= Solution
Two nearby directions separated by $\theta_R$ focus at transverse separation $s\simeq f_m\theta_R$. To avoid a receiver covering both geometric image centres at once, its <diameter> $d$ should not exceed this separation along the orbital direction:
$$
\boxed{d_{\max}\sim f_m\theta_R
=(0.750)(0.03588)\,\mathrm m=26.9\,\mathrm{mm}.}
$$
This is the usual receiver-size estimate from <angular resolution>. The <Airy disk> first-minimum <diameter> is $2f_m\theta_R=53.8\,\mathrm{mm}$, but that is a spot-size definition, not the same as the receiver's allowed angular acceptance. At the <Rayleigh criterion>, the images already overlap; integrating over a finite receiver further reduces contrast. Consequently $26.9\,\mathrm{mm}$ is an order-of-resolution upper scale, not a sharp universal boundary for a specified signal discrimination probability. A detailed limit would require the receiver response and a resolving criterion.
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