58 research outputs found
From electrons to stars : modelling and mitigation of radiation damage effects on astronomical CCDs
The work presented in this thesis is part of an on-going effort to understand and mitigate the effects of radiation damage in astronomical CCDs. My research was motivated by and took place in the challenging context of the European Space Agency__s (ESA) astrometric mission, Gaia, for which radiation damage has been considered since its conception as one of the most important threats to its scientific performance. In this context my research focused primarily on the modelling of the effects of radiation-induced Charge Transfer Inefficiency (CTI) supported by the analysis of the experimental test data. I developed the most detailed model to date of CTI in CCDs that enables simulating the operation of irradiated devices (Chapter 2). Using this model I have been able to verify and enhance our current understanding of CTI as well as support the characterization of CCDs and the understanding of experimental results (Chapters 3 and 6). As part of this research I conducted the comprehensive re-assessment of the performance of Gaia taking into account radiation damage (Chapters 3 and 4). Finally I took part in the effort of countering CTI by elaborating, testing, and improving a forward modelling approach at the image processing level to mitigate the CTI effects on the Gaia easurements (Chapters 3 and 5), as well as test and explore the potential of a specific hardware mitigation tool (Chapter 6).European Marie-Curie research training network ELSA (MRTN-CT-2006-033481)UBL - phd migration 201
Modelling Gaia CCD pixels with Silvaco 3D engineering software
Gaia will only achieve its unprecedented measurement accuracy requirements
with detailed calibration and correction for radiation damage. We present our
Silvaco 3D engineering software model of the Gaia CCD pixel and two of its
applications for Gaia: (1) physically interpreting supplementary buried channel
(SBC) capacity measurements (pocket-pumping and first pixel response) in terms
of e2v manufacturing doping alignment tolerances; and (2) deriving electron
densities within a charge packet as a function of the number of constituent
electrons and 3D position within the charge packet as input to microscopic
models being developed to simulate radiation damage.Comment: 4 pages, 3 figures, contributed poster, appearing in proceedings of
the ELSA conference: Gaia, at the frontiers of astrometry, 7-11 June 2010,
S\`evres, Pari
Detecting stars, galaxies, and asteroids with Gaia
(Abridged) Gaia aims to make a 3-dimensional map of 1,000 million stars in
our Milky Way to unravel its kinematical, dynamical, and chemical structure and
evolution. Gaia's on-board detection software discriminates stars from spurious
objects like cosmic rays and Solar protons. For this, parametrised
point-spread-function-shape criteria are used. This study aims to provide an
optimum set of parameters for these filters. We developed an emulation of the
on-board detection software, which has 20 free, so-called rejection parameters
which govern the boundaries between stars on the one hand and sharp or extended
events on the other hand. We evaluate the detection and rejection performance
of the algorithm using catalogues of simulated single stars, double stars,
cosmic rays, Solar protons, unresolved galaxies, and asteroids. We optimised
the rejection parameters, improving - with respect to the functional baseline -
the detection performance of single and double stars, while, at the same time,
improving the rejection performance of cosmic rays and of Solar protons. We
find that the minimum separation to resolve a close, equal-brightness double
star is 0.23 arcsec in the along-scan and 0.70 arcsec in the across-scan
direction, independent of the brightness of the primary. We find that, whereas
the optimised rejection parameters have no significant impact on the
detectability of de Vaucouleurs profiles, they do significantly improve the
detection of exponential-disk profiles. We also find that the optimised
rejection parameters provide detection gains for asteroids fainter than 20 mag
and for fast-moving near-Earth objects fainter than 18 mag, albeit this gain
comes at the expense of a modest detection-probability loss for bright,
fast-moving near-Earth objects. The major side effect of the optimised
parameters is that spurious ghosts in the wings of bright stars essentially
