Now, lets turn the Ray-aiming back to “Paraxial” (not “Real”). The difference between the real and paraxial Y marginal ray coordinate at the stop surface is practically zero, as reported in the raytrace calculation, for all fields and wavelengths.

Ray-aiming Paraxial

As mentioned previously, with Ray-aiming set to “Paraxial”, the real marginal rays intercepts the “paraxial” stop surface height. Sometimes a user might want to specify the size to the stop that the real ray should fill, just like the previous Double Gauss example. To explicitly specify the location where the real marginal ray should intercept at the stop surface location, in addition to Ray-aiming, the system aperture type can be set to “float by stop size” (System >General> Aperture) and semi-diameter values for the stop entered manually in the Les Data Editor.

For moderately aberrated systems, the stop size determined using paraxial ray will generally be slightly different than one determined using real rays. Usually, the difference between the paraxial and real stop surface height will be too small for the effect due to the difference to be worth considering. Nonetheless, the “aberrated” option for Ray-aiming under System>General>Ray Aiming uses real rays rather than paraxial to calculate the stop height. This option is almost never necessary since you can set the system aperture type to “float by stop size” and manually set the stop size to explicitly specify where the real rays should be aimed at. We at ZDC have yet to see a clear case where this feature is absolutely needed. Of course, this does not mean that there isn’t a system out there that genuinely requires “aberrated” Ray-aiming, hence the reason it is still available as an option. Please consult the manual carefully before using “aberrated” Ray-aiming. The “Real” Ray-aiming option should not be considered as a superior alternative to "Paraxial" Ray-aiming.

By default the “use Ray-aiming cache” option is checked and the “Robust Ray-aiming” unchecked. These setting should not be modified as they are rarely needed. For more information about these setting, please refer to chapter 6 of the manual.