Package airvacuumvald

Python implementation of the air-to-vacuum and vacuum-to-air wavelength conversion used by VALD (Vienna Atomic Line Database).

Quoting http://www.astro.uu.se/valdwiki/Air-to-vacuum%20conversion:

VALD3 stores vacuum wavelengths of all the transitions. This allows uniform handling of
extraction across the whole spectral range. On the other hand, the selection tools in VALD3
include options for returning the air wavelengths. Furthermore, some of the original line lists
include wavelengths measured in the air. This calls for conversion tools.
Such tools must be uniformly accurate and reversible across the whole spectral range.

For the vacuum to air conversion the formula from Donald Morton (2000, ApJ. Suppl., 130, 403)
is used for the refraction index, which is also the IAU standard:

n = 1 + 0.0000834254 + 0.02406147 /
    (130 - s2) + 0.00015998 /
    (38.9 - s2)

, where s = 104 / λvac and λvac is in Ångströms.

The conversion is then: λair = λvac / n.

This formula comes from Birch and Downs (1994, Metrologia, 31, 315) and applies to dry air at
1 atm pressure and 15ºC with 0.045% CO2 by volume. The corrections to
Edlén (1953, J. Opt. Soc. Am., 43, 339) are less than
0.0001 Å at 2000 Å and less than 0.001 Å at 30000 Å.

The opposite conversion (air-to-vacuum) is less trivial because n depends on λvac and
conversion equations with sufficient precision are not readily available.
VALD3 tools use the following solution derived by N. Piskunov:

n = 1 + 0.00008336624212083 + 0.02408926869968 /
    (130.1065924522 - s2) + 0.0001599740894897 /
    (38.92568793293 - s2), where s = 104 / λair

and the conversion is: λvac = λair * n.

Installation

pip install airvacuumvald

Example

The following code reproduces the figure shown in VALD3 Wiki (http://www.astro.uu.se/valdwiki/Air-to-vacuum%20conversion) ("comparison of the Morton and the inverse transformation by NP between 2000 Å and 100000 Å.")

import matplotlib.pyplot as plt
import numpy as np
import airvacuumvald as avv
lambda_vac = 10**np.linspace(np.log10(2000), np.log10(1000000), 2000)
y = ph.air_to_vacuum(ph.vacuum_to_air(lambda_vac))-lambda_vac
plt.semilogx(lambda_vac, y)
plt.xlabel("$\lambda$ in Angstroem")
plt.ylabel("$\Delta\lambda$")
plt.xlim([lambda_vac[0]-50, lambda_vac[-1]])
plt.title("air_to_vacuum(vacuum_to_air($\lambda_{vac}$))-$\lambda_{vac}$")
plt.tight_layout()
plt.show()

Running this code generates the following figure:

More examples

The following code converts wavenumber=2000 cm**-1 to wavelength in Angstrom:

>>> import airvacuumvald as avv
>>> print(avv.vacuum_to_air(1e8/2000))
49986.36934549974

The following code converts wavelength=5500 Angstrom to wavenumber in cm**-1:

>>> import airvacuumvald as avv
>>> print(1e8/avv.air_to_vacuum(5500.))
18176.768046090445

Source code

airvacuumvald/airvacuum.py