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ovac2air stis.ovac2air



ovac2air -- Convert wavelengths from vacuum to air.


ovac2air input <output>


The task ovac2air will convert the wavelengths in the WAVELENGTH column of 1-D extracted STIS spectra from vacuum to air. Only wavelengths longer than 2000 Angstroms will be converted; shorter wavelengths will not be modified. See below for further details.

If both input and output file names are given, the input file is first copied to output, and ovac2air operates on the output file. If only one name is given, ovac2air modifies that file in-place.

The input file should be an _x1d.fits or _sx1.fits file, i.e. the table written by 1-D extraction. There can actually be multiple tables in the one file, e.g. for repeatobs data; all tables in the file will be processed. After correction, a HISTORY record will be added to the table header (not to the primary header) saying that the correction was applied.

The expression for the index of refraction of air at "standard conditions" is

    ns = 1 + 643.28e-7 + 294981.e-7 / (146. - sigma**2) +
                           2554.e-7 / (41. - sigma**2)

where sigma is the wavenumber, the reciprocal of the wavelength in micrometers. This is for dry air with 0.03 percent carbon dioxide, at a pressure of 101325 Pa (760 mm mercury) and a temperature of 288.15 K (15 C).

To correct to air wavelengths at a different temperature or pressure, use tcalc instead of ovac2air, and modify (ns-1) using either of the following expressions:

    n - 1 = (ns - 1) * (P / Ps) * (Ts / T)


        T = temperature in Kelvins
        P = pressure in Pascals
        Ts = 288.15 K
        Ps = 101325 Pa
        ns = index of refraction of air given above
        (see Cox, p 262)

    n - 1 = (ns - 1) * P * (1 + P * beta(T)) * (1 + 15*alpha) /
                     (760 * (1 + 760*beta15) * (1 + T*alpha))


        T = temperature in degrees C
        Ts = 15 degrees C
        P = pressure in mm of mercury
        Ps = 760 mm
        alpha = 0.00366
        beta(T) = (1.049 - 0.015*T)*1.e-6
        beta15 = 8.13e-7
        ns = index of refraction of air given above
        (see AIP Handbook, p 6-111)


Input file name containing 1-D extracted spectra.
If this parameter is null, the input file will be modified in-place. If an output file was specified, the input file will be copied to output, and the WAVELENGTH column in that copy will be modified.


1. Convert the wavelengths in o47s01k9m_x1d.fits from vacuum to air, writing the output to o47s_air_x1d.fits:

	cl> ovac2air o47s01k9m_x1d.fits o47s_air_x1d.fits



This task was written by Phil Hodge.

The expression for the index of refraction of air is from Edlen, B., 1953, J Opt Soc Amer, vol 43, p 339; or see 2000, Allen's Astrophysical Quantities, edited by A.N. Cox, p 262; or the American Institute of Physics Handbook, edited by D.E. Gray, p 6-111.



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