number or sequence)

Vacuum wavelengths with an astropy.unit.

methodstr, optional

Method used to convert wavelengths. Options are:

‘Morton2000’ (default) - from Morton (2000, ApJS, 130, 403), eqn 8. Used by VALD,

the Vienna Atomic Line Database. Very similar to Edlen (1966).

‘Griesen2006’ - from Greisen et al. (2006, A&A 446, 747),

eqn. 65, standard used by International Union of Geodesy and Geophysics

‘Edlen1953’ - from Edlen (1953, J. Opt. Soc. Am, 43, 339). Standard

adopted by IAU (resolution No. C15, Commission 44, XXI GA, 1991), which refers to Oosterhoff (1957) that uses Edlen (1953). Also used by Morton (1991, ApJS, 77, 119), which is frequently cited as IAU source.

‘Edlen1966’ - from Edlen (1966, Metrologia 2, 71), rederived constants

from optical and near UV data.

‘PeckReeder1972’ - from Peck & Reeder (1972, J. Opt. Soc. 62), derived

from additional infrared measurements (up to 1700 nm).

‘Ciddor1996’ - from Ciddor (1996, Appl. Opt. 35, 1566). Based on

Peck & Reeder (1972), but updated to account for the changes in the international temperature scale and adjust the results for CO2 concentration. Arguably most accurate conversion available.

Note that all options except for ‘Griesen2006’ have singularities in the far UV. ‘Griesen2006’ gives values that are slightly inconsistent with the other methods (~0.07 Angstrom difference at visible wavelengths), but it is the best option in the FUV due to the mathematical singularities in the others. See https://specutils.readthedocs.io/en/latest/wcs_utils.html for more detail, or https://github.com/astropy/specutils/issues/1162 for additional context.

co2number, optional

CO2 concentration in ppm. Only used for method=’Ciddor1996’. If not given, a default concentration of 450 ppm is used.

Returns:
refrnumber or sequence

Index of refraction at each given air wavelength.