Illustration of the correction. Red dots show the initial extinction for all targets of the photometric catalogue above 40° Galactic latitude as a function of the optical thickness of the interstellar dust at 353 GHz, as measured by Planck (Planck Collaboration XI 2014). Black dots show the corrected extinction for the same targets. A linear fit to Planck τ353 gives a significantly lower χ² value for the corrected extinctions, showing that some biases have been corrected.