6 research outputs found
HVOX: Scalable Interferometric Synthesis and Analysis of Spherical Sky Maps
Analysis and synthesis are key steps of the radio-interferometric imaging
process, serving as a bridge between visibility and sky domains. They can be
expressed as partial Fourier transforms involving a large number of non-uniform
frequencies and spherically-constrained spatial coordinates. Due to the data
non-uniformity, these partial Fourier transforms are computationally expensive
and represent a serious bottleneck in the image reconstruction process. The
W-gridding algorithm achieves log-linear complexity for both steps by applying
a series of 2D non-uniform FFTs (NUFFT) to the data sliced along the so-called
frequency coordinate. A major drawback of this method however is its
restriction to direction-cosine meshes, which are fundamentally ill-suited for
large field of views. This paper introduces the HVOX gridder, a novel algorithm
for analysis/synthesis based on a 3D-NUFFT. Unlike W-gridding, the latter is
compatible with arbitrary spherical meshes such as the popular HEALPix scheme
for spherical data processing. The 3D-NUFFT allows one to optimally select the
size of the inner FFTs, in particular the number of W-planes. This results in a
better performing and auto-tuned algorithm, with controlled accuracy guarantees
backed by strong results from approximation theory. To cope with the
challenging scale of next-generation radio telescopes, we propose moreover a
chunked evaluation strategy: by partitioning the visibility and sky domains,
the 3D-NUFFT is decomposed into sub-problems which execute in parallel, while
simultaneously cutting memory requirements. Our benchmarking results
demonstrate the scalability of HVOX for both SKA and LOFAR, considering
state-of-the-art challenging imaging setups. HVOX is moreover computationally
competitive with W-gridder, despite the absence of domain-specific
optimizations in our implementation
La grotte des rhinocéros (Casablanca, Maroc) : le remplissage et son âge
International audienc
Les Arcs-sur-Argens: Saint-Pierre
Bilan scientifique. RĂ©gion Provence-Alpes-CĂ´te d'Azur 2008, p. 175-17
Contribution of ESR/U-series dating to the chronology of late Middle Palaeolithic sites in the middle RhĂ´ne valley, southeastern France
International audienceThe establishment of a chronology for late Middle Palaeolithic sites on the right bank of the Rhone valley in southeastern France is important for the knowledge of Neandertal dynamics and their demise in this area. The suite of dating methods that are directly applicable to fossils is limited for this period, especially around 50 ka where radiocarbon dating is beyond its technical limits. Currently applied to Middle and Lower Pleistocene periods, the use of combined ESR/U-series dating on Upper Pleistocene samples led to new issues, such as the acquisition of an age for samples yielding low equivalent doses and low uranium content in dental tissues. The gamma dose rate measurement thus plays a key role in age calculation. Beyond the discussion on methodological issues, the present study contributes to the establishment of a chronological framework that covers the Neandertal occupations between MIS 5 and MIS 3 for this area