Correcting for the Effects of Interstellar Extinction

This paper addresses the issue of how best to correct astronomical data forthe wavelength-dependent effects of Galactic interstellar extinction. The maingeneral features of extinction from the IR through the UV are reviewed, alongwith the nature of observed spatial variations. The enormous range ofextinction properties found in the Galaxy, particularly in the UV spectralregion, is illustrated. Fortunately, there are some tight constraints on thewavelength dependence of extinction and some general correlations betweenextinction curve shape and interstellar environment. These relationshipsprovide some guidance for correcting data for the effects of extinction.Several strategies for dereddening are discussed along with estimates of theuncertainties inherent in each method. In the Appendix, a new derivation of thewavelength dependence of an average Galactic extinction curve from the IRthrough the UV is presented, along with a new estimate of how this extinctionlaw varies with the parameter R = A(V)/E(B-V). These curves represent the truemonochromatic wavelength dependence of extinction and, as such, are suitablefor dereddening IR--UV spectrophotometric data of any resolution, and can beused to derive extinction relations for any photometry system.Comment: To appear in PASP (January 1999) 14 pages including 4 pages of figures Uses emulateapj style. PASP, in press (January 1999