This chapter is devoted to Robert Hanbury Brown (1916-2002) and Richard Quintin Twiss (1920-2005). Sections 19.1 and 19.2 (written by William J. Tango who worked with Twiss) are their biographies. Hanbury Brown and Twiss first carried out experiments on observation of photon correlations (bunching). Section 19.3 contains an excerpt of several pages from the book of R. Hanbury Brown "Boffin: A Personal Story of the Early Days of Radar, Radio Astronomy and Quantum Optics" in which he describes the story of the discovery of photon correlations. The Hanbury Brown-Twiss effect marks the beginning of modern quantum optics.

Boksenberg, A., in Auxiliary Instrumentation for Large Telescopes. Proc. ESO/CERN Conf., May 1972 (Ed. E. Lausten and A. Reiz), p. 295 (1972). Gardner, F.F., and Whiteoak, J.B., Mon. Not. R. Astron. Soc, 175, 9P (1975). Gardner, F.F., and Whiteoak, J.B., Proc. Astron. Soc. Aust., 3, 63 (1976). Graham, J.A., Astrophys. J. (in press) (1979). Kunkel, W.E., and Bradt, H.V., Astrophys. J., 170, L7 (1971). Shobbrook, R.R., and Shaver, P.A., Observatory, 87, 169 (1967). Wade, CM., Hjellming, R.M., Kellermann, K.I., and Wardle, J.F.C., Astrophys. J., 170, Lll (1971). Whiteoak, J.B., in Structure and Properties of Nearby Galaxies, Proc. IAU Symp. No. 77, August 1977 (Ed. E.M. Berkhuijsen and R. Wielebinski), p. 131 (1978). Whiteoak, J.B., and Gardner, F.F., Astrophys. J., 195, L81 (1975). Whiteoak, J.B., and Gardner, F.F., Aust. J. Phys., 30, 187 (1977). Contributions 321

SUSI (Sydney University Stellar Interferometer) is currently being fitted with a 2nd beam combiner, MUSCA (Micro-arcsecond University of Sydney Companion Astrometry), for the purpose of narrow-angle astrometry. With an aim to achieve similar to 10 micro-arcseconds of angular resolution at its best, MUSCA allows SUSI to search for planets around bright binary stars, which are its primary targets. While the first beam combiner, PAVO (Precision Astronomical Visible Observations), is used to track stellar fringes during an observation, MUSCA will be used to measure separations of binary stars. MUSCA is a Michelson interferometer and its setup at SUSI will be described in this poster.

The conventional approach to high-precision narrow-angle astrometry using a long baseline interferometer is to directly measure the fringe packet separation of a target and a nearby reference star. This is done by means of a technique known as phase-referencing which requires a network of dual beam combiners and laser metrology systems. Using an alternative approach that does not rely on phase-referencing, the narrow-angle astrometry of several closed binary stars (with separation less than 2"), as described in this paper, was carried out by observing the fringe packet crossing event of the binary systems. Such an event occurs twice every sidereal day when the line joining the two stars of the binary is is perpendicular to the projected baseline of the interferometer. Observation of these events is well suited for an interferometer in Antarctica. Proof of concept observations were carried out at the Sydney University Stellar Interferometer (SUSI) with targets selected according to its geographical location. Narrow-angle astrometry using this indirect approach has achieved sub-100 micro-arcsecond precision.

We present the first multiplicity-dedicated long-baseline optical interferometric survey of the Scorpius-Centaurus-Lupus-Crux association. We used the Sydney University Stellar Interferometer to undertake a survey for new companions to 58 Sco-Cen B-type stars and have detected 24 companions at separations ranging from 7 to 130 mas, 14 of which are new detections. Furthermore, we use a Bayesian analysis and all available information in the literature to determine the multiplicity distribution of the 58 stars in our sample, showing that the companion frequency is f = 1.35 +/- 0.25 and the mass ratio distribution is best described by q(gamma) with gamma = -0.46, agreeing with previous Sco-Cen high-mass work and differing significantly from lower mass stars in Tau-Aur. Based on our analysis, we estimate that among young B-type stars in moving groups, up to 23 per cent are apparently single stars. This has strong implications for the understanding of high-mass star formation, which requires angular momentum dispersal through some mechanism such as formation of multiple systems.

