Digital Correlators for Intensity Interferometry and High-Speed Astrophysics
  Dainis Dravins, Lund Observatory, Box 43, SE-22100 Lund, Sweden

Abstract: 

Methods and instrumentation for high time-resolution observations in optical 
astrophysics are being developed, using various photon-counting detectors 
(photomultipliers, avalanche photodiodes) and electronics for real-time 
calculation of the photon arrival statistics (autocorrelation, 
cross-correlation, photon distribution functions).  Such instrumentation has 
been used both in the laboratory and at various observatories, e.g., for 
studying atmospheric intensity scintillation from La Palma (Canary Islands), 
for observing rapidly variable sources such as the Crab pulsar from the 
Skinakas Observatory (Crete), and for testing intensity interferometry with 
the atmospheric Cherenkov telescopes of VERITAS (Arizona).  A sequence of 
photon-correlators from different manufacturers have reached successively
higher performance, and current units permit handling very high photon-count 
rates (>100 MHz) , have a time resolution close to 1 ns, while simultaneously 
handling the input from many detectors measuring different wavelength bands 
or placed at different locations.  This performance is now believed to be 
adequate for carrying out digital intensity interferometry connecting groups 
of atmospheric Cherenkov telescopes.