Astronomy with SUSI

Stellar astronomy programs

The observing programs for SUSI concentrate on stellar astronomy, including:

  • Single stars - measuring emergent fluxes, effective temperatures, radii and luminosities.
  • Binary stars - as for single stars, plus measuring distances and masses.
  • Variable stars (e.g. Cepheids and Miras) - as for single stars, plus distances.
  • Emission line stars (e.g. Be and Wolf-Rayet stars) - measure the relative sizes of star and emission regions, emergent fluxes, effective temperatures.
  • Stellar rotation.
  • Limb darkening.
  • Interstellar extinction.

These programs can be tackled with an interferometer that measures the angular diameters of stars and the angular separations of binary systems without the need to form images.

SUSI has been designed with this in mind and light is combined from two apertures at a time in the current configuration. This is in contrast to instruments such as the Cambridge Optical Aperture Synthesis Telescope (COAST) that combine the light from three or more apertures simultaneously to enable images to be constructed using phase-closure techniques.

Basic design specifications

In order to achieve the science objectives, SUSI has the following performance targets:

  • A limiting V magnitude of +7.5.
  • The capability of measuring angular sizes down to 75 micro-arc seconds (7.5 x 10^-5 arc seconds) at a wavelength of 450nm to enable a sample of hot O-type stars to be measured.
  • An accuracy in angular size measurements =<2% (+/- 5% would be the lowest useful accuracy).
  • A spectral range from 400 to 900nm, with the possibility of extension to longer wavelengths.

Current Status

SUSI currently operates with baselines out to 80 m and at a wavelength of 442 nm. Recent work has concentrated on commissioning an additional beam combination system (the 'red table') operating at the red end of the spectrum. This will have the advantage of relaxed constaints imposed by the atmospheric seeing at longer wavelengths. The red table will also feature a new data processing scheme which allows the optical bandwidth to be increased and therefore promises a substantial increase in sensitivity.

Potential future developments of SUSI include an eastern array of siderostat stations. In the first instance any station on this arm could be combined with any station on the north or south arms to give improved parallactic angle cover for the study of binary systems, rotating stars etc. In the longer term, it is possible to upgrade the system to combine the light from three or more apertures using phase-closure techniques to form images.

Recent Observations

SUSI has been involved in a semi-continuous programme of observations, concentrating in particular on close binary systems. Of particular interest has been the Beta Centauri system whose orbit has now been well determined for the first time.