Showing papers by "Jeffrey H. Shapiro published in 1976"

Propagation-medium limitations on phase-compensated atmospheric imaging*

Abstract: There has long been interest in reducing or circumventing the limitations imposed by atmospheric turbulence on optical imaging systems. Recent studies have shown that irradiance or speckle interferometry, or real-time atmospheric compensation, may be used to regain diffraction-limited performance. In this paper, the relationship between real-time phase compensation and the optimum channel-matched filter compensator is developed, with emphasis on the fundamental limits imposed by the propagation medium. In particular, the effects of uncompensated amplitude fluctuations and finite isoplanatic diameter are evaluated. It is shown that the former does not usually present any limitation on imaging performance, whereas the latter may severely limit the field of view over which diffraction-limited imaging can be realized. Results are presented for both coherently and incoherently illuminated objects.

Diffraction-limited atmospheric imaging of extended objects*

Abstract: Recent studies have shown that the use of predetection processing can greatly improve the quality of images formed through atmospheric turbulence. It is generally assumed that the object to be imaged lies within a single isoplanatic patch, and it is recognized that the angular subtense of the isoplanatic patch may be quite small, comparable to the seeing limit. In this paper the use of predetection processing for objects which comprise many isoplanatic elements is considered. It is shown that an atmosphere–lens system behaves macroscopically as though it consisted microscopically of a collection of uncorrelated scatterers with random scattering strengths which contribute image components with varying spatial and spatial-frequency displacements. In communication theory such systems are referred to as wide-sense stationary uncorrelated scatter (WSSUS) channels and are characterized by a macroscopic scattering function. The macroscopic scattering function for the atmosphere is derived and used to show that under typical turbulence-strength conditions the seeing limit can resolve the isoplanatic patch. Thus the fields from different isoplanatic patches of an extended object may be separated in the image plane of an objective lens and processed individually to obtain a diffraction-limited image.