STIS Instrument Handbook for Cycle 25
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Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 25 > Chapter 6: Exposure Time Calculations > 6.7 Extinction Correction

6.7
Extinction can dramatically alter the counts expected from your source, particularly in the UV. Extinction curves, however, have a strong metallicity and environment dependence, and this variation is largest at 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 bump, which is so prominent in the galactic extinction curve, disappears and Aλ/E(B–V) increases at UV wavelengths. However, as discussed by Gordon et al. 2003 (ApJ, 594, 279), the extinction curve for any sight line depends on the local environment, and even within the Magellenic clouds, many examples can be found that are closer to the typical Galactic extinction law than to the Koornneef & Code curve.
The ETC now offers four Galactic extinction curves from Cardelli, Clayton, & Mathis 1989 (ApJ, 345, 245), with RV values ranging from 2.1 to 5.0. Examples of these curves are shown in Figure 6.3. Of these, the “Milky Way Diffuse” curve with RV=3.1 is the default setting and a good choice for an average Galactic extinction curve. Also available are curves for the LMC Average, the LMC Supershell, and the SMC bar extinction from Gordon (2003), as well as the extragalactic Starburst attenuation law of Calzetti. Further details of the extinction curves implemented in the ETC can be found at:
http://etc.stsci.edu/etcstatic/users_guide/1_ref_7_ebv.html
Figure 6.3: Galactic Extinction versus Wavelength from Cardelli, Clayton, & Mathis (1989) for a variety of RV values.

Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 25 > Chapter 6: Exposure Time Calculations > 6.7 Extinction Correction

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