MEASURING REDDENING WITH SLOAN DIGITAL SKY SURVEY STELLAR SPECTRA AND RECALIBRATING SFD

We present measurements of dust reddening using the colors of stars with\nspectra in the Sloan Digital Sky Survey. We measure reddening as the difference\nbetween the measured and predicted colors of a star, as derived from stellar\nparameters from the SEGUE Stellar Parameter Pipeline (Lee et al. 2008a). We\nachieve uncertainties of 56, 34, 25, and 29 mmag in the colors u-g, g-r, r-i,\nand i-z, per star, though the uncertainty varies depending on the stellar type\nand the magnitude of the star. The spectrum-based reddening measurements\nconfirm our earlier "blue tip" reddening measurements (Schlafly et al. 2010,\nS10), finding reddening coefficients different by -3%, 1%, 1%, and 2% in u-g,\ng-r, r-i, and i-z from those found by the blue tip method, after removing a 4%\nnormalization difference. These results prefer an R_V=3.1 Fitzpatrick (1999)\nreddening law to O'Donnell (1994) or Cardelli et al. (1989) reddening laws. We\nprovide a table of conversion coefficients from the Schlegel et al. (1998) maps\nof E(B-V) to extinction in 88 bandpasses for 4 values of R_V, using this\nreddening law and the 14% recalibration of SFD first reported by S10 and\nconfirmed in this work.\n