Example of a quasar with a clean SDSS detection at α = 192.00,δ = 10.17 (lower panel, colour image constructed form u, g and r fiters) for which the WISE-derived concentration parameter (w1mag13≡w1mag˙1−w1mag˙3) is 1.13 because of blending with a nearby star (upper panel, single-band image with W1 flux). Concentric circles mark the aperture in which the w1mag˙1 (5.5′′) and w1mag˙3 (11′′) magnitudes were measured in WISE.