Very Long Baseline Interferometry (VLBI) is a technique used in radio astronomy that combines multiple radio telescopes located at different locations on Earth to create a virtual telescope with the resolution equivalent to that of a much larger telescope. VLBI allows astronomers to observe celestial objects with high precision and resolution.
The wavelength that can be observed using interferometry, including VLBI, depends on the frequency range of the radio telescopes involved. VLBI typically operates in the radio frequency range, which has longer wavelengths compared to visible light.
The shortest wavelength that can be observed using VLBI depends on the frequency range of the telescopes and the capabilities of the receiver systems. In general, VLBI systems can observe radio waves with wavelengths as short as a few millimeters or less. At higher frequencies, such as in the millimeter or submillimeter wavebands, VLBI can achieve extremely high angular resolutions.
It's important to note that interferometry techniques can be used at shorter wavelengths, such as in the optical or infrared range, but these techniques differ from VLBI and have their own specific applications and limitations.