Calibrating Milky Way dust extinction using cosmological sources
  We constrain the light extinction properties of Milky Way dust. We
investigated the correlations between dust column density as inferred from
infrared data and the observed colours of celestial objects at cosmological
distances with low levels of colour dispersion. Results derived using colours
of quasars, brightest central galaxies, and luminous red galaxies are broadly
consistent, indicating a proportionality constant between the reddening
E(B-V)=A_B-A_V and the dust column density D^T (given in units of MJy/sr) of
p=E(B-V)/D^T=0.02 and a reddening parameter R_V=A_V/E(B-V)=3 with fractional
uncertainties of approximately 10%. The data do not provide any evidence for
spatial variations in the dust properties, except for a possible hint of
scatter in the dust extinction properties at the longest optical wavelengths.