pass unfiltered.Comment: Accepted for publication in A&
Digging supplementary buried channels: investigating the notch architecture within the CCD pixels on ESA's Gaia satellite
The European Space Agency (ESA) Gaia satellite has 106 CCD image sensors
which will suffer from increased charge transfer inefficiency (CTI) as a result
of radiation damage. To aid the mitigation at low signal levels, the CCD design
includes Supplementary Buried Channels (SBCs, otherwise known as `notches')
within each CCD column. We present the largest published sample of Gaia CCD SBC
Full Well Capacity (FWC) laboratory measurements and simulations based on 13
devices. We find that Gaia CCDs manufactured post-2004 have SBCs with FWCs in
the upper half of each CCD that are systematically smaller by two orders of
magnitude (<50 electrons) compared to those manufactured pre-2004 (thousands of
electrons). Gaia's faint star (13 < G < 20 mag) astrometric performance
predictions by Prod'homme et al. and Holl et al. use pre-2004 SBC FWCs as
inputs to their simulations. However, all the CCDs already integrated onto the
satellite for the 2013 launch are post-2004. SBC FWC measurements are not
available for one of our five post-2004 CCDs but the fact it meets Gaia's image
location requirements suggests it has SBC FWCs similar to pre-2004. It is too
late to measure the SBC FWCs onboard the satellite and it is not possible to
theoretically predict them. Gaia's faint star astrometric performance
predictions depend on knowledge of the onboard SBC FWCs but as these are
currently unavailable, it is not known how representative of the whole focal
plane the current predictions are. Therefore, we suggest Gaia's initial
in-orbit calibrations should include measurement of the onboard SBC FWCs. We
present a potential method to do this. Faint star astrometric performance
predictions based on onboard SBC FWCs at the start of the mission would allow
satellite operating conditions or CTI software mitigation to be further
optimised to improve the scientific return of Gaia.Comment: Accepted for publication in MNRAS, 16 pages, 19 figure
Two novel human cytomegalovirus NK cell evasion functions target MICA for lysosomal degradation
NKG2D plays a major role in controlling immune responses through the regulation of natural killer (NK) cells, αβ and γδ T-cell function. This activating receptor recognizes eight distinct ligands (the MHC Class I polypeptide-related sequences (MIC) A andB, and UL16-binding proteins (ULBP)1–6) induced by cellular stress to promote recognition cells perturbed by malignant transformation or microbial infection. Studies into human cytomegalovirus (HCMV) have aided both the identification and characterization of NKG2D ligands (NKG2DLs). HCMV immediate early (IE) gene up regulates NKGDLs, and we now describe the differential activation of ULBP2 and MICA/B by IE1 and IE2 respectively. Despite activation by IE functions, HCMV effectively suppressed cell surface expression of NKGDLs through both the early and late phases of infection. The immune evasion functions UL16, UL142, and microRNA(miR)-UL112 are known to target NKG2DLs. While infection with a UL16 deletion mutant caused the expected increase in MICB and ULBP2 cell surface expression, deletion of UL142 did not have a similar impact on its target, MICA. We therefore performed a systematic screen of the viral genome to search of addition functions that targeted MICA. US18 and US20 were identified as novel NK cell evasion functions capable of acting independently to promote MICA degradation by lysosomal degradation. The most dramatic effect on MICA expression was achieved when US18 and US20 acted in concert. US18 and US20 are the first members of the US12 gene family to have been assigned a function. The US12 family has 10 members encoded sequentially through US12–US21; a genetic arrangement, which is suggestive of an ‘accordion’ expansion of an ancestral gene in response to a selective pressure. This expansion must have be an ancient event as the whole family is conserved across simian cytomegaloviruses from old world monkeys. The evolutionary benefit bestowed by the combinatorial effect of US18 and US20 on MICA may have contributed to sustaining the US12 gene family
On-orbit performance of the Gaia CCDs at L2
The European Space Agency's Gaia satellite was launched into orbit around L2
in December 2013 with a payload containing 106 large-format scientific CCDs.
