eference manual which can be used to generate useful types of
aberration fans. There is an extensive aberration fan plotting
capability described in the GRAPHICS section of the reference
manual. Experiment with these.</p>

<h3>Distortion, Field Curvature, and Astigmatism</h3>

<p>Unlike all other optical design programs, this program correctly
calculates distortion, astigmatism and field curvature for off-axis,
decentered systems. It does this by basing its calculations on
a <em>generalized paraxial ray tra