One of the most basic analysis features in ZEMAX is the spot diagram. This feature launches many rays from a single source point in object space, traces all the rays through the optical system, and plots the (x, y) coordinates of all the rays relative to some common reference.

The sample optical system used here is a single parabolic F/5 mirror with a focal length of 50 mm. The object is at infinity. This system is a simplified Newtonian telescope, and the sample file is PSF_Newtonian.ZMX. Here is what the optical system looks like:

The spot diagram for two field points, one on axis and the other at an angle of 2 degrees is shown below.

Note the spot diagram is a collection of points, with each point representing a single ray. There is no interaction or interference between the rays. The spot diagram is very effective at showing the effects of the geometric, or ray aberrations of the telescope. The off axis geometric PSF clearly shows the coma and astigmatism of the system. On axis however, the spot diagram predicts perfect imagery. Is this an accurate representation of the optical system performance? To answer this question for the spot diagram results, we need to compare the spot distribution to the diffraction limited response.

A quick way to compare the geometric aberrations to the diffraction limit is to add an Airy disk reference ellipse to the spot diagram. Open the settings box and change Show Scale to "Airy Disk":



Now the spot diagram will indicate the size of the Airy disk relative to the geometric spot distribution:



On axis, the spot is much smaller than the Airy Disk, while off axis the spot is much larger than the Airy Disk. This indicates the spot diagram is a useful and reasonable indicator of performance off axis only. To compute a more accurate PSF both on and off axis, a consideration of diffraction will be required.

Generally speaking, the spot diagram is useful if the aberrations are large compared to the diffraction limited performance of the system.