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An astronomical interferometer combines observations from separated collecting locations to infer angular source structure. Its projected interferometric baseline determines the angular spatial frequency sampled. Amplitude and intensity interferometers measure different coherence observables; a Michelson stellar interferometer combines optical field amplitudes.

fringe tracking estimates changing optical delay or phase and adjusts delay compensation to keep an astronomical interferometer near a useful interference condition. The delay must first lie inside the finite-bandwidth coherence envelope; rapid tracking then controls atmospheric piston. This is optical-path stabilization, with sensitivity and reference-source limitations.

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 visibilityThe resolving scale is $\lambda /B$, 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 $\Theta$,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: ${\varphi }_{12}+{\varphi }_{23}+{\varphi }_{31}$ 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.^[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.

The Very Large Telescope is an observatory array at Paranal with four $8.2$-metre Unit Telescopes and four movable $1.8$-metre Auxiliary Telescopes. The Unit Telescopes can observe independently; selected beams can also be combined in an astronomical interferometer. Combining beams coherently changes the angular information available and requires an appropriate observing instrument.^[ref]

For exoplanets, adaptive optics helps concentrate light for exoplanet direct imaging, while instruments such as CRIRES resolve molecular spectral lines. Spatial separation and Doppler shift provide complementary ways to distinguish a faint planet from its host star. The capability belongs to the telescope-plus-instrument combination, rather than to aperture size alone.