Abstract

The Knox-Thompson (KT), triple-correlation (TC), and phase-gradient (PG) algorithms have been shown capable of reconstructing the phase of stellar spatial spectra upto the diffraction limit of large telescopes. Combined with the modulus measurements of Labeyrie’s speckle interferometry, these techniques offer from 10 to 50 times improvement in image resolution over the 2-sec of arc limit imposed on conventional astronomical observations by atmospheric turbulence. We report results of a comparative study in which the performance of the three techniques is evaluated with a common set of photon-limited speckle-image data from both simulated and real astronomical sources. The regime of interest is < 1 photon/ speckle and <500 photons/image. In these conditions PG gives excellent reconstructed images and is the most computationally efficient of the three. KT gives slightly poorer results in severe atmospheric conditions, can be improved by image centroiding, but experiences a further distorting effect at very low photon levels. TC requires the greatest computational effort but appears to be more robust with respect to certain types of data defect. Extension of TC can give a small improvement in image quality but at a high computational cost.

© 1988 Optical Society of America