Abstract

The bispectrum (triple correlation) and Knox Thompson (KT) algorithms are capable of achieving nearly diffraction limited astronomical imaging when applied to large numbers of short-exposure speckle images. Previous comparisons of these algorithms have shown that at low signal levels the spectral noise at spatial frequencies above the atmospheric cut-off are essentially the same for the bispectrum and KT in the near-axis region. However, we have discovered a previously unreported difference in the behavior of their atmospherically-induced phase variances that dominate at high signal levels. The bispectrum phase variance decreases to zero while the KT phase variance remains almost constant. At high signal levels these differences are important and can result in images obtained from the bispectrum having as much as an order of magnitude lower error than those obtained from KT crossspectra. We have studied the phenomenon by simulations and by calculation of the bispectrum variances at low spatial frequencies with an extension of the method Von der Luhe¹ used to determine the variance of the KT crossspectra.

© 1993 Optical Society of America