Extinction Correction

Extinction can dramatically alter the counts expected from your source, particularly in the ultraviolet. Figure 6.3 shows A_v / E (B - V) values applicable to our galaxy, taken from Seaton (MNRAS, 187, 73P, 1979).

Extinction curves, however, have a strong metallicity dependence, particularly at the UV wavelengths. Sample extinction curves can be seen in Koornneef and Code, ApJ, 247, 860 1981 (LMC); Bouchet et al., A&A, 149, 330 1985 (SMC); and Calzetti, Kinney and Storchi-Bergmann, ApJ, 429, 582, 1994, and references therein. At lower metallicities, the 2200 Å bump which is so prominent in the galactic extinction curve disappears, and A_v / E (B - V) increases at UV wavelengths.

The easiest way to understand how to determine the extinction correction for your source is to work through an example; see Echelle Spectroscopy of a Bright Star with Large Extinction (Sk -69° 215).

Figure 6.3: Extinction versus Wavelength