Very Long Baseline Interferometry (VLBI) is a technique used in astronomy to achieve extremely high angular resolution by combining observations from multiple telescopes separated by long baselines. The resolution of VLBI depends on the wavelength of the observed radiation.
In VLBI, the shortest observable wavelength is determined by the maximum baseline length between the telescopes used in the interferometric array. The angular resolution achieved by an interferometer is given by the formula:
θ = λ / D
where θ is the angular resolution, λ is the wavelength of the observed radiation, and D is the baseline length.
To observe shorter wavelengths, telescopes with longer baselines are needed. However, there are practical limitations to the baseline length due to the geographical separation between telescopes, Earth's curvature, and technical constraints. Currently, the most extended baselines used in VLBI are achieved by including radio telescopes located across the globe.
In the radio frequency range, VLBI observations typically operate at wavelengths ranging from centimeters to millimeters. For example, typical radio observatories operating in the VLBI mode use wavelengths in the range of a few millimeters (e.g., 3 mm or 1 mm). However, it's worth noting that technological advancements and the development of new instruments may enable observations at even shorter wavelengths in the future.