Correcting for the effects of interstellar extinction

This paper addresses the issue of how best to correct astronomical data for the wavelength-dependent effects of Galactic interstellar extinction. The main general features of extinction from the IR through the UV are reviewed , along with the nature of observed spatial variations. The enormous range of extinction propel-ties found in the Galaxy, particularly in the UV spect ral region, is illustrated. Fortunately, there are some tight constraints o n the wavelength dependence of extinction and some general correlations bet ween extinction curve shape and interstellar environment. These relationshi ps provide some guidance for correcting data for the effects of extinction. Several strategies for dereddening are discussed along with estimates of t he uncertainties inherent in each method. In the Appendix, a new derivation of the wavelength dependence of an average Galactic extinction curve from the TR through the UV is presented, along with a new estimate of how this e xtinction law varies with the parameter R = A (V)/E(B-V). These curves repr esent the true monochromatic wavelength dependence of extinction and, as su ch, are suitable for dereddening IR-UV spectrophotometric data of any resol ution and can be used to derive extinction relations for any photometry sys tem.