Intensity interferometer

The Intensitätsinterferometer, also called Korrelationsinterferometer, is an optical device ( interferometer ) to determine the angular diameter of astronomical objects (typically stars) that can not be resolved on direct observation.

Principle

The Intensitätsinterferometer consists of two spatially separated telescopes to which each case by means of a photomultiplier, the arrival times of arriving there by a bright star photons are registered, what happens in practice through the recording of the time course of the two currents arising there. They are brought together in a correlator, which examines the two timings on coincidences out. From the correlation between the two streams can be concluded that the angular diameter of the star (see further explanations below).

Robert Hanbury Brown and Richard Twiss recognized mid-1950s that the escape times of electrons are correlated at different points of a illuminated by a plane wave photocathode. They showed that this effect - which is named after its discoverers Hanbury Brown Twiss effect - can be interpreted both by the classical wave theory of light as well as by its quantum nature.

According to the work of 1957 reflect the mutually correlated escape times of electrons seen classic uncorrelated intensity fluctuations resist, which occur at different points of the incident wave. These are due to interference of various frequency components of the incident light. In quantum mechanics, the effect seen is due to the fact that photons of the Bose -Einstein statistics follow and thus tend to occur more frequently (which is often referred to as photon bunching ).

In a subsequent paper, the two researchers set, how can the use of them discovered effect for the measurement of the angular diameter of a star. If the two currents are strongly correlated with each other ( which means that photons arrive often simultaneously on both receivers ), so the observed star is not yet resolved. This is the case when the distance of the two telescopes, is too low. If this is increased, taking the correlation between the streams (i.e., the arrival times of photons) from. From the waste of the correlation with increasing distance of the angular diameter of the star can be determined. This is a function of the expression (hereinafter, the wavelength of the incident light ). Has a star, for example, twice the angle in the diameter compared to another, one has to pull apart the telescopes only half as far in order to observe the same correlation waste.

Finally it should be mentioned also a practical example with the stars α Lyrae ( Vega ) and β Crucis ( Mimosa ). In the former, the correlation of the individual streams falls even with a telescope distance of about 20 m from practically to zero. In the latter case one has to remove the about 100 m from each other to achieve the same effect. Consequently Wega has a much larger angular diameter than Mimosa.

Measurement accuracy

The due to the arrival times of the photons correlated intensity fluctuations are superimposed by far stronger not uncorrelated fluctuations. One source of this additional variation is the air turbulence that causes everyday sparkle of the stars ( scintillation ), another shot noise emanating from the two photomultipliers streams. The consequent fluctuations exceed the actually sought correlated fluctuations by far, by about a factor of 100,000! The uncorrelated additive fluctuations due to very long observation times be " averaged out ", but even in very bright, easily visible with the naked eye stars (not less than 2 size) were exposure times of up to 100 hours required! An extraordinary stability of the measurement electronics, in particular of the correlator is therefore extremely important for the instrument.

Fortunately, the air turbulence has little impact on how quickly drops the correlation between the individual streams with increasing distance between the telescopes. Thus, although the air turbulence enforces very long exposure times, but does not distort the appearance of the curve (and thus the derived angular diameter of the star ), which describes the correlation as a function of and.

The accuracy with which the angular diameter of a star can be determined, is given by the ratio of fiber length to maximum Telekopabstand. With = 440 nm and = 188 m ( for the instrument of the 60s ) = 0.0005 is expected. " In practice, Hanbury Brown still reached a slightly better accuracy, namely in the Middle 0.0002 ". Thus, the Intensitätsinterferometer surpasses the classical Michelson Sterninterferometer by far and also allows the inclusion of main-sequence stars close to the Sun. While the resolution of the Michelson Sterninterferometers is limited by the air turbulence ( here it comes to the image of the star in the form of an interference pattern on ), this is not the case with the Intensitätsinterferometer. It only interested in the intensity of the star, not its visual representation.

