Milky Way dust extinction measured with QSOs
  We investigate reddening by Milky Way dust in the low-extinction regime of
$E_{B-V}<0.15$. Using over 50,000 QSOs at $0.5<z<2.5$ from the SDSS DR7 QSO
Catalogue we probe the residual SDSS colours after dereddening and correcting
for the known spectroscopic redshifts. We find that the extinction vector of
Schlafly & Finkbeiner (2011) is a better fit to the data than that used by
Schlegel et al. (1998, SFD). There is evidence for a non-linearity in the SFD
reddening map, which is similarly present in the V1.2 map of the Planck
Collaboration. This non-linearity is similarly seen when galaxies or stars are
used as probes of the SFD map.
