Etching a concave body enlargens the body. In this process facets in the slowest etching directions are formed. Here the example of an etched hemispherical pit. The left diagram is a polar plot of the normal directions at the finite difference nodes. Each red dot represents one node.
Triangles represent the directions, squares the directions and rectangles the directions.
The directions are the slowest directions. The red dots should cluster at the triangles. However, only the central direction is expressed because the others are to close to the border of the pit.