The mask consists of three separate grids, positioned in such a way that the grids produce three angulated diffraction spikes at the focal plane of the instrument for each bright image element (star). As the instrument's focus is changed the central spike appears to move from one side of the star to the other. In reality, all three spikes move but the central spike moves in an opposite direction to the two spikes forming the 'X'. Optimum focus is achieved when the middle spike is centred on the star and symmetrically positioned between the other two spikes. Small deviations from optimal focus are easily visible.

In the example below, the centre pattern shows good focus. The central spike is noticeably displaced from the centre position in the left and right images. The direction of this displacement provides information on which way focus needs to be made. The operator will place the mask over the front aperture of the telescope in the same orientation each time. He/she will then become familiar with the direction clue provided by the central spike. In the example below, the left hand image would indicate an anti-clockwise rotation of the focus knob was needed. The right hand image would need a clockwise rotation. The usual 'trial and error' involved with focussing is then removed. Rotating the mask through 180 degrees will reverse the direction of spike movement, hence the need to place the mask on the scope with consistent orientation. The mask must be removed after accurate focussing is achieved.