Tank - A simple variation on Brierley largely solves the duty cycle and air resistance problems. In Brierley’s design the axis of rotation was a line coming out of the slide. Here the axis of rotation is a vertical line in the plane of the slide. The sweep range is set when the mirrors are fixed to the shaft. The general problem with this design is that alignment between two fairly large mirrors must be maintained to 0.5 arcseconds while they spin at 0.5 kHz.