Recovery of Diffraction-Limited Object Autocorrelations from Astronomical Speckle Interferograms using the CLEAN Algorithm

We present a new technique for processing speckle interferometric data is presented, which uses the CLEAN algorithm, originally developed for the removal of the effects of incomplete spatial frequency coverage in aperture synthesis radio maps. Since Labeyrie first noted in 1970 that the autocorrelation of a speckle-gram preserves information up to the diffraction limit of an optical telescope, a number of different techniques have been applied to recover this information, usually by Fourier deconvolution of the average power spectrum of the specklegrams with a similar spectrum for a known point source. An alternative is to deconvolve the average autocorrelation of the specklegrams directly, rather than the power spectrum; this is where CLEAN, which has been shown to be a powerful tool in deconvolution problems, may be used. Also, because of the immunity of CLEAN to gaps in the spatial frequency coverage of the power spectrum, deconvolution is robust under conditions where regions of low signal-to-noise ratio i the raw speckle data effectively introduce such gaps. We find that CLEAN is straightforward to apply, and yields results that exceed the quality of recoveries made using at least one other existing technique. Diffraction-limited and near-diffraction-limited results are presented, using photon-noise-limited specklegrams taken with the Hale 5-m telescope at Palomar under a variety of seeing conditions.