In order to interpret the measurement results of the Intensitätsinterferometers correctly, must be like in the Michelson Sterninterferometer the limb darkening ( the drop in the intensity of the stellar disk from the center to the edge ) are observed. According to Hanbury Brown's publications from 1967 this has the consequence that the correlation of the two individual flows slower in pulling apart the telescopes decreases, as one would expect in a uniformly bright stellar disk, ie the angular diameter is underestimated. However, the nature of the mathematical law between correlation and telescopic distance remains unchanged, they will only receive a longer scale with respect. Thus, the interferometer provides an effective angular diameter - which the star would have if the disc uniformly illuminated at the same total intensity. In order to determine from the effective diameter to the actual angle, a model of the star atmosphere is required. The Hanbury Brown ( 1967b ) measured main-sequence stars, the modification of the angular diameter is only a few percent.

History

After the theoretical groundwork in the 1950s soon also managed a successful test measurement of Sirius. The first fully operational device of its kind was in 1962 in Narrabri ( Australia) in operation. The construction and commissioning of the interferometer proved to be a difficult challenge, only this phase lasted for almost two years. It was followed by a turn two-year phase of test measurements of several main-sequence stars, in 1965 launched the actual observation program. 1967 was the first publication of angular diameters of 15 main-sequence stars .. Overall, the angular diameters were determined to 1972 of 32 main-sequence stars.

The interferometer consisted of two reflectors, each 6.7 m in diameter, which were composed of 252 hexagonal each individual mirrors. The two instruments could be removed on a track circle up to 188 m from each other.

1990 was put into operation as a successor instrument to the Sydney University Stellar Interferometer SUSI. It is located on the Observatory Culgoora near Narrabi. This is however not a Intensitätsinterferometer, but a modern Michelson interferometer with adaptive optics for the lack of classical interferometer, the disturbance of the interference image by air turbulence overcomes. At the same time it is far more sensitive than the Narrabi interferometer allows the measurement of stars to about the eighth magnitude. The maximum spacing of the interference brought to light paths is 640 m. A description of the SUSI can be found at Michelson Sterninterferometer.

The measurement of the angular diameter of a star and thus at a known distance and the radius of which inevitably leads to the question, what is meant is actually below the surface of a star. This, in the absence of a solid crust is anything but trivial problem is discussed under stellar surface.

Swell

• Hanbury Brown R.: A Test of a new Type of Stellar Interferometer on Sirius, in: Nature Volume 178, pp. 1046ff, 1956
• Hanbury Brown R., Twiss RG: Correlation in between Photons in two Coherent Beams of Light, in: Nature Volume 177, pp. 27ff, 1956
• Hanbury Brown R., Twiss RG: Interferometry of the Intensity Fluctuations in Light. I. Basic Theory: the correlation in between Photons in Coherent Beams of Radiation, in: Proceedings of the Royal Society of London Volume 242, pp. 300ff, 1957
• Hanbury Brown R., Twiss RG: Interferometry of the Intensity Fluctuations in Light. III. Applications to Astronomy, in: Proceedings of the Royal Society of London Volume 248, p 199ff, 1958
• Hanbury Brown R., Davis J., Allen LR: The Stellar Interferometer at Narrabi Observatory. I. A Description of the instrument and the observational procedure, in: Monthly Notices of the Royal Astronomical Society, Volume 137, p 3 75ff, 1967a
• Hanbury Brown R., Davis J., Allen LR, Rome JM: The Stellar Interferometer at Narrabi Observatory. II The Angular Diameters of 15 Stars, in: Monthly Notices of the Royal Astronomical Society, Volume 137, pp. 393ff, 1967b
• Hanbury Brown R., Davis J., Allen LR: The Angular Diameters of 32 Stars, in: Monthly Notices of the Royal Astronomical Society, Volume 167, p 121ff, 1974
• J. Davis, WJ Tango, AJ Booth, ten Brummelaar TA, Minard AR, Owens SM: The Sydney University Stellar Interferometer - I. The instrument, in: Monthly Notices of the Royal Astronomical Society, Volume 303, p 773ff, 1999