I have been working with millimeter-wave / radio optics, like the ALMA array, or the ARO, and the most important metric for these types of systems is usually the amount of light that the detector sees that isn’t coming from a cold source. This is called the warm spillover, and is evaluated by using a time-reversed model, where the detectors are treated as the sources. Then, any light that reaches a non-cold source will contribute to the noise in the system. Evaluating this quantity is difficult due to the frequencies of radio systems.
At the long wavelengths of radio optics, we are in a quasi-optical regime where the wave nature of light becomes very relevant, but can still be well represented by geometrical ray tracing. Therefore, to evaluate the spillover metric, we should use a beam propagation method and account for the diffraction that occurs on finite apertures.
However, when designing such systems, it is really challenging to create a merit function that captures this behavior! So, has anyone else dealt with this kind of consideration, and how have you gone about it?