= Uniform-disc binary-star visibility
{title2=$V(Y)=\left|\frac{2J_1(x)}x\cos(\pi\beta Y/\lambda)\right|$}
Two identical uniform circular discs of <angular diameter> $\alpha$ and equal <flux> have $g_\star(Y)=2J_1(x)/x$, $x=\pi\alpha Y/\lambda$, with limiting value one at $Y=0$. Multiplication by $\cos(\pi\beta Y/\lambda)$ gives their signed binary coherence. Its first single-disc envelope zero is $Y=1.21967\lambda/\alpha$, while the binary modulation has a separate scale $\lambda/\beta$. <Limb darkening> and other nonuniform profiles change the envelope, not the elementary Fourier shift factor.
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