The reason that ranges of modulation frequencies were quoted is that retardation and optical velocity in this interferometer, and in most completely rotating designs, are sinusoidal functions of time. There is then a fundamental tradeoff between duty cycle and velocity variation. A duty cycle efficiency of 66% was chosen, which corresponds to a factor of two variation in velocity. The extremes of retardation must be slightly larger than desired range of operation according to the duty cycle. At 90% duty cycle the velocity varies by a factor of 4.5 from the middle of the scan to the end.