= Solution
The <photon> relation gives $\lambda=hc/E=2.4797\times10^{-12}\,\mathrm m$. The diffraction-limited angular scale of an <aperture> of diameter $D$ is therefore
$$
\boxed{\theta\sim\lambda/D=2.48\times10^{-11}\,\mathrm{rad}.}
$$
For a uniformly illuminated circular <aperture>, the <Rayleigh criterion> instead gives $1.22\lambda/D=3.03\times10^{-11}\,\mathrm{rad}$. Both express the same requested estimate. This is an ideal <diffraction limit>, conditional on the effective diameter behaving as a coherent imaging <aperture>; <photon energy> and diameter alone do not specify the performance of a practical gamma-ray telescope.
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