  Measurements of the spectrum of the fluctuations of the output current of the
quadratic detector of a telescope can be used to find unresolved astronomical
gravitational lenses and determine time delays between their image components.
These time delays can be used for astronomical studies. The spatial correlation
coefficient of a source is an important parameter that quantifies the loss of
contrast, caused by the extendedness of the source, in the spectral modulation
of the intensity fluctuations. This work shows that the correlation coefficient
must not be evaluated at the frequency of observation, but must instead be
evaluated at the much lower beat frequencies of the spectrum of the
fluctuations. This opens up a powerful novel technique to find unresolved
gravitational lenses and to study the lensing event and the source.
