2,762 research outputs found
Exploiting spatial sparsity for multi-wavelength imaging in optical interferometry
Optical interferometers provide multiple wavelength measurements. In order to
fully exploit the spectral and spatial resolution of these instruments, new
algorithms for image reconstruction have to be developed. Early attempts to
deal with multi-chromatic interferometric data have consisted in recovering a
gray image of the object or independent monochromatic images in some spectral
bandwidths. The main challenge is now to recover the full 3-D (spatio-spectral)
brightness distribution of the astronomical target given all the available
data. We describe a new approach to implement multi-wavelength image
reconstruction in the case where the observed scene is a collection of
point-like sources. We show the gain in image quality (both spatially and
spectrally) achieved by globally taking into account all the data instead of
dealing with independent spectral slices. This is achieved thanks to a
regularization which favors spatial sparsity and spectral grouping of the
sources. Since the objective function is not differentiable, we had to develop
a specialized optimization algorithm which also accounts for non-negativity of
the brightness distribution.Comment: This version has been accepted for publication in J. Opt. Soc. Am.
PeX 1. Multi-spectral expansion of residual speckles for planet detection
The detection of exoplanets in coronographic images is severely limited by
residual starlight speckles. Dedicated post-processing can drastically reduce
this "stellar leakage" and thereby increase the faintness of detectable
exoplanets. Based on a multi-spectral series expansion of the diffraction
pattern, we derive a multi-mode model of the residuals which can be exploited
to estimate and thus remove the residual speckles in multi-spectral
coronographic images. Compared to other multi-spectral processing methods, our
model is physically grounded and is suitable for use in an (optimal) inverse
approach. We demonstrate the ability of our model to correctly estimate the
speckles in simulated data and demonstrate that very high contrasts can be
achieved. We further apply our method to removing speckles from a real data
cube obtained with the SPHERE IFS instrument.Comment: accepted for publication in MNRAS on 25th of August 2017, 17 pages,
15 figure
Image Reconstruction in Optical Interferometry
This tutorial paper describes the problem of image reconstruction from
interferometric data with a particular focus on the specific problems
encountered at optical (visible/IR) wavelengths. The challenging issues in
image reconstruction from interferometric data are introduced in the general
framework of inverse problem approach. This framework is then used to describe
existing image reconstruction algorithms in radio interferometry and the new
methods specifically developed for optical interferometry.Comment: accepted for publication in IEEE Signal Processing Magazin
Mixed models for longitudinal left-censored repeated measures
Longitudinal studies could be complicated by left-censored repeated measures.
For example, in Human Immunodeficiency Virus infection, there is a detection
limit of the assay used to quantify the plasma viral load. Simple imputation of
the limit of the detection or of half of this limit for left-censored measures
biases estimations and their standard errors. In this paper, we review two
likelihood-based methods proposed to handle left-censoring of the outcome in
linear mixed model. We show how to fit these models using SAS Proc NLMIXED and
we compare this tool with other programs. Indications and limitations of the
programs are discussed and an example in the field of HIV infection is shown
Holomorphic Approximation via Dolbeault Cohomology
The purpose of this paper is to study holomorphic approximation and
approximation of -closed forms in complex manifolds of complex
dimension . We consider extensions of the classical Runge theorem and
the Mergelyan property to domains in complex manifolds for the smooth and the
topology. We characterize the Runge or Mergelyan property in terms of
certain Dolbeault cohomology groups and some geometric sufficient conditions
are given
High dynamic range imaging with a single-mode pupil remapping system : a self-calibration algorithm for redundant interferometric arrays
The correction of the influence of phase corrugation in the pupil plane is a
fundamental issue in achieving high dynamic range imaging. In this paper, we
investigate an instrumental setup which consists in applying interferometric
techniques on a single telescope, by filtering and dividing the pupil with an
array of single-mode fibers. We developed a new algorithm, which makes use of
the fact that we have a redundant interferometric array, to completely
disentangle the astronomical object from the atmospheric perturbations (phase
and scintillation). This self-calibrating algorithm can also be applied to any
- diluted or not - redundant interferometric setup. On an 8 meter telescope
observing at a wavelength of 630 nm, our simulations show that a single mode
pupil remapping system could achieve, at a few resolution elements from the
central star, a raw dynamic range up to 10^6; depending on the brightness of
the source. The self calibration algorithm proved to be very efficient,
allowing image reconstruction of faint sources (mag = 15) even though the
signal-to-noise ratio of individual spatial frequencies are of the order of
0.1. We finally note that the instrument could be more sensitive by combining
this setup with an adaptive optics system. The dynamic range would however be
limited by the noise of the small, high frequency, displacements of the
deformable mirror.Comment: 11 pages, 7 figures. Accepted for publication in MNRA
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