Space Telescope Science Institute
COS Instrument Handbook
help@stsci.edu
Table of Contents Previous Next Index Print


Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 7: Exposure-Time Calculator (ETC) > 7.5 Extinction Correction

7.5
Extinction can dramatically reduce the observed intensity of your source, particularly in the ultraviolet. Figure 7.3 shows Aλ/AV values applicable to our Galaxy, taken from Cardelli, Clayton, & Mathis (1989, ApJ, 345, 245) assuming RV = 3.1. This corresponds to the Milky Way Diffuse (Rv=3.1) selection of the ETC.
Extinction curves have a strong metallicity dependence, particularly at ultraviolet wavelengths. Sample extinction curves are presented in Gordon et al. [2003, ApJ, 594, 279 (LMC Average, LMC 30 Dor Shell, and SMC Bar)], Calzetti et al. [2000, ApJ, 533, 682 (starburst galaxies)], and references therein. At lower metallicities, the 2200 bump that is so prominent in the Galactic extinction curve disappears, and Aλ/E(B–V) increases at shorter UV wavelengths.
The ETC allows the user to select among a variety of extinction curves and to apply the extinction correction either before or after the input spectrum is normalized.
Figure 7.3: Extinction in Magnitude as a Function of Wavelength
The Galactic extinction model of Cardelli et al. (1989), computed for RV = 3.1.

Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 7: Exposure-Time Calculator (ETC) > 7.5 Extinction Correction

Table of Contents Previous Next Index Print