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10.5 Extinction Correction

Extinction caused by the solid component of the interstellar medium can significantly distort flux distributions and line intensities in the ultraviolet. The IUEDAC has three procedures to correct for this. None of them are actually specific to IUE data and may be used with other spectra.

The procedure UNRED will correct for the effects of interstellar extinction using any of nine different extinction laws.


Table 10.2: UV Extinction Laws
Law Reference #
Galactic Avg. Savage and Mathis (1979) Ann. Rev. A&A 17, 73 1
Galactic Avg. Seaton (1979) MNRAS 187, 73 2
Galactic Avg. Nandy et al.  (1975) A&A 44, 195 3
Theta 2B Ori Bohlin and Savage (1981) ApJ 249, 109 4
SMC Avg. Hutchings (1982) ApJ 255, 70 5
LMC Avg. Nandy et al.  (1981) MNRAS 196, 955 6
LMC Avg. Koornneef and Code (1981) ApJ 247, 860 7
LMC 30 Dor Fitzpatrick (1985) ApJ 299, 219 8
LMC non-30 Dor Fitzpatrick (1985) ApJ 299, 219 9

Before using any of these extinction laws, you should familiarize yourself with these references. You will also need to estimate the color excess E(B-V) for the source you wish to de-redden. Use UNRED to correct your data.


Omission of the extinction law number will result in use of Savage and Mathis (1979).

UNRED will add remarks to the FITS header describing the dereddening done. If an IUESIPS H vector is included, elements 91 and 92 will be updated. If H is a scalar or undefined, it will be set to a string array containing the same remarks that would have been added to a FITS header.

Another dereddening procedure parameterized by Cardelli, Clayton, and Mathis (Ap.J. Vol 345, p. 245, 1989) is available. The parameterization is valid from 0.3 to 10 microns.


The ratio of total selective extinction R(V)=A(V)/E(B-V) may be entered via the R_V keyword. This ratio should be in the 2.75-5.3 range. If not specified, 3.1 is used.

The CCM curve shows good agreement with the Savage & Mathis (1979) UV curve longward of 1400 Å but may be better between 1200-1400 ÅUNRED_CCM does not alter the IUESIPS H vector. However, if a FITS header is entered via the HEADER keyword, remarks will be added describing the dereddening done.

A separate correction is available for interstellar Lyman-alpha absorption (see Savage 1979, Ann. Rev. Astr. Ap.):


which allows you to correct the flux for a Lorentzian absorption profile at Lyman alpha. Note that for exposure times longer than approximately 15-30 minutes, the Lyman alpha profile is contaminated by emission from the Earth's geocorona. You must correct for this emission (if present) before using LYMANA.

Both UNRED and LYMANA can be used with both IUESIPS and NEWSIPS data. In the case of UNRED the header (h) will be updated to reflect the unreddening. The header will be updated if the ``header'' keyword is used in LYMANA.

next up previous contents
Next: 10.6 Continuum Normalization Up: 10 MORE WAYS TO Previous: 10.4 Coadding Spectra