= Solution
An <astronomical interferometer> combines light from collectors separated by projected <interferometric baseline> $\mathbf B$. For a distant, narrowband, spatially incoherent source, the <Van Cittert-Zernike theorem> gives normalized <complex visibility>
$$
\mathcal V(\mathbf B/\lambda)=\frac{\int I(\boldsymbol\theta)e^{-2\pi i\mathbf B\cdot\boldsymbol\theta/\lambda}\,d^2\theta}{\int I(\boldsymbol\theta)\,d^2\theta}.
$$
The resolving scale is \b[$\lambda/B$], while the individual collector and beam combiner restrict the field of view. Delay lines compensate geometric path differences. To see broadband fringes, residual <optical path length> mismatch must lie within the <coherence length> of order $\lambda^2/\Delta\lambda$; <fringe tracking> follows rapidly changing <atmospheric piston in interferometry>. Baseline changes and <Earth-rotation aperture synthesis> sample additional <spatial frequencies>.
For an ideal uniform circular <star> of full <angular diameter> $\Theta$,
$$
\mathcal V=\frac{2J_1(x)}x,\qquad x=\frac{\pi B\Theta}{\lambda},\qquad
\boxed{\Theta=1.2197\,\lambda/B_{\rm first\ null}}.
$$
Real <limb darkening> changes the conversion. Several baselines constrain binaries, <stellar radii> and circumstellar structure; orbital angular separations combined with <radial velocities> can give physical orbital scales. <calibration> <stars> test contrast losses, alongside <wavelength>, photometric imbalance, <polarization> and <bandwidth> controls. For three collectors the <closure phase> cancels telescope-based phase errors: $\phi_{12}+\phi_{23}+\phi_{31}$ retains source asymmetry information. Sparse baseline coverage still leaves image/model ambiguities.
In <stellar intensity interferometry>, thermal-light correlations instead give $g^{(2)}(0)-1=|g^{(1)}|^2$ for an ideal single-polarization resolved temporal mode. The Sirius experiment correlated separate detector outputs rather than optically combining the beams.<A Test of a New Type of Stellar Interferometer on Sirius by Robert Hanbury Brown and Richard Quentin Twiss>[Nature 178, 1046–1048 (1956)]{ref} Pure atmospheric phase shifts do not alter local <intensities>, but finite detector <bandwidth> dilutes the correlation and <photon> statistics limit sensitivity; the two-detector observable also loses the complex phase.
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