A method capable of delivering relative optical path length metrology with nanometer precision is demonstrated. Unlike conventional dual-wavelength metrology, which employs heterodyne detection, the method developed in this work utilizes direct detection of interference fringes of two He-Ne lasers as well as a less precise stepper motor open-loop position control system to perform its measurement. Although the method may be applicable to a variety of circumstances, the specific application in which this metrology is essential is in an astrometric optical long baseline stellar interferometer dedicated to precise measurement of stellar positions. In our example application of this metrology to a narrow-angle astrometric interferometer, measurement of nanometer precision could be achieved without frequency-stabilized lasers, although the use of such lasers would extend the range of optical path length the metrology can accurately measure. Implementation of the method requires very little additional optics or electronics, thus minimizing the cost and effort of implementation. Furthermore, the optical path traversed by the metrology lasers is identical to that of the starlight or science beams, even down to using the same photodetectors, thereby minimizing the noncommon path between metrology and science channels.

The Sydney University Stellar Interferometer (SUSI) now incorporates a new beam combiner, called the Microarc-second University of Sydney Companion Astrometry instrument (MUSCA), for the purpose of high precision differential astrometry of bright binary stars. Operating in the visible wavelength regime where photon-counting and post-processing fringe tracking is possible, MUSCA will be used in tandem with SUSI's primary beam combiner, Precision Astronomical Visible Observations (PAVO), to record high spatial resolution fringes and thereby measure the separation of fringe packets of binary stars. In its current phase of development, the dual beam combiner configuration has successfully demonstrated for the first time a dual-star phase-referencing operation in visible wavelengths. This paper describes the beam combiner optics and hardware, the network of metrology systems employed to measure every non-common path between the two beam combiners and also reports on a recent narrow-angle astrometric observation of δ Orionis A (HR 1852) as the project enters its on-sky testing phase.

This paper presents an overview of recent progress at the Sydney University Stellar Interferometer (SUSI). Development of the third-generation PAVO beam combiner has continued. The MUSCA beam combiner for high-precision differential astrometry using visible light phase referencing is under active development and will be the subject of a separate paper. Because SUSI was one of the pioneering interferometric instruments, some of its original systems are old and have become difficult to maintain. We are undertaking a campaign of modernization of systems: (1) an upgrade of the Optical Path Length Compensator IR laser metrology counter electronics from a custom system which uses an obsolete single-board computer to a modern one based on an FPGA interfaced to a Linux computer -in addition to improving maintainability, this upgrade should allow smoother motion and higher carriage speeds; (2) the replacement of the aged single-board computer local controllers for the siderostats and the longitudinal dispersion compensator has been completed; (3) the large beam reducing telescope has been replaced with a pair of smaller units with separate accessible foci. Examples of scientific results are also included.

Abstract The Sydney University Stellar Interferometer (SUSI) has been used to make a new determination of the angular diameter of Sirius A. The observations were made at an effective wavelength of 694.1 nm and the new value for the limb-darkened angular diameter is 6.048 ± 0.040 mas (± 0.66%). This new result is compared with previous measurements and is found to be in excellent agreement with a conventionally calibrated measurement made with the European Southern Observatory's Very Large Telescope Interferometer (VLTI) at 2.176 μm (but not with a second globally calibrated VLTI measurement). A weighted mean of the SUSI and first VLTI results gives the limb-darkened angular diameter of Sirius A as 6.041 ± 0.017 mas (± 0.28%). Combination with the Hipparcos parallax gives the radius equal to 1.713 ± 0.009 R⊙. The bolometric flux has been determined from published photometry and spectrophotometry and, combined with the angular diameter, yields the emergent flux at the stellar surface equal to (5.32 ± 0.14) × 108 W m−2 and the effective temperature equal to 9845 ± 64 K. The luminosity is 24.7 ± 0.7 L⊙.

The Sydney University Stellar Interferometer uses embedded processors to control each siderostat station as well as other major components of the instrument. The maintenance of the original controllers has become a significant issue and we set out to design a new system that would be inexpensive, suitable for the relatively harsh operating environment and simple to maintain. We have demonstrated that the new system works satisfactorily and we are currently replacing the existing controllers with new ones.

The University of Sydney has a long history in optical stellar interferometry. The first project, in the 1960s, was the Narrabri Stellar Intensity Interferometer, which measured the angular diameters of 32 hot stars and established the temperature scale for spectral classes O - F. That instrument was followed by the Sydney University Stellar Interferometer (SUSI), which is now undergoing a third-generation upgrade, to use the multi-wavelength PAVO beam combiner. SUSI operates at visible rather than IR wavelengths and has baselines up to 160 m, so it is well suited to the study of hot stars. A number of studies have been carried out, and more are planned when commissioning of the PAVO system is complete. Conversion of the system to allow remote operation will allow larger scientific projects to be undertaken.

