Experimental measurements of estimator bias and the signal-to-noise ratio for deconvolution from wave-front sensing.
1997
Deconvolution from wave-front sensing (DWFS) has been proposed as a
method for achieving high-resolution images of astronomical objects from
ground-based telescopes. The technique consists of the simultaneous
measurement of a short-exposure focal-plane speckled image, as well as the
wave front, by use of a Shack–Hartmann sensor placed at the pupil plane.
In early studies it was suspected that some problems would occur in poor
seeing conditions; however, it was usually assumed that the technique would
work well as long as the wave-front sensor subaperture spacing was less than
r0
(L/
r0 < 1). Atmosphere-induced
phase errors in the pupil of a telescope imaging system produce both phase
errors and magnitude errors in the effective short-exposure optical transfer
function (OTF) of the system. Recently it has been shown that the commonly
used estimator for this technique produces biased estimates of the magnitude
errors. The significance of this bias problem is that one cannot properly
estimate or correct for the frame-to-frame fluctuations in the magnitude of
the OTF but can do so only for fluctuations in the phase. An auxiliary
estimate must also be used to correct for the mean value of the magnitude
error. The inability to compensate for the magnitude fluctuations results in a
signal-to-noise ratio (SNR) that is less favorable for the technique than was
previously thought. In some situations simpler techniques, such as the
Knox-Thompson and bispectrum methods, which require only speckle gram data
from the focal plane of the imaging system, can produce better results. We
present experimental measurements based on observations of bright stars and
the Jovian moon Ganymede that confirm previous theoretical
predictions.
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