  We present measurements of the second order spatial coherence function of
thermal light sources using Hanbury-Brown and Twiss interferometry with a
digital correlator. We demonstrate that intensity fluctuations between
orthogonal polarizations, or at detector separations greater than the spatial
coherence length of the source, are uncorrelated but can be used to reduce
systematic noise. The work performed here can readily be applied to existing
and future Imaging Air-Cherenkov Telescopes used as star light collectors for
Stellar Intensity Interferometry (SII) to measure spatial properties of
astronomical objects.
