Intensity interferometer

An intensity interferometer is usually used to determine the angular dimensions of astronomical objects, such as stars, which cannot be measured by direct observation. The principle of its work was proposed by R. H. Brown in 1949 in the course of solving the problem of measuring the angular dimensions of two extraterrestrial radio sources, Cygnus A and Cassiopeia A ^[1] . Later, in 1954, the theory of the device received a mathematical model formulated by R. H. Brown and R. K. Twiss ( born RG Twiss ) ^[2] .

Originally created for the needs of radio astronomy , the method of intensity interferometry in this area has received limited application. The reason was the requirement that the measured signal exceed the background noise. However, in optical astronomy , where these conditions are feasible, the use of an intensity interferometer ensured a large practical return, which made it possible to overcome the problems of atmospheric turbulence .

The principle of operation of the intensity interferometer is based on the idea of correlating the fluctuations in the intensity of the received signal at two close receivers aimed at the object under study. The second name of this device is associated with this - the correlation interferometer. A decrease in such a correlation with an increase in the distance between the receivers allows one to calculate the angular size of the object under study ^[3] .

The idea was first successfully tested in 1950 to measure the angular size of the Sun at the Jodrell Bank Observatory using an interferometer operating at a frequency of 125 MHz. ^[4] In 1956, an intensity interferometer from two parabolic mirrors with a diameter of 1.56 m and with a variable base of up to 14 m was first used to measure the angular diameter of Sirius ^[5] . Each of the mirrors separately gave a blurred image projected onto the cathode of the photomultiplier . The received signals were amplified and their amplitudes multiplied. A measure of the correlation between fluctuations in the light intensity on two mirrors was the average value of the indicated product over several hours. The angular diameter of Sirius, calculated from the decrease in the correlation with the increase in the base, turned out to be, with good accuracy, equal to the value predicted by theoretical astrophysics ^[1] .

Currently, the largest intensity interferometer has been built and operated since 1990 at the Observatory of the Institute of Astronomy of the University of Sydney . Modern equipment and tunable receiver diversity base from 5 m to 160 m allow you to explore objects up to 8 magnitude . At the same time, preparatory work was carried out to increase the tunable base to 640 m. However, due to the lack of demand for science so far achieved results, namely, the achieved size of 0.2 angular milliseconds , the project is temporarily frozen ^[6] .