Circular Analysis Surface

FRED has two types of analysis surfaces: 

1. The "standard" analysis surface type, which is rectangular.
2. The detector entity type, which is available in non-rectangular shapes such as sphere, cone, and cylinder. 

When a plane, circular shaped analysis surface is the most suitable configuration, use the cone shaped detector entity and set the height of the cone to zero, as shown below.  Note: Detector entities can only be used for incoherent raytraces.