Measuring Reddening with SDSS Stellar Spectra and Recalibrating SFD
  We present measurements of dust reddening using the colors of stars with
spectra in the Sloan Digital Sky Survey. We measure reddening as the difference
between the measured and predicted colors of a star, as derived from stellar
parameters from the SEGUE Stellar Parameter Pipeline (Lee et al. 2008a). We
achieve uncertainties of 56, 34, 25, and 29 mmag in the colors u-g, g-r, r-i,
and i-z, per star, though the uncertainty varies depending on the stellar type
and the magnitude of the star. The spectrum-based reddening measurements
confirm our earlier "blue tip" reddening measurements (Schlafly et al. 2010,
S10), finding reddening coefficients different by -3%, 1%, 1%, and 2% in u-g,
g-r, r-i, and i-z from those found by the blue tip method, after removing a 4%
normalization difference. These results prefer an R_V=3.1 Fitzpatrick (1999)
reddening law to O'Donnell (1994) or Cardelli et al. (1989) reddening laws. We
provide a table of conversion coefficients from the Schlegel et al. (1998) maps
of E(B-V) to extinction in 88 bandpasses for 4 values of R_V, using this
reddening law and the 14% recalibration of SFD first reported by S10 and
confirmed in this work.
