to some central interferometry point. For this reason many telescopes instead record the radio waves onto a storage medium. The recordings are then transferred to a central correlator station where the waves are interfered. Historically the recordings were analog and were made on magnetic tapes. This was quickly superseded by the current method of digitizing the radio waves, and then either storing the data onto computer hard disks for later shipping, or streaming the digital data directly over a telecommunications network e.g. over the Internet to the correlator station. Radio arrays with a very broad bandwidth, and also some older arrays, transmit the data in analogue form either electrically or through fibre-optics. A similar approach is also used at some submillimetre and infrared interferometers, such as the Infrared Spatial Interferometer. Some early radio interferometers operated as intensity interferometers, transmitting measurements of the signal intensity over electrical cables to a central correlator. A similar approach was used at optical wavelengths by the Narrabri Stellar Intensity Interferometer to make the first large-scale survey of stellar diameters in the 1970s. At the correlator station, the actual interferometer is synthesized by processing the digital signals using correlator hardware or software. Common correlator types are the FX and XF correlators. The current trend is towards software correlators running on consumer PCs or similar enterprise hardware. There also exists some amateur radio astronomy digital interferometers, such as the ALLBIN of the European Radio Astronomy Club. 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. The output of both a digital and analog correlator can be used to computationally synthesize the interferometer aperture in the same way as with direct detection interferometers (see above). Using gamma-ray telescopes Optical intensity interferometry has been revived, measuring the width of giant stars using the optical instruments of the Cherenkov Telescope Array, a ground-based Cherenkov-radiation-based gamma-ray telescope normally intended to observe atmospheric Cherenkov-radiation so as to detect gamma-ray photons. See also Aperture synthesis Astronomical interferometer List of astronomical interferometers at visible and infrared wavelengths List of types of interferometers Multiple satellite imaging Optical coherence tomography Very-long-baseline interferometry Wave interference References P. Hariharan, Optical Interferometry, 2nd edition, Academic Press, San Diego, USA, 2003. E. Hecht, Optics, 2nd Edition, Addison-Wesley Publishing Co., Reading, Mass, USA, 1987. Further reading