Snapshot of the main OSP panel showing the location of the mask superimposed on a sky image and with several slits allocated to targets (blue squares) and fiducial objects (red crosses). The background image is a UKIDSS field located in the Galactic plane at 30° longitude. This example was used to construct a mask for the GALEP project (see Sect. 5), where the targets are selected in the infrared colour-magnitude diagram as candidates for red giant stars.