Title:Towards a Square-Kilometer Optical Telescope: 
The Potential of Intensity Interferometry

Abstract:
Many stars begin to appear as surface objects for baselines of hundreds of meters, while kilometric 
ones are desired for imaging stellar-disk features or for resolving more remote objects.  Since 
atmospheric turbulence makes phase interferometry challenging for such long baselines, kilometric 
space telescope clusters have been proposed.  The complexities of such projects make their 
realization uncertain but comparable imaging could be realized by ground-based intensity 
interferometry, at least for brighter and hotter stars (S/N increases with temperature).  The 
method is insensitive to both atmospheric turbulence and to imperfections in telescope optics, but 
requires large flux collectors, such as being set up as arrays of atmospheric Cherenkov telescopes 
for studying energetic gamma rays. High-speed detectors and digital signal handling enable very 
many baselines to be synthesized between many pairs of telescopes, while stars may be tracked 
across the sky by electronic time delays.  In essence, this becomes a digital revival of the 
technique pioneered by Hanbury Brown & Twiss long ago, now forming an optical software telescope 
somewhat analogous to LOFAR in the radio.  Following laboratory work, first test observations with 
digitally combined optical instruments have now been made with pairs of 12-meter telescopes of the 
VERITAS array in Arizona.  Observing at short wavelengths adds no problems, and similar techniques 
on E-ELT could achieve diffraction-limited imaging down to the atmospheric cutoff, achieving a 
spatial resolution superior to that feasible by adaptive optics operating in the (infra)red. 

A working group on intensity interferometry was recently set up within the IAU Commission 54 
(“Optical and Infrared Interferometry”), a workshop on the technique will be held in Salt Lake 
City now in January, and a study is in progress to investigate possible design constraints for 
enabling a use of the planned CTA, Cherenkov Telescope Array, for also stellar intensity 
interferometry.   CTA, foreseen as an array of perhaps 50 to 80 optical dishes of 10-20 meter size, 
spread over some square km, was identified in the recent ASTRONET study as one of the priority 
projects for European astronomy.  Its main task of observing atmospheric Cherenkov light induced by 
high-energy gamma rays is probably only feasible during dark-Moon periods while the bright time 
might be made available for optical intensity interferometry, thus perhaps realizing the first 
kilometric-scale optical imager.