Separation supplies baselines
Signals from separated telescopes can be combined using the baseline between them. The combination supplies information about angular structures beyond one individual aperture’s resolution.
Longer baselines generally probe finer angles, but wavelength also matters. One separation does not give identical angular resolution at every wavelength.
References: [1]
Combining signals in different bands
ESO contrasts digital radio correlation at ALMA with physical beam combination in the near- and mid-infrared VLTI. Both use interference but differ in recording and control.
Arrival paths and times must be reconciled. VLTI delay lines adjust optical path lengths; pointing telescopes at one target alone does not establish a usable interferometric measurement.
References: [1]
Reconstruction needs diverse sampling
Baseline orientations and lengths sample different structure. Additional combinations, telescope positions or Earth’s rotation improve coverage; sparse data may constrain geometric properties without reconstructing a detailed image.
Separate mirrors do not fill the virtual aperture’s collecting surface. Very long baselines can also miss extended structure, so fine resolution, light collection and coverage of larger scales require separate assessment.
References: [1]