572 research outputs found
A conjugate gradient algorithm for the astrometric core solution of Gaia
The ESA space astrometry mission Gaia, planned to be launched in 2013, has
been designed to make angular measurements on a global scale with
micro-arcsecond accuracy. A key component of the data processing for Gaia is
the astrometric core solution, which must implement an efficient and accurate
numerical algorithm to solve the resulting, extremely large least-squares
problem. The Astrometric Global Iterative Solution (AGIS) is a framework that
allows to implement a range of different iterative solution schemes suitable
for a scanning astrometric satellite. In order to find a computationally
efficient and numerically accurate iteration scheme for the astrometric
solution, compatible with the AGIS framework, we study an adaptation of the
classical conjugate gradient (CG) algorithm, and compare it to the so-called
simple iteration (SI) scheme that was previously known to converge for this
problem, although very slowly. The different schemes are implemented within a
software test bed for AGIS known as AGISLab, which allows to define, simulate
and study scaled astrometric core solutions. After successful testing in
AGISLab, the CG scheme has been implemented also in AGIS. The two algorithms CG
and SI eventually converge to identical solutions, to within the numerical
noise (of the order of 0.00001 micro-arcsec). These solutions are independent
of the starting values (initial star catalogue), and we conclude that they are
equivalent to a rigorous least-squares estimation of the astrometric
parameters. The CG scheme converges up to a factor four faster than SI in the
tested cases, and in particular spatially correlated truncation errors are much
more efficiently damped out with the CG scheme.Comment: 24 pages, 16 figures. Accepted for publication in Astronomy &
Astrophysic
Use of Docker for deployment and testing of astronomy software
We describe preliminary investigations of using Docker for the deployment and
testing of astronomy software. Docker is a relatively new containerisation
technology that is developing rapidly and being adopted across a range of
domains. It is based upon virtualization at operating system level, which
presents many advantages in comparison to the more traditional hardware
virtualization that underpins most cloud computing infrastructure today. A
particular strength of Docker is its simple format for describing and managing
software containers, which has benefits for software developers, system
administrators and end users.
We report on our experiences from two projects -- a simple activity to
demonstrate how Docker works, and a more elaborate set of services that
demonstrates more of its capabilities and what they can achieve within an
astronomical context -- and include an account of how we solved problems
through interaction with Docker's very active open source development
community, which is currently the key to the most effective use of this
rapidly-changing technology.Comment: 29 pages, 9 figures, accepted for publication in Astronomy and
Computing, ref ASCOM19
High-resolution tungsten spectroscopy relevant to the diagnostic of high-temperature tokamak plasmas
The x-ray transitions in Cu- and Ni-like tungsten ions in the 5.19â5.26 A wavelength range that are relevant Ë as a high-temperature tokamak diagnostic, in particular for JET in the ITER-like wall configuration, have been studied. Tungsten spectra were measured at the upgraded Shanghai- Electron Beam Ion Trap operated with electron-beam energies from 3.16 to 4.55 keV. High-resolution measurements were performed by means of a flat Si 111 crystal spectrometer equipped by a CCD camera. The experimental wavelengths were determined with an accuracy of 0.3â0.4 mA. The wavelength of the ground-state transition in Cu-like tungsten from the Ë 3p53d104s4d [(3/2,(1/2,5/2)2]1/2 level was measured. All measured wavelengths were compared with those measured from JET ITER-like wall plasmas and with other experiments and various theoretical predictions including COWAN, RELAC, multiconfigurational Dirac-Fock (MCDF), and FAC calculations. To obtain a higher accuracy from theoretical predictions, the MCDF calculations were extended by taking into account correlation effects (configuration-interaction approach). It was found that such an extension brings the calculations closer to the experimental values in comparison with other calculations.National Magnetic Confinement Fusion Program of China 2015GB117000National Natural Science Foundation of China 11374061EURATOM 63305
The astrometric core solution for the Gaia mission. Overview of models, algorithms and software implementation
The Gaia satellite will observe about one billion stars and other point-like
sources. The astrometric core solution will determine the astrometric
parameters (position, parallax, and proper motion) for a subset of these
sources, using a global solution approach which must also include a large
number of parameters for the satellite attitude and optical instrument. The
accurate and efficient implementation of this solution is an extremely
demanding task, but crucial for the outcome of the mission. We provide a
comprehensive overview of the mathematical and physical models applicable to
this solution, as well as its numerical and algorithmic framework. The
