signal to cause optical ouput signal 33 to expose film alternatively pixel clock hardware can count down to supply signal based on controller supplied offset data vertical pixel position timing signals are sent from controller 36 to modulator 30 which impresses a signal on incident laser beam 27 controller 36 also issues commands typically consisting of a microstep count to move optical head to the next desired raster line and timing control for microstepping events to stepper motor controller 40 stepper motor controller 40 issues microstep control signals to stepper motor controller also monitors and controls system status safety interlocks and ancilliary subsystems and components conversion of input data from store 37 typically after initial reading of magnetic tape input proceeds at an average rate equal to one raster line per drum revolution i e real time operation during plotting fig2 illustrates the positioning system calibration process a test pattern 190 is defined typically a square grid pattern on one inch centers with diagonal lines at 45 degrees right and left of vertical passing through the square grid centers exposed on film and the film processed all under closely held environmental conditions to produce one or more test patterns 192 the grid intersection points are measured to an accuracy good enough to determine machine output performance and the grid point position errors are computed 194 alternatively the measured grid positions are input to the controller computer where resident auxilliary software performs the comparison the position error matrix is stored in the controller permanent memory 39 fig1 where it is typically similar to 204 converted by resident software to a form suitable for real time manipulation to produce offset commands the measured error data is corroborated by producing one or more corrected.