Due to the way sequential ray tracing works (rays are traced from one surface to the next in a specific order), modeling a double pass system requires that the optical system be modeled twice; once for each pass.  Different solve types, such as position and pickup, are essential for making sure that the optics are mapped appropriately for the return path of the beam.  Great care should be taken to ensure the model has been setup correctly, especially for tolerancing. 

In a double pass system, the tolerance events are not totally independent.  In other words, if an element is tilted by some amount in the first pass, the element should have the same exact physical tilt in the return path, otherwise the ray tracing results from ZEMAX in the tolerance analysis will be an incorrect representation of reality. 

To model and tolerance such a configuration, careful placement of coordinate breaks, pickups, and user-defined tolerances are required.  This tutorial is designed to cover all of the fundamental aspects of this process, starting with a single lens relay in double pass.