... OM images are affected by a fixed modulo-8 pattern originating from the on-board centroiding of photon events to one eighth of the physical CCD pixel (see [ 1 ] or [ 3 ]) . The task ommodmap implements modulo-8 pattern correction by redistributing the photon counts over the -pixel areas of the image. ... With this, the total number of the photon events is conserved, as in the first method. ... Figure 2: OM grism image with a bright spectrum affected by the modulo-8 noise and coincidence losses. ...