Discussion
What limits how far a feed can be displaced from the focus before the beam is unusable?
Answer: D
Lateral displacement introduces an odd order phase error across the aperture, producing asymmetric coma lobes and a falling gain, and a few beamwidths of scan is usually the practical limit. Longer focal length reflectors tolerate more. Mechanical collision is a layout detail, coherence length is irrelevant at these scales, and polarisation is only slightly affected. Naming the aberration and the aperture error that produces it is what allows the limit to be predicted rather than discovered by experiment.
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