The Chatterton Astronomy Department of the University of Sydney is proposing the construction of a very high angular resolution stellar interferometer of major astronomical and international significance. Based on the prototype modern Michelson stellar interferometer which the Department has developed, the new instrument will be used in a wide range of astrophysical programmes. The siting, specification, design and costing of the instrument are outlined.

The Sydney University Stellar Interferometer (SUSI) uses siderostats (plane, alt–azimuth mounted mirrors) to reflect starlight into the instrument. The analysis of the pointing and guidance of the siderostats by traditional methods is not practical and vector algebra has been used to develop the required algorithms. Real siderostats will not be ideal and can be modelled by a set of parameters. The methods for determining the model parameters and for incorporating them into the guidance and pointing algorithms are described. Field rotation also affects the operation of SUSI, and the field rotation angle and related quantities are also determined.

The Chatterton Astronomy Department of Sydney University, in collaboration with the Astronomy Group of Imperial College (London), has carried out a program of speckle interferometry on the 3.9 m Anglo-Australian Telescope (AAT). Over 2 km of film were exposed during the two observing sessions in November and December, 1978, and the analysis of this very large amount of data is being carried out at the present time.

New angular diameter determinations for the bright southern F8 supergiant δ CMa enable the bolometric emergent flux and effective temperature of the star to be determined with improved accuracy. The spectral flux distribution and bolometric flux have been determined from published photometry and spectrophotometry and combined with the angular diameter to derive the bolometric emergent flux ℱ = (6.50 ± 0.24) × 107 Wm−2 and the effective temperature T eff = 5818 ± 53 K. The new value for the effective temperature is compared with previous interferometric and infrared flux method determinations. The accuracy of the effective temperature is now limited by the uncertainty in the bolometric flux rather than by the uncertainty in the angular diameter.

A new beam-combination and detection system has been installed in the Sydney University Stellar Interferometer working at the red end of the visual spectrum (λλ 500–950 nm) to complement the existing blue-sensitive system (λλ 430–520 nm) and to provide an increase in sensitivity. Dichroic beam-splitters have been introduced to allow simultaneous observations with both spectral systems, albeit with some restriction on the spectral range of the longer wavelength system (λλ 550–760 nm). The blue system has been upgraded to allow remote selection of wavelength and spectral bandpass, and to enable simultaneous operation with the red system with the latter providing fringe-envelope tracking. The new system and upgrades are described and examples of commissioning tests presented. As an illustration of the improvement in performance the measurement of the angular diameter of the southern F supergiant δ CMa is described and compared with previous determinations.

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 SUSI control system is a distributed real-time system currently consisting of 17 processors. A custom real-time operating system and network protocols ensure synchronous operation of servo loops across multiple processors.

A new 1.87 m fixed-baseline interferometer designed for binary star observation is now in operation at the Italian outstation of the Royal Observatory Edinburgh. It has a theoretical resolving power of 0.015 arcsee and a limiting B magnitude of 6.5. Atmospheric seeing is dynamically corrected, allowing a quantitative measurement of fringe visibility.