Locations of background stars on the optical depth τ(250  μm) map from Herschel. The contour labels give τ(250  μm) values multiplied by 1000. Different symbols have been used to distinguish between stars from the three near-IR surveys: 2MASS (green asterisks), HAWK-I (red crosses), and SOFI (white plus signs). Of the HAWK-I and 2MASS stars only those within a radius of the map centre are shown here.