The resolution of conventional astronomical images made with large, ground-based telescopes is
limited to about 0.3 to 1 arcsec by atmospheric turbulence. Much higher resolution can be obtained by
the speckle masking method13 . Speckle masking yields images with diffraction-limited resolution, for
example, 0.03 arcsec resolution for a 3.6-rn telescope. In speckle masking a large number of speckle
interferograms (short-exposure photographs; exposure time 0.05 sec) has to be evaluated. The first
image processing step is the calculation of the ensemble average bispectrum or the ensemble average
triple correlation of all speckle interferograms. The speckle interferograms of faint astronomical objects
consist of only a small number of photon events. For example in the NGC 1068 experiment described
below, each of the 10 000 evaluated speckle interferograms consisted of about 400 photon events only.
As described in the next section, triple correlation or bispectrum processing can be applied to the
photon-counting speckle interferograms.