The results discussed above strictly pertain to Milky Way extinction for wavelengths longward of about 1150 Å (the IUE satellite cutoff). Observers of extragalactic objects must contend with foreground Milky Way extinction as well as extinction due to dust grains local to the object of interest. It is reasonable to assume that the properties of interstellar extinction in external galaxies are fully as complex as found for the Milky Way. However, only for the nearby Large Magellanic Cloud (LMC) do observed extinction curves for individual sightlines approach the accuracy needed to study spatial extinction variations (see Fitzpatrick 1998 and references within). The properties of Milky Way extinction along the high galactic latitude sightlines relevant to most extragalactic observations are also not well-determined. One of the few quantitative studies of high latitude extinction, that by Kiszkurno-Koziej & Lequeux (1987) utilizing ANS satellite data, suggests that halo extinction is slightly steeper in the far-UV and has a weaker 2175 Å bump than the typical extinction found in the plane of the Galaxy. Attempts to eliminate galactic foreground reddening by ironing out the bump may lead to systematic underestimates of the total amount of foreground extinction.
The properties of extinction in the wavelength range 912-1150 Å are poorly known, but the work that has been done (e.g., York et al. 1973; Snow et al. 1990) suggests that the far-UV rise continues at least down to wavelengths of 950 Å or so. An intrinsic problem with measuring extinction in this wavelength range is the high density of H I and H2 absorption lines which mimic continuous absorption in low-resolution data. CCM suggest that the wavelength dependence of extinction in this region is reasonably well-represented by an extrapolation of the fitting functions used in the IUE wavelength range. At the present, the best dereddening strategy for 912-1150 Å data would be to adopt such an extrapolation, based if possible on IUE extinction curves derived for the sightline of interest. The uncertainties listed in Table 1 for 1000 Å are based on this simple extrapolation.
Acknowledgements
I thank my partner-in-interstellar-grime, Derck Massa, for many years of enjoyable collaboration. I also acknowledge the contributions of the late Jason Cardelli to the study of interstellar extinction. Future progress in the field will proceed at a slower pace without Jason's enthusiasm and scientific insight. This work was supported in part by NASA grant no. NAG5-7113.