6,556 research outputs found
Towards optical intensity interferometry for high angular resolution stellar astrophysics
Most neighboring stars are still detected as point sources and are beyond the
angular resolution reach of current observatories. Methods to improve our
understanding of stars at high angular resolution are investigated. Air
Cherenkov telescopes (ACTs), primarily used for Gamma-ray astronomy, enable us
to increase our understanding of the circumstellar environment of a particular
system. When used as optical intensity interferometers, future ACT arrays will
allow us to detect stars as extended objects and image their surfaces at high
angular resolution.
Optical stellar intensity interferometry (SII) with ACT arrays, composed of
nearly 100 telescopes, will provide means to measure fundamental stellar
parameters and also open the possibility of model-independent imaging. A data
analysis algorithm is developed and permits the reconstruction of high angular
resolution images from simulated SII data. The capabilities and limitations of
future ACT arrays used for high angular resolution imaging are investigated via
Monte-Carlo simulations. Simple stellar objects as well as stellar surfaces
with localized hot or cool regions can be accurately imaged.
Finally, experimental efforts to measure intensity correlations are
expounded. The functionality of analog and digital correlators is demonstrated.
Intensity correlations have been measured for a simulated star emitting
pseudo-thermal light, resulting in angular diameter measurements. The StarBase
observatory, consisting of a pair of 3 m telescopes separated by 23 m, is
described.Comment: PhD dissertatio
LOFAR Sparse Image Reconstruction
Context. The LOw Frequency ARray (LOFAR) radio telescope is a giant digital
phased array interferometer with multiple antennas distributed in Europe. It
provides discrete sets of Fourier components of the sky brightness. Recovering
the original brightness distribution with aperture synthesis forms an inverse
problem that can be solved by various deconvolution and minimization methods
Aims. Recent papers have established a clear link between the discrete nature
of radio interferometry measurement and the "compressed sensing" (CS) theory,
which supports sparse reconstruction methods to form an image from the measured
visibilities. Empowered by proximal theory, CS offers a sound framework for
efficient global minimization and sparse data representation using fast
algorithms. Combined with instrumental direction-dependent effects (DDE) in the
scope of a real instrument, we developed and validated a new method based on
this framework Methods. We implemented a sparse reconstruction method in the
standard LOFAR imaging tool and compared the photometric and resolution
performance of this new imager with that of CLEAN-based methods (CLEAN and
MS-CLEAN) with simulated and real LOFAR data Results. We show that i) sparse
reconstruction performs as well as CLEAN in recovering the flux of point
sources; ii) performs much better on extended objects (the root mean square
error is reduced by a factor of up to 10); and iii) provides a solution with an
effective angular resolution 2-3 times better than the CLEAN images.
Conclusions. Sparse recovery gives a correct photometry on high dynamic and
wide-field images and improved realistic structures of extended sources (of
simulated and real LOFAR datasets). This sparse reconstruction method is
compatible with modern interferometric imagers that handle DDE corrections (A-
and W-projections) required for current and future instruments such as LOFAR
and SKAComment: Published in A&A, 19 pages, 9 figure
Intermittency of interstellar turbulence: extreme velocity-shears and CO emission on milliparsec scale
The condensation of diffuse gas into molecular clouds occurs at a rate driven
largely by turbulent dissipation. This process still has to be caught in action
and characterized. A mosaic of 13 fields was observed in the CO(1-0) line with
the IRAM-PdB interferometer in the translucent environment of two low-mass
dense cores. The large size of the mosaic compared to the resolution (4 arcsec)
is unprecedented in the study of the small-scale structure of diffuse molecular
gas. Eight weak and elongated structures of thicknesses as small as 3 mpc (600
AU) and lengths up to 70mpc are found. These are not filaments because once
merged with short-spacing data, they appear as the sharp edges of larger-scale
structures. Six out of eight form quasi-parallel pairs at different velocities
and different position angles. This cannot be the result of chance alignment.
The velocity-shears estimated for the three pairs include the highest ever
measured far from star forming regions (780 km/s/pc). Because the large scale
structures have sharp edges, with little or no overlap, they have to be thin
CO-layers. Their edges mark a sharp transition between a CO-rich component and
a gas undetected in the CO line because of its low CO abundance, presumably the
cold neutral medium. We propose that these sharp edges are the first
directly-detected manifestations of the intermittency of interstellar
turbulence. The large velocity-shears reveal an intense straining field,
responsible for a local dissipation rate several orders of magnitude above
average, possibly at the origin of the thin CO-layers.Comment: 16 pages, 11 figures, Accepted for publication in Astronomy and
Astrophysic
Intensity invariant complex encoded colour correlation
Optical correlation has traditionally processed monochromatic grey scale images. This paper develops a new encoding mechanism that uses the chromaticy of the input signal. It is then not only possible to detect different coloured objects but the system is invariant to changes in the brightness of the lighting, including variations across the object
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