which are supplied to the modulator control 11 so as to allow the optical beam generator 12 to produce optical pulses at a predetermined repetition rate in accordance with the signals provided by the modulator control 11 at the same time the divider 22 or similar apparatus monitors the pulses produced by the clock generator and after a predetermined number of these pulses for example the number of pulses employed in one 34 scan 34 an output is produced which disables the clock generator 20 from producing further pulses simultaneously however the auxiliary clock generator 23 is enabled through enabled gate 26 and it produces pulses at a second predetermined rate which as will be explained below is preferably at a higher rate than the rate of pulses produced by the clock generator 20 up down counter 24 therefore begins counting preferably from zero at this point in time mirror 15 is reaching its other travel end point and when that point is reached and the mirror again reverses direction the l pulse is produced switching the action of the counter 24 so that it now counts down from the count it reached in counting up after a sufficient number of pulses from the auxiliary clock generator 23 have been.