Speckle masking (or bispectral analysis) is a speckle imaging method which involves estimation of the bispectrum or closure phases from each of the short exposures. The "average bispectrum" can then be calculated and then inverted to obtain an image. With a normal telescope aperture, a large number of short exposures must be averaged to remove the effects of the atmosphere. If a non-redundant aperture mask is used the bispectrum can be calculated without any noise contribution from the atmosphere (but still with a photon-shot noise error contribution). In this arrangement a ground-based telescope aperture is blocked by astronomers apart from a few holes which allow light through, creating a small optical interferometer with the resolving power of a diffraction-limited telescope rather than being limited by the astronomical seeing.