Spatial variation of the PSF in our best-seeing stack of the HAWK-I K_s observationsof RCS2 J2327: each whisker indicates the measured polarisation e^* of a star, while the circle indicates its half-light radius  from analyseldac (see the reference whisker and circles at the top for the absolute scale). In this stack north is up and east is left, matching the orientation of the input frames (observations obtained with a default 0^∘ position angle).