At the beginning of the 21st century, the VLTI and Keck Interferometer large-telescope arrays came into operation, and the first interferometric measurements of the brightest few extra-galactic targets were performed.
One of the first astronomical interferometers was built on the Mount Wilson Observatory's reflector telescope in order to measure the diameters of stars.
This method was extended to measurements using separated telescopes by Johnson, Betz and Townes (1974) in the infrared and by Labeyrie (1975) in the visible.
Techniques from Very Long Baseline Interferometry (VLBI), in which a large aperture is synthesized computationally, were implemented at optical and infrared wavelengths in the 1980s by the Cavendish Astrophysics Group.
Additional results included direct measurements of the sizes of and distances to Cepheid variable stars, and young stellar objects.
It is often said that an interferometer achieves the effect of a telescope the size of the distance between the apertures; this is only true in the limited sense of angular resolution.
However, they have proven useful for making very high precision measurements of simple stellar parameters such as size and position (astrometry) and for imaging the nearest giant stars.
The very large distances do not always allow any usable transmission of radio waves received at the telescopes to some central interferometry point.
As most radio astronomy interferometers are digital they do have some shortcomings due to the sampling and quantization effects as well as the need for much more computing power when compared to analog correlation.