DIFFRACTION-LIMITED IMAGING WITH PARTIALLY REDUNDANT MASKS .2. OPTICAL IMAGING OF FAINT SOURCES

In a recent paper [J. Opt. Soc. Am. A 9, 203 (1992)] the benefits of p upil apodization were examined for the near-infrared imaging of bright sources. In the current paper we extend these considerations to optic al speckle imaging, in which photon noise rather than detector readout noise is important. We demonstrate that a one-dimensional pupil geome try (i.e., a thin slit) has several advantages over an unapodized aper ture when faint sources are being observed through atmospheric turbule nce. The use of a slit aperture does not decrease the signal-to-noise ratios of the power-spectrum and bispectrum measurements, and in many cases it increases them, despite the large reduction in signal level. The disadvantage of this apodization is a reduction in Fourier-plane c overage, which must be compensated for by observations with the slit a ligned at several position angles. The performance of many of the curr ent generation of photon-counting imaging detectors deteriorates at th e high counting rates that can be experienced even when one is observi ng sources that are approaching the limiting magnitude of the speckle imaging technique. Under such conditions, we recommend the use of an a podized pupil, in contrast to the current preference for employing a n eutral-density filter to reduce the detector count rate.