= Finite-grating incoherent doublet profile
{title2=$I(\theta)\propto\sum_j\bigl[\sin(Nu_j)/(N\sin u_j)\bigr]^2$}
For narrow slits and equally bright mutually incoherent spectral lines, $u_j=\pi p\sin\theta/\lambda_j$ and the <intensities> add as displayed. The <Rayleigh criterion> uses a main-peak-to-first-zero separation $\delta\lambda\simeq\lambda/(|m|N)$. A slightly sub-Rayleigh doublet can still have two local maxima: the criterion is not a theorem of statistical non-identifiability. <photon> counts, background, unequal strengths and uncertainty in the instrumental profile govern actual inference.
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