= Solution
Under the <Rayleigh criterion>, two equally bright, mutually incoherent <point sources> are just resolved when the central maximum of one <diffraction pattern> lies at the first minimum of the other. Their <intensities>, rather than their electric-field amplitudes, add. For an ideal circular aperture of diameter $D$, the first dark ring of the <Airy disk> gives
$$
\boxed{\theta_R\simeq1.22\frac{\lambda}{D}}.
$$
The angle is in radians in the small-angle limit. This is a conventional resolution criterion for a specified aperture and <wavelength>; it is not an absolute prohibition on estimating a source separation by fitting noisy data under additional assumptions.
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