The primary goal of the mission is to repeatedly obtain high-precision
astrometric and photometric measurements of one thousand million stars over the
course of five years. The scientific value of the down-linked data, and the
operation of the onboard autonomous detection chain, relies on the high
performance of the detectors. As Gaia slowly rotates and scans the sky, the
CCDs are continuously operated in a mode where the line clock rate and the
satellite rotation spin-rate are in synchronisation. Nominal mission operations
began in July 2014 and the first data release is being prepared for release at
the end of Summer 2016.
In this paper we present an overview of the focal plane, the detector system,
and strategies for on-orbit performance monitoring of the system. This is
followed by a presentation of the performance results based on analysis of data
acquired during a two-year window beginning at payload switch-on. Results for
parameters such as readout noise and electronic offset behaviour are presented
and we pay particular attention to the effects of the L2 radiation environment
on the devices. The radiation-induced degradation in the charge transfer
efficiency (CTE) in the (parallel) scan direction is clearly diagnosed;
however, an extrapolation shows that charge transfer inefficiency (CTI) effects
at end of mission will be approximately an order of magnitude less than
predicted pre-flight. It is shown that the CTI in the serial register
(horizontal direction) is still dominated by the traps inherent to the
manufacturing process and that the radiation-induced degradation so far is only
a few per cent. Finally, we summarise some of the detector effects discovered
on-orbit which are still being investigated.Comment: Published in A&A Gaia special featur
The Gaia mission
Gaia is a cornerstone mission in the science programme of the EuropeanSpace Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page. http://www.cosmos.esa.int/gai
The XXL Survey . IV. Mass-temperature relation of the bright cluster sample
Part of the first data release of the XXL Survey. Associated data is accessible via CDS and via the XXL Database hosted at IASF-MIContext. The XXL Survey is the largest survey carried out by XMM-Newton. Covering an area of 50 deg2, the survey contains ~450 galaxy clusters out to a redshift ~2 and to an X-ray flux limit of ~ 5 × 10-15 erg s-1 cm-2. This paper is part of the first release of XXL results focussed on the bright cluster sample. Aims: We investigate the scaling relation between weak-lensing mass and X-ray temperature for the brightest clusters in XXL. The scaling relation discussed in this article is used to estimate the mass of all 100 clusters in XXL-100-GC. Methods: Based on a subsample of 38 objects that lie within the intersection of the northern XXL field and the publicly available CFHTLenS shear catalog, we derive the weak-lensing mass of each system with careful considerations of the systematics. The clusters lie at 0.1 Results: The mass-temperature relation fit (M ∝ Tb) to the XXL clusters returns a slope and intrinsic scatter σlnM|T≃ 0.53; the scatter is dominated by disturbed clusters. The fit to the combined sample of 96 clusters is in tension with self-similarity, b = 1.67 ± 0.12 and σlnM|T ≃ 0.41. Conclusions: Overall our results demonstrate the feasibility of ground-based weak-lensing scaling relation studies down to cool systems of ~1 keV temperature and highlight that the current data and samples are a limit to our statistical precision. As such we are unable to determine whether the validity of hydrostatic equilibrium is a function of halo mass. An enlarged sample of cool systems, deeper weak-lensing data, and robust modelling of the selection function will help to explore these issues further. Based on observations obtained with XMM-Newton, an ESA sci- ence mission with instruments and contributions directly funded by ESA Member States and NASA. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme 089.A-0666 and LP191.A-0268.The Master catalogue is available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/592/A
Variational approach to regularity of optimal transport maps : general cost functions
We extend the variational approach to regularity for optimal transport maps
initiated by Goldman and the first author to the case of general cost
functions. Our main result is an -regularity result for optimal
transport maps between H\"older continuous densities slightly more quantitative
than the result by De Philippis-Figalli. One of the new contributions is the
use of almost-minimality: if the cost is quantitatively close to the Euclidean
cost function, a minimizer for the optimal transport problem with general cost
is an almost-minimizer for the one with quadratic cost. This further highlights
the connection between our variational approach and De Giorgi's strategy for
-regularity of minimal surfaces.Comment: In this version we added an extension of our main
-regularity result to general almost-minimisers (among optimal
transport maps), see Theorem 1.18. 51 pages, 2 figure
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