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- How to Design Afocal Systems
How to Design Afocal Systems
- By Mark Nicholson
- Published 14 September 2005
- Sequential Ray Tracing , First Time Users
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Analysing data in angular units
So how good is our afocal system? Look in the merit function, at the value of the REAY operand. It should show a value of exactly 12.5. So, we are getting the beam expansion we asked for. Then open the OPD, Ray-Fan, Point Spread Function and Modulation Transfer Function windows.
The OPD should appear like so:

this shows focus, spherical and higher-order spherical all balancing, and the system's PTV wavefront error to be less than l/1000. The ray-fan plot is also interesting:
Note the focus, spherical and higher order spherical are also clearly shown, but also note that the units used are µR: microradians. This plot is showing the deviation from perfect collimation directly. The spot diagram also shows this:

The RMS angular deviation is 0.265 µR. Of course, diffraction effects are much larger: on the settings of the spot diagram, click the button "Show Airy Disc". Diffraction effects limit the resolution to about 30 µR.
To see this, look at Analysis > PSF > FFT PSF Cross-section:
This shows that diffraction effects produce an Airy disc of around 30 µR. Analysis > MTF > FFT MTF shows the contrast ratio of the system in units of cycles per milliradian: