Stellar Intensity Interferometry - The Background
John Davis
Sydney Institute for Astronomy, School of Physics, University of Sydney, 
NSW, Australia

The origins of stellar intensity interferometry will be outlined starting 
from Robert Hanbury Brown’s original idea that was conceived to tackle a 
particular problem in radio astronomy. By the time a radio intensity 
interferometer had been proved experimentally, the radio problem had been 
solved by amplitude interferometry but it was observed that the intensity 
interferometric signal was unaffected by scintillation. Hanbury Brown and 
Richard Twiss had the insight to realise that the technique might work at  
optical wavelengths and overcome the problems that had prevented the 
development of the classical Michelson stellar interferometer.   In a series 
of laboratory experiments, that were controversial at the time, they 
demonstrated the feasibility of intensity interferometry at optical 
wavelengths.   Hanbury Brown went further and illustrated its potential 
by measuring the angular diameter of Sirius in the presence of strong 
scintillation from Jodrell Bank where the elevation of Sirius at transit 
is less than 20 degrees.

The success of the Sirius measurement led to the development of the Narrabri 
Stellar Intensity Interferometer (NSII), a joint project between the 
Universities of Manchester and Sydney, which was located in Australia.  The 
NSII will be described, some of the practical problems encountered will be 
presented, and the achievements summarised.   As it neared the end of its 
useful life, set by its limiting sensitivity, plans were developed for a next 
generation Very Large Stellar Intensity Interferometer (VLSII).   For 
reasons that will be outlined, the plans for the VLSII were abandoned in 
favour of an amplitude interferometer and, after the development and testing 
of a prototype instrument, the Sydney University Stellar Interferometer 
(SUSI) was built.  SUSI will be described briefly and some of its achievements 
summarised.

Finally, some thoughts on the potential future of stellar intensity 
interferometry will be presented.