The Foucault test experiment can be reproduced and analyzed with BEAM3 too. Use three files to do this: PARA.RAY, PARA.OPT, and FOUC.OPT (no media file). The PARA files define a parabola with about the same focal length as the mirror in the lab. A point source is at its center of curvature, and the light is reflected back toward the same point. The parabola is tipped a little, so that the image point is just below the object in Layout.
Load PARA.OPT and PARA.RAY. Run Layout. Then inspect the focal
plane with Plot. The rays do not all return to the same point because
a parabola does not image a point back on itself perfectly. A sphere will,
however. To see this, edit the optics file and change the surface parameter
from 0.0 (a parabola) to 1.0 (a sphere). The run Plot again.
You'll see this scatter of rays converge to almost one point. The slight
error remaining is due to astigmatism, because of the tip of the mirror.
To make that disappear, edit the optics file and change the pitch of the
mirror to 
.
Now keep the PARA.RAY file but fetch FOUC.OPT for a new optics file. The difference is that the FOUC.OPT file has a mask in it. The mask is actually a ring with inner and outer radii that you can change the location to isolate different rays, just as you changed the mask in the laboratory experiment. I recommend that you keep the difference in these radii fixed, and modify the inner radius a centimeter at a time.
Run Layout and you'll see the rays heading toward the mirror, but only
two get through the mask and reflect back to the screen. When you Plot
their positions on the screen you will see that they do not come to a single
point. You can adjust the position of the screen by editing the optics
table and changing the value of Z
.