To illustrate the implementation of the swap procedure, the M10 roof mirror is shown as seen by the technical CCD of the auxiliary telescope. Left: in the normal mode configuration, the light of the primary star P falls onto the left side of M10 and feeds IP2 or IP4, thus FSUB, whereas the light of the secondary star feeds FSUA. The derotator ensures that the objects are kept on the dashed line at a distance equivalent to 2′′ from the mirror edge. Right: the situation in swapped mode, where the derotator has rotated the field by 180°. The primary star is fed to FSUA and the secondary star to FSUB.