Aperture synthesis is possible only if both the amplitude and the phase of the incoming signal are measured by each telescope. For radio frequencies, this is possible by electronics, while for optical frequencies, the electromagnetic field cannot be measured directly and correlated in software, but must be propagated by sensitive optics and interfered optically. Accurate optical delay and atmospheric wavefront aberration correction is required, a very demanding technology that became possible only in the 1990s. This is why imaging with aperture synthesis has been used successfully in radio astronomy since the 1950s and in optical/infrared astronomy only since the turn of the millennium. See astronomical interferometer for more information.
In order to produce a high quality image, a large number of different separations between different telescopes is required (the projected separation between any two telescopes as seen from the radio source is called a baseline) - as many different baselines as possible are required in order to get a good quality image. The number of baselines (nb) for an array of n telescopes is given by nb=(n2- n)/2. For example, the Very Large Array has 27 telescopes giving 351 independent baselines at once, and can give high quality images.