astrometric core solution is a simultaneous least-squares estimation of about
half a billion parameters, including the astrometric parameters for some 100
million well-behaved so-called primary sources. The global nature of the
solution requires an iterative approach, which can be broken down into a small
number of distinct processing blocks (source, attitude, calibration and global
updating) and auxiliary processes (including the frame rotator and selection of
primary sources). We describe each of these processes in some detail, formulate
the underlying models, from which the observation equations are derived, and
outline the adopted numerical solution methods with due consideration of
robustness and the structure of the resulting system of equations. Appendices
provide brief introductions to some important mathematical tools (quaternions
and B-splines for the attitude representation, and a modified Cholesky
algorithm for positive semidefinite problems) and discuss some complications
expected in the real mission data.Comment: 48 pages, 19 figures. Accepted for publication in Astronomy &
Astrophysic
Added-value interfaces to asteroid photometric and spectroscopic data in the Gaia database
Abstract We present two added-value interfaces (AVIs) for analyzing photometric and spectroscopic data observed by the Gaia satellite. The Gaia Added-Value Interface for Temporal Analysis (GAVITEA) is used to calculate an estimate for the spin state and shape of an asteroid from its photometric data, and the Gaia Added-Value Interface for Spectral Classification (GAVISC) provides tools to define the taxonomic type and surface absorption coefficient based on spectroscopic asteroid data. Computations are mainly carried out using well-known methods of asteroid data analysis but the AVIs also offer the possibility to test novel methods that are specifically developed for analyzing temporally sparse photometric data, typical for Gaia.Peer reviewe
Preparing Red-Green-Blue (RGB) Images from CCD Data
We present a new, and we believe arguably correct, algorithm for producing
Red-Green-Blue (RBG) composites from 3-band astronomical images. Our method
ensures that an object with a specified astronomical color (e.g. g-r and r-i)
has a unique color in the RGB image, as opposed to the burnt-out white stars to
which we are accustomed. A natural consequence of this is that we can use the
same colors to code color-magnitude diagrams, providing a natural `index' to
our images. We also introduce the use of an asinh stretch, which allows us to
show faint objects while simultaneously preserving the structure of brighter
objects in the field, such as the spiral arms of large galaxies. We believe
that, in addition to their aesthetic value, our images convey far more
information than do the traditional ones, and provide examples from Sloan
Digital Sky Survey (SDSS) imaging, the Hubble Deep Field (HDF), and Chandra to
support our claims. More examples are available at
http://www.astro.princeton.edu/~rhl/PrettyPicture
Relativistic quantum backflow
In this paper we discuss relativistic quantum backflow. The general theory of relativistic
 backflow is written down and it is shown that the backflow can be written as a function of
 a simple parameter epsilon which is defined in terms of fundamental constants and the 
 backflow period. Backflow eigenfunctions are determined numerically for a range of values 
 of epsilon and an explicit expression for the relativistic backflow eigenvalue in terms of the 
 non-relativistic backflow constant is presented. Then backflow eigenvectors are fitted with 
 some standard functions which lead to substantially higher backflow than has been found 
 previously with fitting procedures, for some values of epsilon. In analysing the non-relativistic 
 limit of the theory we show that this problem is one of those rare cases where the relativistic 
 theory is intrinsically more simple than the non-relativistic theory
Gold nanoparticles electrodeposited on glassy carbon using cyclic voltammetry: Application to Hg(II) trace analysis
The electrochemical determination of Hg(II) at trace level using gold nanoparticlesâmodified glassy carbon (AuNPsâGC) electrodes is described. Starting from HAuCl4 in NaNO3, gold nanoparticles (AuNPs) were deposited onto Glassy Carbon (GC) electrodes using Cyclic Voltammetry (CV). Different deposits were obtained by varying the global charge consumed during the whole electroreduction step, depending on the number of cyclic potential scans (N). AuNPs were characterized as a function of the charge using both CV in H2SO4 and Field Emission Gun Scanning Electron Microscopy (FEG-SEM). The AuNPsâGC electrodes were then applied to determine low Hg(II) concentrations using Square Wave Anodic Stripping Voltammetry (SWASV). The AuNPsâGC electrodes provided significantly improved performances in Hg(II) determination compared to unmodified GC and bare Au electrodes. It was shown that the physico-chemical properties of the deposits are correlated to the performances of the AuNPsâGC electrode with respect to Hg(II) assay. The best results were obtained for four electrodeposition cyclic scans, where small-sized particles (36 ± 13 nm) with high density (73 particles ÎŒm-ÂČ) were obtained. Under these conditions, a linearity range from 0.64 to 4.00 nM and a limit of detection of 0.42 nM were obtained
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