You may already know that the Footprint Plot (FOO) option helps you learn about the used area of the surfaces in your model. But did you also know there is a sub-feature of FOO to check the clearance of the surfaces in your model? This can help you make sure there is no physical conflict between adjacent surfaces. The clearance checking helps detect overlap of surfaces in non-rotationally symmetric systems, as well as with non-spherical surfaces.
For decentered systems, clearance checking is automatically enabled. When FOO detects a physical conflict, a table in the analysis output will be displayed that shows the surface numbers where there is a conflict, the height where surface overlap is detected, and the worst case virtual ray length between the two surfaces. For example, consider the example of a singlet with a 45-degree fold mirror. In this example, there is not enough clearance to fit the fold mirror without coming into contact with the singlet.