= Binary separation from visibility-zero spacing
{title2=$\Delta Y=\lambda/\beta$}
For two identical equally bright stars, zeros of the binary factor occur at $Y_j=(j+1/2)\lambda/\beta$, where $\beta$ is separation projected along the baseline. Their repeated spacing is $\Delta Y=\lambda/\beta$. Multiple samples separate these rapid zeros from zeros of a resolved stellar envelope. Baseline rotation is needed to recover an unknown two-dimensional separation direction; the first zero alone does not determine both diameter and separation.
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