893 research outputs found
Wavefront sensing with a brightest pixel selection algorithm
Astronomical adaptive optics systems with open-loop deformable mirror control
have recently come on-line. In these systems, the deformable mirror surface is
not included in the wavefront sensor paths, and so changes made to the
deformable mirror are not fed back to the wavefront sensors. This gives rise to
all sorts of linearity and control issues mainly centred on one question: Has
the mirror taken the shape requested? Non-linearities in wavefront measurement
and in the deformable mirror shape can lead to significant deviations in mirror
shape from the requested shape. Here, wavefront sensor measurements made using
a brightest pixel selection method are discussed along with the implications
that this has for open-loop AO systems. Discussion includes elongated laser
guide star spots and also computational efficiency.Comment: 10 pages, 12 figures, accepted by MNRA
Bayesian photon counting with electron-multiplying charge coupled devices (EMCCDs)
The EMCCD is a CCD type that delivers fast readout and negligible detector
noise, making it an ideal detector for high frame rate applications. Because of
the very low detector noise, this detector can potentially count single
photons. Considering that an EMCCD has a limited dynamical range and negligible
detector noise, one would typically apply an EMCCD in such a way that multiple
images of the same object are available, for instance, in so called lucky
imaging. The problem of counting photons can then conveniently be viewed as
statistical inference of flux or photon rates, based on a stack of images. A
simple probabilistic model for the output of an EMCCD is developed. Based on
this model and the prior knowledge that photons are Poisson distributed, we
derive two methods for estimating the most probable flux per pixel, one based
on thresholding, and another based on full Bayesian inference. We find that it
is indeed possible to derive such expressions, and tests of these methods show
that estimating fluxes with only shot noise is possible, up to fluxes of about
one photon per pixel per readout.Comment: Fixed a few typos compared to the published versio
Basic principles of SSM modelling: an examination of CATWOE from a soft perspective
This paper examines the SSM technique CATWOE, which focuses on defining necessary elements that together constitute a human activity system from a certain perspective. Despite its recognition within the literature and its numerous uses, there are few studies on how the technique can be improved. This research reflects on each of the elements both from a theoretical and a practical perspective. Findings point to the fact that some of the terms have a meaning in everyday language that differs from its definition within CATWOE. Other concepts are not well-defined. This is unfortunate and may both lead to misunderstandings and limit analysis. The paper points to a number of ways in which the use of CATWOE can be developed in order to further support the process of eliciting novel ideas for future actions. Hence, the overall conclusion is that the elements need to be rethought and some of them renamed
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