The previous example shows clearly that the mirror is tilting about its vertex, which would be fine if we are modelling a galvanometer or similar type mirror. But, if the mirror is part of a polygon scanner, for example, then it is being tilted about a point some distance behind the vertex. How do we model that?

We need to place the pivot point of the mirror at the center of the polygon scanner. Let's say the distance from the vertex of the mirror to the center of the polygon is 50 mm. Make the following changes to the galvanometer file:

This simply offsets the point about which the tilts are done by 50 mm. Then double-click on the mirror surface, and in its Draw tab set it to draw the substrate of the mirror 50 mm away:

and the tilt about the offset point can be clearly seen: