2. The role and development of radio interferometry

Interferometry is the key technique for achieving much higher angular resolution than is possible with single apertures. Since its modest beginnings in the 1950s and 1960s, larger and more complex interferometer systems have been built to map the brightness distribution in the sky. Some focus on imaging small-diameter radio sources, and their angular resolution can far exceed the resolving power of the largest optical telescopes on the ground or in space. In contrast, low-resolution arrays with many baselines can image faint, scattered radio emissions. The most important link between interferometer observations and the brightness distribution of a source is the Fourier transform, which we introduce in chapter 2.

Here is a brief overview of the evolution of synthesis techniques:

  • The Michelson stellar interferometer: Between 1890 and 1921, the first measurements using optical interferometer instruments were made by using two separated receiving apertures measuring fringe amplitudes to determine stellar angular sizes.
  • The first astronomical observations with a two-element radio interferometer: In 1946 Ryle and Vonberg measured the correlation of radio flux emitted by the sun with the appearance of sunspots.
  • The first phase-switching interferometer: In 1952 the first successor to the previously used \textit{adding interferometer}, the phase-switching interferometer, was implemented and adopted by multiple arrays during that time. This significantly reduced common issues with the adding interferometer. A more detailed analysis of this will be given in chapter 2.
  • Beginning of astronomical calibration: During the 1950s and 1960s optical identifications of known, small-diameter, radio sources allowed for accurate calibration of interferometer baselines and instrumental phases. Single dish radio observations cannot reach the resolution capabilities of optical telescopes, which is why only accurate radio interferometers are capable to cross correlate astrometry with optical data, because here resolution capabilities can be similar.
  • Early measurements of angular dimensions: From 1952 onward, variable-baseline interferometers were used more and more often in measurements.
  • Use of solar arrays: From the mid-1950s onwards, multiantenna arrays sensitive to centimeter-wavelengths were developed to provide detailed maps of the solar disk.
  • Use of arrays of tracking antennas: From around 1960 onward, meter-wavelength arrays with non-tracking antennas became less prominent while centimeter-wavelength arrays with tracking antennas became more common due to improved computation capabilities. As well as that, separate correlators for each baseline were implemented, instead of a central one for all baselines.
  • Earth-rotation synthesis: Introduced in 1962 by Ryle and Neville, the first measurements using earth rotation synthesis could be made. The parallel advancements in computer technology during that time were crucial in providing the necessary tools to process the data, especially performing Fourier transforms.
  • Spectral line studies: The aforementioned advancements in electronic engineering allowed for spectral line studies with radio interferometer instruments from around 1962.
  • Development of image-processing techniques: From around 1974 onward, efforts were made to develop imaging techniques to translate the interferometric visibilities in spatial brightness distributions in a standardized fashion. The first approaches were also based on phase and amplitude closure as well as nonlinear deconvolution.
  • Very long baseline Interferometry (VLBI): The first measurements making use of VLBI were made in 1967. After a few years of improvements and successful operation of multiple programs, the first detection of superluminal motion in active galactic nuclei was discovered by Whitney et al. (1971). Christodoulidis et. al (1986) were able to detect contemporary plate motion, and more accurate astrometry was attained by defining and adopting the International Celestial Reference Frame (Feissel and Mignard, 1998).
  • Millimeter-wavelength instruments: From the mid-1980s onward, major developments were made in interferometry arrays covering 100 to 300 GHz.
  • Orbiting VLBI (OVLBI) / Space VLBI: The first array that included space satellites in the U.S. Tracking and Data Relay Satellite System (TDRSS) experiment took place from 1986 to 1988. Later, in 1997, the first space VLBI dedicated mission was launched by Japan: an 8-meter radio dish, operated by the VLBI Space Observatory Programme (VSOP). In 2011, the Russian 10-meter radio satellite was launched in order to perform VLBI measurements with ground based telescopes by the program called RadioAstron.

Of course, this list is a small sample of the many developments and accomplishments that took place in this field. This is also a list that could be continuously updated. But we want to show only an extremely brief list of important milestones, relevant to this course, to help the reader keep a historic overview of general developments.