= Stellar angular diameter from a visibility envelope
{title2=$\alpha=1.21967\lambda/Y_0$}
For a uniform circular stellar disc, the first zero of $J_1$ at $x=3.831706\ldots$ gives $Y_0=1.21967\lambda/\alpha$ when $\alpha$ is full <angular diameter>. In contrast a one-dimensional top-hat angular profile of full width $\alpha$ has a sinc envelope with $Y_0=\lambda/\alpha$. Diameter estimates therefore require a brightness-profile convention. A binary additionally multiplies the envelope by its separation-dependent <interference> factor.
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