Observations of the southern Cepheid l Car to yield the mean angular diameter and angular pulsation amplitude have been made with the Sydney University Stellar Interferometer at a wavelength of 696 nm. The resulting mean limb-darkened angular diameter is 2.990 +/- 0.017 mas (i.e. +/- 0.6 per cent) with a maximum-to-minimum amplitude of 0.560 +/- 0.018 mas corresponding to 18.7 +/- 0.6 per cent in the mean stellar diameter. Careful attention has been paid to uncertainties, including those in measurements, in the adopted calibrator angular diameters, in the projected values of visibility squared at zero baseline, and to systematic effects. No evidence was found for a circumstellar envelope at 696 nm. The interferometric results have been combined with radial displacements of the stellar atmosphere derived from selected radial velocity data taken from the literature to determine the distance and mean diameter of l Car. The distance is determined to be 525 +/- 26 pc and the mean radius 169 +/- 8 R(circle dot). Comparison with published values for the distance and mean radius shows excellent agreement, particularly when a common scaling factor from observed radial velocity to pulsation velocity of the stellar atmosphere (the p-factor) is used.

The binary star delta Sco (HD143275) underwent remarkable brightening in the visible in 2000, and continues to be irregularly variable. The system was observed with the Sydney University Stellar Interferometer (SUSI) in 1999, 2000, 2001, 2006 and 2007. The 1999 observations were consistent with predictions based on the previously published orbital elements. The subsequent observations can only be explained by assuming that an optically bright emission region with an angular size of greater than or similar to 2 +/- 1 mas formed around the primary in 2000. By 2006/2007 the size of this region grew to an estimated >= 4 mas.We have determined a consistent set of orbital elements by simultaneously fitting all the published interferometric and spectroscopic data as well as the SUSI data reported here. The resulting elements and the brightness ratio for the system measured prior to the outburst in 2000 have been used to estimate the masses of the components. We find M(A) = 15 +/- 7M(circle dot) and M(B) = 8.0 +/- 3.6 M(circle dot). The dynamical parallax is estimated to be 7.03 +/- 0.15 mas, which is in good agreement with the revised Hipparcos parallax.

We have used the Sydney University Stellar Interferometer (SUSI) to measure the angular diameter of the F9V star beta Virginis (beta Vir). After correcting for limb darkening and combining with the revised Hipparcos parallax, we derive a radius of 1.703 +/- 0.022 R-circle dot (1.3 per cent). We have also calculated the bolometric flux from published measurements which, combined with the angular diameter, implies an effective temperature of 6059 +/- 49K (0.8 per cent). We also derived the luminosity of beta Vir to be L = 3.51 +/- 0.08 L-circle dot (2.1 per cent). Solar-like oscillations were measured in this star by Carrier et al. and using their value for the large frequency separation yields the mean stellar density with an uncertainty of about 2 per cent. Our constraints on the fundamental parameters of beta Vir will be important to test the theoretical models of this star and its oscillations.

The first orbital solution for the spectroscopic pair in the multiple star system σ Scorpii, determined from measurements with the Sydney University Stellar Interferometer (SUSI), is presented. The primary component is of β Cephei variable type and has been one of the most intensively studied examples of its class. The orbital solution, when combined with radial velocity results found in the literature, yields a distance of 174 −18 pc, which is consistent with, but more accurate than the Hipparcos value. For the primary component we determine 18.4±5.4M⊙, −4.12±0.34 mag and 12.7 ± 1.8R⊙ for the mass, absolute visual magnitude and radius respectively. A B1 dwarf spectral type and luminosity class for the secondary is proposed from the mass determination of 11.9 ± 3.1M⊙ and the estimated system age of 10Myr.

Observations of the southern Cepheids ℓCar and β Dor to yield the mean angular diameters and angular pulsation amplitudes have been made with the Sydney University Stellar Interferometer (SUSI)at a wavelength of 700 nm. The results of a preliminary analysis are compared with those obtained with the VLTI at 2.2 μm and excellent agreement between the results from the two instruments is found for ℓCar but there are significant differences for βDor.

We have used the Sydney University Stellar Interferometer to measure the angular diameter of α Cir. This is the first detailed interferometric study of a rapidly oscillating A (roAp) star, α Cir being the brightest member of its class. We used the new and more accurate Hipparcos parallax to determine the radius to be 1.967 ± 0.066 R⊙. We have constrained the bolometric flux from calibrated spectra to determine an effective temperature of 7420 ± 170 K. This is the first direct determination of the temperature of an roAp star. Our temperature is at the low end of previous estimates, which span over 1000 K and were based on either photometric indices or spectroscopic methods. In addition, we have analysed two high-quality spectra of α Cir, obtained at different rotational phases and we find evidence for the presence of spots. In both spectra we find nearly solar abundances of C, O, Si, Ca and Fe, high abundance of Cr and Mn, while Co, Y, Nd and Eu are overabundant by about 1 dex. The results reported here provide important observational constraints for future studies of the atmospheric structure and pulsation of α Cir.