Editing Aperture synthesis (section)

== Technical issues ==
Aperture synthesis is possible only if both the [[amplitude]] and the [[phase (waves)|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 are 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 (''n''<sub>b</sub>) for an array of ''n'' telescopes is given by ''n''<sub>b</sub>=(''n''<sup>2</sup>&nbsp;−&nbsp;''n'')/2. (This is <math>\binom{n}{2}</math> or [[Binomial coefficient|<sup>''n''</sup>C<sub>2</sub>]]).  For example, the [[Very Large Array]] has 27 telescopes giving 351 independent baselines at once, and can give high quality images.

[[File:Earth rotation aperture synthesis.jpg|thumb|upright=1.2|Most aperture synthesis interferometers use the rotation of the Earth to increase the number of baseline orientations included in an observation. In this example with the Earth represented as a grey sphere, the baseline between telescope A and telescope B changes angle with time as viewed from the radio source as the Earth rotates. Taking data at different times thus provides measurements with different telescope separations.]]

In contrast to radio arrays, the largest optical arrays currently have only 6 telescopes, giving poorer image quality from the 15 baselines between the telescopes.

Most radio frequency aperture synthesis interferometers use the rotation of the Earth to increase the number of different baselines included in an observation (see diagram on right). Taking data at different times provides measurements with different telescope separations and angles without the need for additional telescopes or moving the telescopes manually, as the rotation of the Earth moves the telescopes to new baselines.

The use of Earth rotation was discussed in detail in the 1950 paper ''A preliminary survey of the radio stars in the Northern Hemisphere''.<ref>[http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1950MNRAS.110..508R&amp;db_key=AST A preliminary survey of the radio stars in the Northern Hemisphere]</ref> Some instruments use artificial rotation of the interferometer array instead of Earth rotation, such as in [[Aperture Masking Interferometry|aperture masking interferometry]].