ZEMAX's Non-Sequential ray tracing capabilties can further be extended to finding rays which have specific characteristcs or properties.  For example, imagine you are studying the stray light in a telescope:

• How significant are rays which "ghost" reflect off of various surfaces (both mechanical and optical)?
• Rays which are experience multiple reflections may be important, but how signficant are those which experience more than four reflections? 
• How effective is a strategically placed baffle in terms of limiting the amount of stray light on the detector? 

Each of these, and many similar questions, may be answered conveniently using what are known as Filter Strings in ZEMAX.  Filter strings are a means of defining a specific "test" that each ray must pass before it is displayed (on the Layouts or Detector Viewer) or reported in the Ray Database. 

It is important to note that filter strings can be used to perform a full Opto-Mechanical assessment of stray light.  Therefore, ZEMAX will consider partial reflections from optics, reflections from mechanical components, and scattering from both optics and mechanics. 

The filter string syntax consists of logical operations between flags that indicate if one segment within a ray hit, missed, reflected, refracted, scattered, diffracted, or ghost reflected from an object within an NSC group.  A complete listing of the availabe filter string flags and discussion may be found in "The filter string" section of Chapter 12 in the ZEMAX Users's Guide.

In this article, we will demonstrate where filter strings are entered, how they are applied, and how they may be used to analyze rays with certain optical properties.  To do so, we will evaluate the amount of "moon" light that gets into the image area of a Cassegrain-type telescope when viewing a distant star.  The basic concept?  A large number of rays will be traced and saved to a database.  We will then "dig" through this ray database to find rays which have explicit properties.