Imagine viewing a star with a telescope during a full moon.  Some of the light from the moon, though off-axis (where the axis is described as the distance from the star to the vertex of the telescope barrell), reaches the detector plane (camera) of the telescope.  We need to determine exactly how much stray light from the moon is reaching the detector.

This scenario is simulated using the following purely Non-Sequential (NS) ZEMAX lens file.  Download the file attached to this article and open it from within ZEMAX.

FILE:  "Stray Light_Telescope.ZMX"

Note:  For the purposes of demonstrating filter strings in ZEMAX, it is assumed that you are familiar with the standard procedure of performing a Non-Sequential (NS) Ray Trace and viewing the results on the Detector Viewer.  If you are unfamiliar with these concepts, please refer to the article on Creating a Basic Non-Sequential System.



The moon in the telescope model is represented by an off-axis Source Ellipse.  The moon is approximated as a collimated source, so rays from the moon (in green above) are parallel to one another.  Similarly, the object of interest is represented by an on-axis collimated Source Ellipse.  As in a typical Cassegrain-like telescope design, collimated rays (the blue rays in the layout above) from an on axis field focus to a well-corrected spot at the image plane.  However, some of the rays(from both the moon and the star) do not follow the desired sequence of optical surfaces, yet still reach the detector.

Perform an initial NS ray trace with Use Polarization, Ignore Errors, Split Rays, Scatter Rays, and Save Rays all checked.  Save the rays with any desired file name.  Note that rays MUST be saved first before filter strings can be applied to the Ray Database and Detector Viewers!  For the time being, we will leave the "Filter" entry blank since filters may also be applied later on in the settings of individual analysis features.



Note that the "Lost Energy" values due to thresholds and errors are very important, and should be reviewed with great care to ensure accurate evaluation of the current ray sample.  If a ray falls below the minimum energy threshold for tracing, the ray will be terminated.  The minimum relative and absolute energy thesholds are defined by the "Minimum Relative Ray Intensity" and "Minimum Absolute Ray Intensity" entries under the Non-Sequential tab of the System General dialog.  To reduce the caluclation times in the current example, the Minimum Relative Ray Intensity is set to 1.00E-007.  This value may need to be reduced in some applications in order to reduce the lost energy due to thresholds.