While they are often used to do project vector graphics, they are not as simple to operate as a vector CRT. They exhibit some intersting problems warranting a complex control system. Depending on geometry, they have a slight hyperbolic abberation because of the seperation of the mirrors from each other. They exhibit a funny response; the physics analogy is that a mass at the end of a spring dipped in a viscous fluid and rubbing against the bottom. This is the reason it is easy to make them do lissajuos curves and more difficult to do other things. They naturally follow a sine wave.