The quasi-linear intensity interferometer

A new type of radio intensity interferometer (fourth-order correlation system) suitable for use at long baselines is described. It provides both amplitude and phase information about the distribution of radio sources which excite its two antennas. There is no RF connection between the two widely separated antennas, and consequently no RF phase instabilities are present to vitiate the correlation of the two outputs. Use is also made of the bandwidth of the signals received so that in an aperture synthesis the baselines increase exponentially in length, thereby reducing considerably the total number of baselines needed for an aperture synthesis. Conventional types of intensity interferometers suffer from output signal-to-noise ratios (SNR´s) which vary quadratically with the SNR of the familiar second-order Michelson-type interferometer. It is shown that except for a constant factor the output SNR of the new interferometer varies linearly with the SNR of the Michelson interferometer, hence the name quasi-linear intensity interferometer. Such a behavior in large measure overcomes the major drawback of intensity interferometers, i.e., their poor output SNR when the sources are themselves very weak.