The atmosphere of the earth restricts the resolution of conventional astrophotography to " 1 arcsec.
Much higher resolution can be obtained by interferometric speckle techniques. Bispectrum analysis'3
(also called speckle masking) of many speckle interferograms (short-exposure photographs; exposure time
,,o.o5 sec) yields diffraction-limited images with, for example, 0.02 arcsec resolution for a 5-rn telescope.
After the first processing steps of speckle masking the object bispectrurn o()(u,v) is obtained up to the
cut-off frequency of the telescope. From the object bispectrum a diffraction-limited image of the object
can be reconstructed (a) by using the conventional recursive method2 (the Fourier phase of the object is
extracted recursively from O()(u,v) ) or (b) by using an iterative image reconstruction algorithm presented
in this paper. The iterative algorithm searches for the high-resolution image which has the best agreement
with the measured object bispectrum.
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