tokens: continuum from decay of the metastable 2s level of H i , since that would peak at around 1450 Å and drop toward zero as the wavelength approaches Ly α , which is not observed. Rather, it is dust-scattered galactic background starlight, as also seen with HUT (Raymond et al. 1995 ). That continuum was attributed to dust-scattered light from B stars based on a private communication from J. Murthy, whose UV background observations showed that this area of the sky is unusually bright. 4. Analysis 4.1. Proper Motion We have used H α images for six epochs from 1998 to 2016, as listed in Table 1 , to measure the proper motion of the Balmer filament that delineates the bow shock. All the images were first placed on a common world coordinate system using a few hundred stars from the Gaia First Data Release (Gaia Collaboration et al. 2016 ). We then extracted a 1D profile for each epoch from a section 8′′ long, oriented normal to the shock front at the bow shock position. For each epoch pair separated by ∼4 yr or more, we carried out a fitting procedure to determine the proper motion over the intervening interval—a total of 14 independent epoch pairs. Since the 2015 and 2016 images had significantly better seeing than those at the earlier epochs, it was necessary to smooth the sharper images with an appropriate Gaussian to obtain acceptable fits. Figure 2 shows the bow shock region at three of the epochs, displayed in different colors. All the epoch pairs gave reasonably good fits and results ranging from 0314 ± 0019 yr −1 to 0357 ± 0023 yr −1 , with most of the pairs ranging from 0336 yr −1 to 0349 yr −1 . We took a simple average of all 14 measurements to obtain our final result for the proper motion of bow shock filament: 0341 ± 0010 yr −1 , where the uncertainty should include both statistical errors and possible systematic ones due to misalignment of images from different epochs. This corresponds to −0245 ± 0008 yr −1 in R.A. and +0238 ± 0008 yr −1 in decl. This value is 22% faster than the proper motion of 0280 yr −1 obtained by Winkler et al. ( 2003 ) for the brighter main portion of the NW filament in SN 1006, located a few arcminutes to the NE. It is hardly surprising that the bow shock region is moving faster, since the shock is apparently being driven ahead here by an ejecta fragment. Indeed, it was for this reason that we selected it for study with COS—to study the fastest shock that was also bright enough to enable us to obtain an ultraviolet spectrum in a reasonable amount of time. Combining our proper motion with the distance of 1.84 ± 0.14 kpc from the proper motion and shock speed of the main NW filament, we obtain a shock speed of 3030 ± 250 for the bow shock. Our long sequence of images will enable a more complete study of the proper motions of optical filaments in SN 1006, which will be reported in a separate publication. 4.2. Optical Spectrum We extracted the spectrum of a 15′′ section of the IMACS slit centered on the HST /COS
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