Farfield Monitor - distance from metalic features

I am using a farfield monitor, to calculate the farfield from an aperture (similar to https://kb.lumerical.com/en/sp_bullseye_aperture.html).

My question is if there is a minimal distance between the features and the monitor that is used to calculate the farfield.
I used 200 nm before (lambda=650nm), but got numerical errors.


There is no minimal distance required between the structure and the monitor that is used for far field projections, however, a couple of requirements for the far field projection to be accurate are that the monitor itself should be placed in a homogenous medium, and there are no other structures beyond the monitor that could eventually modify the direction of light propagation beyond the monitor.

Another requirement is that the monitor should have a large enough collection angle that it is measuring all of the light which propagates to the far field, otherwise there can be ripples in the far field projection, as described in more detail on this page:

If you are not seeing the expected result from your far field projection, you can include a copy of your file so that I could check the settings.