For any lens or mirror system you can observe the image in the focal plane,
either by inspecting tables of data or by looking at the graphs. Experiment
with the parabola to see what you can find out.
For example, select Evaluate then Spot Diagram and
Report graphic .... This brings up a small menu. Accept the defaults,
except select ``Show Airy Disk''. You will see the image in the focal plane,
on and off axis, and the image inside and outside the best focus.
Make a slight change to the IMS Thickness to move the image plane and then
right-click on the Spot Diagram graphic, select Update, and you will see the
new image. To find the best focus, use the Autofocus option above and move the
plane back. You can always restart the
process with a File->Exit selection.
You can evaluate the relative size of the coma and defocusing on the graphical
screen. You can also see a numerical value for the diffraction limit with
Evaluate then Spot Diagram and Spot size analysis ...